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Differences of Ballet Turns (Pirouette) Performance Between Experienced and Novice Ballet Dancers a
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Chia-Wei Lin , Shing-Jye Chen , Fong-Chin Su , Hong-Wen Wu & Cheng-Feng Lin a
National Cheng Kung University
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National Taiwan University of Physical Education and Sports Published online: 20 Aug 2014.
To cite this article: Chia-Wei Lin, Shing-Jye Chen, Fong-Chin Su, Hong-Wen Wu & Cheng-Feng Lin (2014) Differences of Ballet Turns (Pirouette) Performance Between Experienced and Novice Ballet Dancers, Research Quarterly for Exercise and Sport, 85:3, 330-340, DOI: 10.1080/02701367.2014.930088 To link to this article: http://dx.doi.org/10.1080/02701367.2014.930088
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Research Quarterly for Exercise and Sport, 85, 330–340, 2014 Copyright q SHAPE America ISSN 0270-1367 print/ISSN 2168-3824 online DOI: 10.1080/02701367.2014.930088
Differences of Ballet Turns (Pirouette) Performance Between Experienced and Novice Ballet Dancers Chia-Wei Lin, Shing-Jye Chen, and Fong-Chin Su National Cheng Kung University
Hong-Wen Wu Downloaded by [Washburn University] at 06:24 02 November 2014
National Taiwan University of Physical Education and Sports
Cheng-Feng Lin National Cheng Kung University
Purpose: This study investigated the different postural control strategies exhibited by experienced and novice dancers in ballet turns ( pirouettes). Method: Thirteen novice and 13 experienced dancers performed ballet turns with dominant-leg support. The peak push force was measured in the double-leg support phase. The inclination angles of rotation axis with respect to vertical axis were calculated in the early single-leg support phase as well as the initiation sequence of ankle, knee, and hip joints on the supporting leg. Moreover, the anchoring index of the head was computed in the transverse plane during turning. Results: The novice dancers applied a greater push force, an increased inclination angle of rotation axis, and an insufficient proximal-to-distal extension sequence pattern. The novice dancers also had a smaller head-anchoring index compared with experienced dancers, which meant novice dancers were not using a space target as a stability reference. Conclusions: A poorer performance in novice dancers could result from higher push force in propulsion, lack of a “proximal-to-distal extension sequence” pattern, and lack of visual spotting for postural stability. Training on sequential initiation of lower-extremity joints and rehearsal of visual spotting are essential for novice dancers to obtain better performance on ballet turns. Keywords: anchoring index, ground reaction force, rotation axis, sequential pattern
A pirouette is known as a ballet turn and requires a rapid full rotation of the body with single-leg support on the toes or balls of the foot. An efficient strategy for maintaining dynamic stability of whole body is essential for performing a pirouette. Because the magnitudes of the push forces for pirouettes are positively correlated with the revolutions of turns (Laws, 2002), quantifying the three-dimensional (3-D) forces can provide insight into how to initiate and control pirouettes. In addition, dancers push their gesture leg off from initially flexed legs with an upright trunk, and then Submitted March 11, 2013; accepted November 25, 2013. Correspondence should be addressed to Cheng-Feng Lin, Department of Physical Therapy, National Cheng Kung University, 1 University Road, 701 Tainan, Taiwan. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/urqe.
they extend their hip joints followed by extending the knee and then ankle plantar flexing at the end. This proximal-todistal moving sequence in ballet dancers has shown its positive effects on performance, which is similar to that in soccer instep kicking (Naito, Fukui, & Maruyama, 2012), sprinting to start (Charalambous, Irwin, Bezodis, & Kerwin, 2012), and jumping to land (Iida, Kanehisa, Inaba, & Nakazawa, 2011). The rotation axis of ballet turns was previously assumed to be a fixed vertical axis (Laws, 1978). However, the rotation axis is not constantly fixed and remaining in the vertical direction, and thus, using the other method to calculate the rotation axis in pirouettes could be a better measurement of performance in ballet turns. An instantaneous rotation axis calculated by the weighted least squares method has been well applied in previous studies to
CONTROL STRATEGY OF BALLET TURNS IN DANCERS
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TABLE 1 Basic Anthropometric Data and Characteristics of Dancers
Age (years) Height (cm) Body weight (kg) Body mass index (kg/m2) Experience in dance (years)a Experience in ballet dance (years) Experience with wearing pointe shoes (yes/no) Years wearing pointe shoes (years) Experience of one-revolution pirouette in soft shoes (years) Years of one-revolution pirouette in pointe shoes (years)
Experienced
Novice
p Value
df
t
Effect Size (Cohen’s d)
17.77 (3.39) 159.27 (4.21) 51.54 (4.66) 20.29 ^ 1.37 11.31 (3.07) 8.69 (3.30) 13/0 4.54 (2.88) 6.08 (3.40) 2.75 (1.71)
12.00 (1.91) 151.89 (11.48) 43.81 (9.68) 18.7 ^ 2.14 4.31 (2.02) 3.23 (1.69) 5/8 0.23 (0.23) 0.92 (0.64) 0.00 (0.00)
,.001* .046* .017* .033* ,.001* ,.001* — ,.001* ,.001* ,.001*
24 15.16 24 24 24 24 — 24 24 24
5.34 2.18 2.58 2.27 6.88 5.31 — 5.37 6.74 5.06
2.10 0.85 1.02 0.88 2.69 2.08 — 2.11 2.11 2.27
*Statistically significant difference between groups ( p , .05). Movement class, folk dance, and some basic training of preballet.
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a
describe the quantity of body rotations around the axis (Salvia et al., 2000; Sommer, 1992; Woltring, 1976, 1980, 1994), and it may further be utilized to calculate the rotation axis in ballet turns. Moreover, aligning the rotation axis with the vertical line is essential for dancers to maintain postural stability of t turns because the postural stability is greatly influenced by the deviations of the rotation axis and may result in a fail of turning (Laws, 2002). The anchoring index (AI), an indicator of how well two adjacent segments coordinate to each other, has been often used to identify walking stability in patients (Mesure, Azulay, Pouget, & Amblard, 1999; Sienko, Balkwill, Oddsson, & Wall, 2008; Sveistrup, Schneiberg, McKinley, McFadyen, & Levin, 2008). Although the relative angle changes of adjacent segments in pirouettes were analyzed previously (Golomer, Rosey, Dizac, Mertz, & Fagard, 2008), the angle changes did not provide sufficient assessments of stability and coordination for the dancers. To our knowledge, there is no study available using the AI to evaluate dancers’ performance by using the critical measure of head-on-trunk orientation during pirouettes (Robert, Blouin, Ruget, & Mouchnino, 2007). Thus, the calculation of AI in head-on-trunk orientation may provide useful information in how dancers maintain their stability. Therefore, the purpose of the current study was to investigate control strategies of ballet dancers during pirouettes using advanced biomechanical tools. The strategies were quantified by (a) the 3-D ground reaction forces of the supporting leg and gesture leg at initial push off; (b) the initiation sequence of the supporting leg in the ankle, knee, and hip joints; (c) the inclination angle changes based on the angles between the instantaneous rotation axis and the vertical global axis; and (d) the performance of AI in head-on-trunk orientation between novice and experienced dancers. We hypothesized that experienced ballet dancers would perform better than novice dancers by showing the fewest changes in the inclination angles of the rotation axis and the AI of the head, accompanied with a smaller 3-D ground reaction
force for turning initiation. We hope that the findings of the current study will provide dance educators with more details about turning to enhance their teaching and to help novice dancers to learn the turning movement in a more efficient and correct way.
METHODS Participants Thirteen experienced female dancers and 13 novice female dancers were recruited to take part in the study (see Table 1). The inclusion criteria for the experienced group were specified as follows: (a) at least 6 years of ballet training; (b) at least 3 hr of weekly routine training; and (c) a single-leg support for two complete revolutions of turning. The inclusion criteria for the novice group were also specified as follows: (a) 2 years to 5 years of ballet training; (b) 1.5 hr and less than 3 hr of weekly routine training; and (c) a single-leg support for one complete revolution of turning but the inability to perform two complete revolutions. The exclusion criteria of the participants were a history of a vestibular or balance problem and previous injuries of the lower extremity and back. This information was obtained from participants’ self-reported history. Each participant read and signed an informed consent approved by the hospital institutional review board. Note that experienced dancers are generally older than novice dancers. This is mainly because dancers start to learn dance at the preschool age, the experienced dancers often have more years of training than the novice dancers, and the balance ability is also positively correlated with age (Kilby & Newell, 2012). Instrumentations An acquisition system with eight Eagle high-speed cameras (Motion Analysis Corporation, Santa Rosa, CA) was
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sampled at 200 Hz to collect 3-D trajectories of makers placed on each participant. A modified Helen Hayes marker set with 36 markers was used and placed on the specific anatomical locations of participants who wore leotards and individual ballet shoes (see Figure 1). The locations of each segment are listed as the following anatomical landmarks: forehead, top head, rear head, sternal notch, xiphoid process, umbilicus, seventh cervical spinal process (C7), sacrum, bilateral acromion process, midpoints of the arm and forearm, lateral epicondyles of the humerus, radial styloid processes, third metacarpal heads, anterior superior iliac spines (ASIS), midpoints of the lateral sides of the thigh and shank, greater trochanters, lateral knee joint lines, lateral malleoli of ankles, midpoints between the first and fifth metatarsal heads, and the posterior heels. Two static standing trials were collected before performing a turn. Eight additional markers were placed on both limbs of the medial humeral epicondyles, ulnar styloid processes, medial knee, and ankle joints, and they were removed during dynamic trials. Two force plates (Kistler Instrument Corporation, Winterhur, Switzerland) were mounted on the ground to record 3-D ground reaction force with sampling at 1,000 Hz and synchronized with the motion acquisition system. To detect the ground reaction force correctly, the performance area was expanded with a customized wooden floor that was 800 mm £ 600 mm £ 40 mm (length £ width £ height), and the floor was covered with vinyl to create a floor surface that the participants were familiar with during their routine practices. Procedures All female participants performed single-revolution pirouette en dehors by spinning the gesture leg outward and backward with support by the dominant leg. The participant’s dominant leg was the leg chosen for kicking an object without a second thought, and all of the
participants were tested with right-leg as the dominant leg in this study. Note that five successful trials of pirouette en dehors were acquired in each dancer, and a successful trial means that participants need to perform pirouette en dehors on the force plates and land on the force plates as well. The kinematic and kinetic parameters were recorded and analyzed during the pirouette en dehors, and the task was divided into five phases (see Figure 2): (a) the preparatory (PRE) phase, (b) turning with double-leg support (TDS), (c) turning with single-leg support in preswing (TSSp), (d) turning with single-leg support in mid-swing (TSSm), and (e) the ending phase (END). At the beginning, an initial ballet fourth foot position was requested by placing a gesture leg (left leg) behind the dominant leg (right leg) as preparation. Meanwhile, an upright trunk and extended knees were positioned prior to turning. An audible cue was delivered to start turning, and a preparatory movement was executed by flexing the knees and shifting body weight to the supporting leg in front of the body for rotation. Then, the TDS phase started, and both knees extended to initiate counterclockwise rotation by raising the gesture leg and pushing off the ground. When the gesture leg was initially pushed off and spun, the TSSp phase began with the singleleg supporting and ended at a ballet retire position with the gesture leg flexed and foot pointed close to the medial knee joint line of the supporting leg. Then, the dancer moved into the TSSm phase. Prior to the end of the rotation, the gesture leg was descending down back to the ground and placed behind the supporting leg. Both legs were flexed and the trunk was upright with arms curled, and the movement was finished when the participants fully extended their knees to the ballet fifth foot position. Data Analysis Five phases of the pirouette en dehors and the duration of each phase were analyzed. At the beginning of the TDS phase to initiate push-off for turning, the peak 3-D ground
FIGURE 1 Marker set. Note. ASIS ¼ anterior superior iliac spines; C7 ¼ seventh cervical spinal process.
CONTROL STRATEGY OF BALLET TURNS IN DANCERS
between the global vertical axis and the instantaneous rotation axis during the TSSp phase. In performing the analysis, the minimum, maximum, average, range, and standard deviation of the ANGLErotation axis were also considered. In addition, the trunk inclination angle (ANGLEtrunk) was measured as the angle between the global vertical axis and the vector from the sacral marker to the C7 marker in the sagittal plane during the END phase. During the single-leg support phase, the joint angles of the hip, knee, and ankle were resampled into 100% with 101 frames, respectively, from the beginning of the TSSp phase to the end of the TSSm phase. The joint angle and time (in percentage) reached to maximum extension angle of the hip, knee, and ankle were identified, respectively. Meanwhile,
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reaction force (i.e., vertical, anterior-posterior, and mediallateral directions) of both the supporting and gesture legs placed on two separate force plates were recorded. The peak resultant ground reaction force was also calculated in the last two phases (i.e., TSSm and END). All the ground reaction forces were normalized to the corresponding body weight of each individual. The instantaneous rotation axis of the torso was calculated using the weighted least squares method reported by Sommer (1992) based on the motion of the landmarks on the torso (namely the C7, xiphoid process, sternal notch, umbilicus, sacrum, and bilateral ASIS markers). Meanwhile, the inclination angle of the instantaneous rotation axis (ANGLErotation axis) was also calculated as the angle
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FIGURE 2 Five phases in pirouette en dehors. Note. SL ¼ supporting leg, GL ¼ gesture leg.
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a proximal-to-distal pattern of the maximum segmental extension angle of the hip, knee, and ankle joints was identified in each trial. Furthermore, an instantaneous angular velocity of the torso was also calculated at the time of the retire position. During each phase of the pirouette en dehors, the AI for the head (AIhead) with respect to trunk was calculated as a ratio of standard deviation differences between relative and absolute angles being divided by a summation of both standard deviations of relative and absolute angles (Equation 1).
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AI head
s gHr 2 s gHa ¼ s gHr þ s gHa
statistical significance was set at p , .05 for the temporalspatial parameters, maximum extension angles, AIhead, peak 3-D ground reaction forces, ANGLEtrunk, and the minimum, maximum, average, and ranges of the ANGLErotation axis. Note the adjusted mean values by ANCOVA were presented. The eta 2 was used to represent the effect size (small, eta 2 ¼ .01; medium, eta 2 ¼ .06; large, eta 2 ¼ .14).
RESULTS Temporal-Spatial Parameters
ð1Þ
where s gH is a standard deviation of the relative angles r of the head with respect to the shoulder, and s gH is a a standard deviation of the absolute angles of the head with respect to a global anterior-posterior axis. Prior to calculating the index, the absolute angles and the H relative angles of the head gH and g , respectively, a r were computed first (see Figure 3a). The absolute angle gH a was defined as the angle between a global anterior-posterior axis and a line of the head formed by the forehead and rear head markers. The relative angle gH was defined as an angle r between two adjacent segments based on the head line and the shoulder line between markers placed on the right and left acromion processes of the shoulders. All variables were analyzed using the Statistical Package for the Social Sciences Version 17.0 (SPSS for Windows, Chicago, IL). Significant differences between the pirouette en dehors performed by novice and experienced dancers were detected by means of an analysis of covariance (ANCOVA) with body height as the covariance to eliminate their confounding effects on the outcome measures. The
The novice group was found to take a longer time (0.80 ^ 0.06 s) during the PRE phase compared with the experienced group (0.42 ^ 0.05 s), F(1, 66) ¼ 22.667, p , .001, eta 2 ¼ .250. However, the novice group spent less time during the TDS phase (novice ¼ 0.52 ^ 0.02 s, experienced ¼ 0.55 ^ 0.02 s), F(1, 66) ¼ 5.72, p ¼ .020, eta 2 ¼ .078, and the TSSp phase (novice ¼ 0.37 ^ 0.02 s, experienced ¼ 0.50 ^ 0.02 s), F(1, 66) ¼ 47.21, p , .001, eta 2 ¼ .410. There was no significant difference in the TSSm phase (novice ¼ 0.44 ^ 0.02 s, experienced ¼ 0.43 ^ 0.02 s), F(1, 66) ¼ 0.14, p ¼ .906, eta 2 , .001, or the END phase (novice ¼ 1.28 ^ 0.06 s, experienced ¼ 1.23 ^ 0.06 s), F(1, 66) ¼ 2.16, p ¼ .156, eta 2 ¼ .031. On the other hand, novice dancers had faster angular velocity of torso rotation at the retire position (novice ¼ 594.33 ^ 26.23 degrees/s, experienced ¼ 549.46 ^ 23.92 degrees/ sec), F(1, 66) ¼ 4.13, p ¼ .046, eta 2 ¼ .057. Joint Angle and Moving Sequence of the Lower Extremity No significant group differences were found in the maximum angles of hip extension (novice ¼ 5.99 ^ 1.54
FIGURE 3 (a) A schematic anchoring index of the head (AIhead), and (b) the index value changes in each phase of one-revolution pirouette en dehors. *Statistically significant difference between groups ( p , .05). rHa ¼ an absolute angle respective to an external anterior-posterior axis and head line; rHr ¼ a relative angle between the head and shoulder lines; PRE ¼ preparatory; TDS ¼ turning with double-leg support; TSSp ¼ single-leg support in preswing; TSSm ¼ single-leg support in mid-swing; END ¼ ending phase.
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CONTROL STRATEGY OF BALLET TURNS IN DANCERS TABLE 2 Ground Reaction Force During the TDS Phase (Normalized to Body Weight)
Supporting leg Peak posterior force Peak medial-lateral force Peak vertical force Gesture leg Peak anterior force Peak medial-lateral force Peak vertical force
F(1, 66)
p Value
Effect Size (Eta2)
Experienced
Novice
0.06 (0.01) 0.12 (0.01) 0.81 (0.03)
0.08 (0.01) 0.15 (0.01) 0.91 (0.03)
6.35 2.09 2.46
.014* .153 .121
.084 .029 .034
0.08 (0.01) 0.21 (0.01) 1.02 (0.03)
0.12 (0.01) 0.22 (0.01) 1.05 (0.04)
13.03 ,0.01 2.45
,.001* .983 .122
.159 ,.001 .034
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Note. TDS ¼ turning with double-leg support. *Statistically significant difference between groups ( p , .05).
FIGURE 4 Extension (þ ) joint angles in the hip, knee, and ankle joints during turning with the single-leg support phase represented by one experienced dancer (top) and one novice dancer (bottom); the arrows indicate the occurrence of the maximum join angle.
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C.-W. LIN ET AL. TABLE 3 Inclination Angle of the Instantaneous Rotation Axis During the TSSp Phase (Degrees)
Average Minimum Maximum Range Standard deviation
Experienced
Novice
F(1, 66)
p Value
Effect Size (Eta2)
3.33 (0.20) 1.43 (0.18) 4.95 (0.36) 3.52 (0.37) 1.04 (0.12)
4.74 (0.22) 2.25 (0.20) 7.24 (0.39) 4.96 (0.41) 1.63 (0.13)
23.10 14.46 17.03 4.48 7.36
,.001* ,.001* ,.001* .038* .008*
.254 .175 .200 .062 .098
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TSSp ¼ turning with single-leg support in preswing. * Statistically significant difference between groups ( p , .05).
degrees, experienced ¼ 8.01 ^ 1.41 degrees), F(1, 66) ¼ 2.13, p ¼ .149, eta 2 ¼ .030, knee extension (novice ¼ 6.98 ^ 1.13 degrees, experienced ¼ 6.02 ^ 1.03 degrees), F(1, 66) ¼ 3.14, p ¼ .081, eta 2 ¼ .044, or ankle plantarflexion (novice ¼ 5.61 ^ 2.27 degrees, experienced ¼ 10.58 ^ 2.07 degrees), F(1, 66) ¼ 3.23, p ¼ .077, eta 2 ¼ .045. In addition, 63.89% of experienced dancers had maximum extension angles in a sequential extension pattern from proximal to distal (hip ! knee !
ankle), whereas only 41.67% of novice dancers exhibited such a sequential pattern (see Figure 4). Ground Reaction Force During the TDS phase, the novice group was found to exert a significantly greater peak posterior force than the experienced group (see Table 2). However, no significant group differences were found in the peak medial-lateral
FIGURE 5 Ground reaction force (GRF) during turning with the double-leg support phase on the supporting leg (left) and gesture leg (right) represented by one experienced dancer and one novice dancer.
CONTROL STRATEGY OF BALLET TURNS IN DANCERS
force and the vertical force. For the gesture leg in the TDS phase, the novice group was found to exert a greater peak anterior propulsion force than that of the experienced group, but no significance was found between groups in the mediallateral and vertical directions (see Figure 5). On the other hand, no significant group difference for the peak landing resultant force was found (novice ¼ 1.57 ^ 0.03 body weight, experienced ¼ 1.39 ^ 0.03 body weight), F(1, 66) ¼ 23.24, p , .001, eta 2 ¼ .252, during the TSSm and END phases.
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Anchoring Index and Inclination Angles The novice group was shown to have a smaller AIhead than that of the experienced group for all five phases of turning (see Figure 3b). A similar trend of decreasing AIhead from the PRE, F(1, 66) ¼ 8.08, p ¼ .006, eta 2 ¼ .106, TDS, F(1, 66) ¼ 30.98, p , .001, eta 2 ¼ .312, and TSSp, F(1, 66) ¼ 28.36, p , .001, eta 2 ¼ .294, phases was observed for both groups, and then the AIhead was increased from the TSSp, TSSm, F(1, 66) ¼ 34.50, p , .001, eta 2 ¼ .337, to the END phases, F(1, 66) ¼ 4.40, p ¼ .040, eta 2 ¼ .061. The positive values of the AIhead across the five phases of turning were shown more in the advanced dancer group than in the novice group. The average, minimum, maximum, range, and standard deviation of ANGLErotation axis were all found to be greater for the novice group in the single-leg support phase compared with the experienced group (see Table 3). The novice dancers were also found to show a greater range of ANGLEtrunk during the END phase compared with the experienced dancers (novice ¼ 7.83 ^ 0.51 degrees, experienced ¼ 4.68 ^ 0.57degrees), F(1, 66) ¼ 28.00, p , .001, eta 2 ¼ .292.
DISCUSSION To fully understand ballet pirouettes from a biomechanical perspective, we investigated performance and strategies of ballet turns in novice and experienced dancers. The findings indicated that the experienced ballet dancers had the fewest changes of the ANGLErotation axis in the preswing phase. The experienced dancers also had greater positive AIhead in all five phases accompanied with a decreased peak ground reaction force in both the supporting leg and gesture leg during the initial push-off compared with the novice dancers. In addition, the experienced group also showed the fewest changes in the range of ANGLEtrunk during the END phase of turning. Temporal-Spatial Parameters The longer preparatory duration exhibited by the novice dancers was consistent with that of a previous study (Laws,
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1978). Their study revealed that the professional dancers take less time in the preparation phase of pirouette en dehors than do the less skilled dancers. Thus, the novice dancers spending more time during the preparation phase prior to turning initiation may suggest that they need more time to adjust their posture before turning. They might also spend more time in the preparation phase for cognitive thinking because they were in the early stages of skill acquisition (Newell, 1991). A greater angular velocity on the retire position but shorter duration in the TDS and TSSp phases were found in the novice group. Because the effect size of the angular parameter was small, we should interpret this finding with caution. Given that the same degrees of angle should be completed in both groups, the novice group spent less time to achieve the same degrees of angle and thus had greater angular velocity compared with the experienced group. Ground Reaction Force To better quantify the necessary forces to initiate turning in ballet dancers, the 3-D ground reaction forces showed that only the anterior-posterior force was found to be different and was greater for the novice dancers during the TDS phase. In this phase, the dancers’ supporting legs were changing from double-leg support to single-leg support, and each leg also revealed different directional forces in either the anterior or posterior direction. The smaller anterior-posterior ground reaction force needed for the turning demonstrated that the experienced dancers exerted smaller peak forces in the anterior-posterior direction for both legs. By contrast, the greater anteriorposterior forces in the novice dancers indicated that they put more effort into maintain their postures and dynamic balance. Therefore, adopting the proper magnitude of ground reaction force in the supporting and gesture legs is essential for performing a poised turn. This also suggests that too much force leads to the body deviating from the rotational axis, while too little force makes it incapable to initiate a turn (Laws, 2002). Furthermore, a proper landing is important for the sake of aesthetic requirement and injury prevention. Although the impulses of landing from ballet turns were not as large as that from jump landing, a greater amount of angular velocity decreased within a short period was also a risk for injury when landing from turns. The foot and ankle joints thus have to absorb the whole body weight as well as the force generated from the angular velocity, and thus, great demands of the lower extremities are noted (Krasnow & Wyon, 2011). In the current study, although no difference in landing force was observed between the groups, the different strategies may be utilized in the experienced and novice dancers, respectively. The novice dancers inclined their trunk forward to compensate
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instability compared with the experienced dancers, who landed with their trunk slightly forward, and this further revealed that the novice dancers had poorer postural control ability.
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Kinematics Even though the two groups did not differ in the maximum extension angles of the hip, knee, and ankle joints during the single-leg support phase, different sequences of time to maximum extension angles in these joints did exist between groups. The majority of experienced dancers used the sequence of the proximal-to-distal pattern (hip ! knee ! ankle joints), but few novice dancers used this sequential extension pattern. When performing pirouette en dehors, the sequential pattern helps dancers to centralize their segmental mass and to align the center of mass (COM) to the center of pressure (COP) efficiently. Furthermore, the sequential pattern was also observed in gymnasts who performed fast backward trunk skill (i.e., bending head and trunk backward as fast as possible) compared with untrained participants (Pedotti, Crenna, Deat, Frigo, & Massion, 1989). The untrained participants had synchronized muscle activation in the back, thighs, and legs, whereas the gymnast adopted the distal-to-proximal sequential pattern when performing the backward trunk skill. Based on this, we suggest that both gymnasts and experienced ballet dancers adapted to their training to perform the movement efficiently. Furthermore, the synchronized muscle activation froze the degrees of freedom and is common in early learning (Vorro & Hobart, 1981), and this condition could also contribute to the lack of sequential pattern in the novice dancers. Anchoring Index The positive AIhead indicates that the head moves with fewer variations relative to the global axis. A positive AIhead also demonstrates that the participants use the global anteriorposterior axis as a stabilization reference (space reference) more often than the adjacent segment (local reference) to control posture of the head segment. However, the negative value shows the participants take local reference, instead of the space reference, as the stabilization reference. This is the so-called “en bloc” strategy, which means moving with adjacent segments together (Golomer, Toussaint, Bouillette, & Keller, 2009). During the TDS phase, the positive AIhead in the experienced group but negative AIhead in the novice group indicated that the experienced dancers utilized visual input as a stabilization strategy during turns. The utilization of vision and gazing at space may have a positive effect on turning, because gazing on a specific target in space prevents vertigo or dizziness (Amblard et al., 2001). It is the so-called “spotting technique” in ballet (Kirstein & Stuart,
2004), which is to focus on the target as long as possible then whip the head around and refocus on the target. This technique decreases head motion and improves sensory processing from the head to minimize body oscillations (Chatfield, Krasnow, Herman, & Blessing, 2007). This also further explains why the dancers depend on vision and constantly use mirrors for postural stabilization (Kilby & Newell, 2012). Inclination Angles The experienced dancers had a smaller ANGLErotation axis during the TSSp phase, indicating a smaller deviation and a better performance. This is consistent with a previous investigation on the differences between dancers and untrained participants during single-leg support with gesture leg raising 45 degrees laterally (Mouchnino, Aurenty, Massion, & Pedotti, 1992). In that study, the dancers used the trunk “translation strategy” (i.e., maintaining the trunk axis vertically by using hip adduction/abduction motion), whereas the untrained participants utilized the “inclination strategy” (i.e., inclining the trunk for postural stability) when performing the gesture-leg raising. The inclination strategy used by novice dancers may be one of reasons for greater ANGLErotation axis during ballet turns in the current study. The novice dancers inclined their trunk to compensate for the rising of the gesture leg, but this inclined trunk would likely abate the visual sensory inputs to postural control (Clark & Iltis, 2008). Given that the inclination strategy leads the COM and COP apart on the transverse plane, imbalance would also occur. By contrast, the translation strategy ensures that the visual sensory inputs are not disturbed and make certain continuous sensory inputs. When utilizing translation strategy, the COM and COP separation on the transverse plane can be limited because the major adjustment was performed by the hip joints. As a result, the translation strategy was suggested to be superior to the inclination strategy while turning in the view of balance control. The higher push force at propulsion along with greater angular velocity resulted in a larger ANGLEtrunk and difficulty in control of postural stability. Given less verticality in novice dancers, aligning rotation axis and gravity reference would be a challenge for novice dancers while maintaining the balance within a small base of support. Therefore, the novice dancers inclined their trunk forward to regain their balance in the END phase. Implication According to current findings, some suggestions are made to novice dancers and dance educators. During training, novice dancers are suggested to reduce the push
CONTROL STRATEGY OF BALLET TURNS IN DANCERS
force of the gesture leg during the TDS phase and to adapt more visual input to maintain their stability, especially during the TDS phase. While turning with single-leg support, extending the hip joint first then the knee joint and later the ankle joint to obtain better postural stability is also preferred. Finally, excessive trunk-forward inclination should be avoided during the END phase.
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Limitations Three limitations of the present study need to be acknowledged. First, the age difference between groups was a concern; experienced dancers are generally older than novice dancers. However, this was reasonable because they start learning dance around the same age, and the ability of balance as positively correlated to age due to different training experiences has been evidenced (Kilby & Newell, 2012). Second, this study considered only the trunk control during pirouette en dehors. Movement of the extremities logically affects performance of ballet turns. Therefore, investigation into the influence of the extremities on ballet turns should be the next step for future study. Third, body height- and body weightmatched controls should be recruited for future study to eliminate their potential effect on movement performance and execution strategy.
Conclusion Experienced dancers had a smaller inclination angle of rotational axis by adapting the “translation strategy,” which implies that adjustment of biomechanical factors was efficient for performing better movements and maintaining postural stability. By contrast, the novice dancers maintained their postural stability through the “inclination strategy” so that the visual sensory inputs were distorted. Furthermore, the novice dancers performed more poorly than experienced dancers because of higher push forces, lack of a “proximal-to-distal extension sequence” pattern in the supporting leg, and insufficient visual spotting stabilization for postural stability. Finally, the novice dancers inclined their trunk forward to regain balance at the END phase.
WHAT DOES THIS ARTICLE ADD? The strategy used to perform ballet turns differed between experienced and novice dancers. Large push force and an insufficient proximal-to-distal extension sequence pattern were potential factors for inferior performance in novice dancers. Moreover, novice dancers utilized trunk-forward inclination to regain balance in the END phase. Note that the
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lack of visual spotting stabilization affected postural stability. REFERENCES Amblard, B., Assaiante, C., Vaugoyeau, M., Baroni, G., Ferrigno, G., & Pedotti, A. (2001). Voluntary head stabilisation in space during oscillatory trunk movements in the frontal plane performed before, during and after a prolonged period of weightlessness. Experimental Brain Research, 137, 170– 179. Charalambous, L., Irwin, G., Bezodis, I. N., & Kerwin, D. (2012). Lower limb joint kinetics and ankle joint stiffness in the sprint start push-off. Journal of Sports Sciences, 30, 1–9. Chatfield, S., Krasnow, D., Herman, A., & Blessing, G. (2007). A descriptive analysis of kinematic and electromyographic relationships of the core during forward stepping in beginning and expert dancers. Journal of Dance Medicine and Science, 11, 76 –84. Clark, S., & Iltis, P. W. (2008). Effects of dynamic head tilts on sensory organization test performance: A comparison between college-age athletes and nonathletes. Journal of Orthopaedic & Sports Physical Therapy, 38, 262–268. Golomer, E., Rosey, F., Dizac, H., Mertz, C., & Fagard, J. (2008). The influence of classical dance training on preferred supporting leg and whole body turning bias. Laterality, 14, 165–177. Golomer, E., Toussaint, Y., Bouillette, A., & Keller, J. (2009). Spontaneous whole body rotations and classical dance expertise: How shoulder–hip coordination influences supporting leg displacements. Journal of Electromyography and Kinesiology, 19, 314 –321. Iida, Y., Kanehisa, H., Inaba, Y., & Nakazawa, K. (2011). Activity modulations of trunk and lower limb muscles during impact-absorbing landing. Journal of Electromyography and Kinesiology, 21, 602– 609. Kilby, M. C., & Newell, K. M. (2012). Intra- and inter-foot coordination in quiet standing: Footwear and posture effects. Gait & Posture, 35, 511–516. Kirstein, L., & Stuart, M. (2004). The classic ballet: Basic technique and terminology. New York, NY: Knopf. Krasnow, D., & Wyon, M. (2011). Biomechanical research in dance: A literature review. Medical Problems of Performing Artists, 26, 3 –23. Laws, K. (1978). An analysis of turns in dance. Dance Research Journal, 11, 12 –19. Laws, K. (2002). Physics and the art of dance: Understanding movement. Oxford, England: Oxford University Press. Mesure, S., Azulay, J. P., Pouget, J., & Amblard, B. (1999). Strategies of segmental stabilization during gait in Parkinson’s disease. Experimental Brain Research, 129, 573– 581. Mouchnino, L., Aurenty, R., Massion, J., & Pedotti, A. (1992). Coordination between equilibrium and head-trunk orientation during leg movement: A new strategy build up by training. Journal of Neurophysiology, 67, 1587–1598. Naito, K., Fukui, Y., & Maruyama, T. (2012). Energy redistribution analysis of dynamic mechanisms of multi-body, multi-joint kinetic chain movement during soccer instep kicks. Human Movement Science, 31, 161–181. Newell, K. M. (1991). Motor skill acquisition. Annual Review of Psychology, 42, 213–237. Pedotti, A., Crenna, P., Deat, A., Frigo, C., & Massion, J. (1989). Postural synergies in axial movements: Short and long-term adaptation. Experimental Brain Research, 74, 3–10. Robert, G., Blouin, J., Ruget, H., & Mouchnino, L. (2007). Coordination between postural and movement controls: Effect of changes in body mass distribution on postural and focal component characteristics. Experimental Brain Research, 181, 159 –171. Salvia, P., Woestyn, L., David, J. H., Feipel, V., Van, S., Jan, S., . . . Rooze, M. (2000). Analysis of helical axes, pivot and envelope in active wrist circumduction. Clinical Biomechanics, 15, 103–111.
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Sienko, K. H., Balkwill, M. D., Oddsson, L. I., & Wall, C. (2008). Effects of multi-directional vibrotactile feedback on vestibular-deficient postural performance during continuous multi-directional support surface perturbations. Journal of Vestibular Research, 18, 273 –285. Sommer, H. (1992). Determination of first and second order instant screw parameters from landmark trajectories. Journal of Mechanical Design, 114, 274 –281. Sveistrup, H., Schneiberg, S., McKinley, P. A., McFadyen, B. J., & Levin, M. F. (2008). Head, arm and trunk coordination during reaching in children. Experimental Brain Research, 188, 237–247.
Vorro, J., & Hobart, D. (1981). Kinematic and myoelectric analysis of skill acquisition: I. 90 cm subject group. Archives of Physical Medicine and Rehabilitation, 62, 575–582. Woltring, H. J. (1976). Calibration and measurement in 3-dimensional monitoring of human motion by optoelectronic means. II. Experimental results and discussion. Biotelemetry, 3, 65–97. Woltring, H. J. (1980). Planar control in multi-camera calibration for 3-D gait studies. Journal of Biomechanics, 13, 39– 48. Woltring, H. J. (1994). 3-D attitude representation of human joints: A standardization proposal. Journal of Biomechanics, 27, 1399–1414.
Research Quarterly for Exercise and Sport Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/urqe20
Differences of Ballet Turns (Pirouette) Performance Between Experienced and Novice Ballet Dancers a
a
a
b
a
Chia-Wei Lin , Shing-Jye Chen , Fong-Chin Su , Hong-Wen Wu & Cheng-Feng Lin a
National Cheng Kung University
b
National Taiwan University of Physical Education and Sports Published online: 20 Aug 2014.
To cite this article: Chia-Wei Lin, Shing-Jye Chen, Fong-Chin Su, Hong-Wen Wu & Cheng-Feng Lin (2014) Differences of Ballet Turns (Pirouette) Performance Between Experienced and Novice Ballet Dancers, Research Quarterly for Exercise and Sport, 85:3, 330-340, DOI: 10.1080/02701367.2014.930088 To link to this article: http://dx.doi.org/10.1080/02701367.2014.930088
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Research Quarterly for Exercise and Sport, 85, 330–340, 2014 Copyright q SHAPE America ISSN 0270-1367 print/ISSN 2168-3824 online DOI: 10.1080/02701367.2014.930088
Differences of Ballet Turns (Pirouette) Performance Between Experienced and Novice Ballet Dancers Chia-Wei Lin, Shing-Jye Chen, and Fong-Chin Su National Cheng Kung University
Hong-Wen Wu Downloaded by [Washburn University] at 06:24 02 November 2014
National Taiwan University of Physical Education and Sports
Cheng-Feng Lin National Cheng Kung University
Purpose: This study investigated the different postural control strategies exhibited by experienced and novice dancers in ballet turns ( pirouettes). Method: Thirteen novice and 13 experienced dancers performed ballet turns with dominant-leg support. The peak push force was measured in the double-leg support phase. The inclination angles of rotation axis with respect to vertical axis were calculated in the early single-leg support phase as well as the initiation sequence of ankle, knee, and hip joints on the supporting leg. Moreover, the anchoring index of the head was computed in the transverse plane during turning. Results: The novice dancers applied a greater push force, an increased inclination angle of rotation axis, and an insufficient proximal-to-distal extension sequence pattern. The novice dancers also had a smaller head-anchoring index compared with experienced dancers, which meant novice dancers were not using a space target as a stability reference. Conclusions: A poorer performance in novice dancers could result from higher push force in propulsion, lack of a “proximal-to-distal extension sequence” pattern, and lack of visual spotting for postural stability. Training on sequential initiation of lower-extremity joints and rehearsal of visual spotting are essential for novice dancers to obtain better performance on ballet turns. Keywords: anchoring index, ground reaction force, rotation axis, sequential pattern
A pirouette is known as a ballet turn and requires a rapid full rotation of the body with single-leg support on the toes or balls of the foot. An efficient strategy for maintaining dynamic stability of whole body is essential for performing a pirouette. Because the magnitudes of the push forces for pirouettes are positively correlated with the revolutions of turns (Laws, 2002), quantifying the three-dimensional (3-D) forces can provide insight into how to initiate and control pirouettes. In addition, dancers push their gesture leg off from initially flexed legs with an upright trunk, and then Submitted March 11, 2013; accepted November 25, 2013. Correspondence should be addressed to Cheng-Feng Lin, Department of Physical Therapy, National Cheng Kung University, 1 University Road, 701 Tainan, Taiwan. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/urqe.
they extend their hip joints followed by extending the knee and then ankle plantar flexing at the end. This proximal-todistal moving sequence in ballet dancers has shown its positive effects on performance, which is similar to that in soccer instep kicking (Naito, Fukui, & Maruyama, 2012), sprinting to start (Charalambous, Irwin, Bezodis, & Kerwin, 2012), and jumping to land (Iida, Kanehisa, Inaba, & Nakazawa, 2011). The rotation axis of ballet turns was previously assumed to be a fixed vertical axis (Laws, 1978). However, the rotation axis is not constantly fixed and remaining in the vertical direction, and thus, using the other method to calculate the rotation axis in pirouettes could be a better measurement of performance in ballet turns. An instantaneous rotation axis calculated by the weighted least squares method has been well applied in previous studies to
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TABLE 1 Basic Anthropometric Data and Characteristics of Dancers
Age (years) Height (cm) Body weight (kg) Body mass index (kg/m2) Experience in dance (years)a Experience in ballet dance (years) Experience with wearing pointe shoes (yes/no) Years wearing pointe shoes (years) Experience of one-revolution pirouette in soft shoes (years) Years of one-revolution pirouette in pointe shoes (years)
Experienced
Novice
p Value
df
t
Effect Size (Cohen’s d)
17.77 (3.39) 159.27 (4.21) 51.54 (4.66) 20.29 ^ 1.37 11.31 (3.07) 8.69 (3.30) 13/0 4.54 (2.88) 6.08 (3.40) 2.75 (1.71)
12.00 (1.91) 151.89 (11.48) 43.81 (9.68) 18.7 ^ 2.14 4.31 (2.02) 3.23 (1.69) 5/8 0.23 (0.23) 0.92 (0.64) 0.00 (0.00)
,.001* .046* .017* .033* ,.001* ,.001* — ,.001* ,.001* ,.001*
24 15.16 24 24 24 24 — 24 24 24
5.34 2.18 2.58 2.27 6.88 5.31 — 5.37 6.74 5.06
2.10 0.85 1.02 0.88 2.69 2.08 — 2.11 2.11 2.27
*Statistically significant difference between groups ( p , .05). Movement class, folk dance, and some basic training of preballet.
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a
describe the quantity of body rotations around the axis (Salvia et al., 2000; Sommer, 1992; Woltring, 1976, 1980, 1994), and it may further be utilized to calculate the rotation axis in ballet turns. Moreover, aligning the rotation axis with the vertical line is essential for dancers to maintain postural stability of t turns because the postural stability is greatly influenced by the deviations of the rotation axis and may result in a fail of turning (Laws, 2002). The anchoring index (AI), an indicator of how well two adjacent segments coordinate to each other, has been often used to identify walking stability in patients (Mesure, Azulay, Pouget, & Amblard, 1999; Sienko, Balkwill, Oddsson, & Wall, 2008; Sveistrup, Schneiberg, McKinley, McFadyen, & Levin, 2008). Although the relative angle changes of adjacent segments in pirouettes were analyzed previously (Golomer, Rosey, Dizac, Mertz, & Fagard, 2008), the angle changes did not provide sufficient assessments of stability and coordination for the dancers. To our knowledge, there is no study available using the AI to evaluate dancers’ performance by using the critical measure of head-on-trunk orientation during pirouettes (Robert, Blouin, Ruget, & Mouchnino, 2007). Thus, the calculation of AI in head-on-trunk orientation may provide useful information in how dancers maintain their stability. Therefore, the purpose of the current study was to investigate control strategies of ballet dancers during pirouettes using advanced biomechanical tools. The strategies were quantified by (a) the 3-D ground reaction forces of the supporting leg and gesture leg at initial push off; (b) the initiation sequence of the supporting leg in the ankle, knee, and hip joints; (c) the inclination angle changes based on the angles between the instantaneous rotation axis and the vertical global axis; and (d) the performance of AI in head-on-trunk orientation between novice and experienced dancers. We hypothesized that experienced ballet dancers would perform better than novice dancers by showing the fewest changes in the inclination angles of the rotation axis and the AI of the head, accompanied with a smaller 3-D ground reaction
force for turning initiation. We hope that the findings of the current study will provide dance educators with more details about turning to enhance their teaching and to help novice dancers to learn the turning movement in a more efficient and correct way.
METHODS Participants Thirteen experienced female dancers and 13 novice female dancers were recruited to take part in the study (see Table 1). The inclusion criteria for the experienced group were specified as follows: (a) at least 6 years of ballet training; (b) at least 3 hr of weekly routine training; and (c) a single-leg support for two complete revolutions of turning. The inclusion criteria for the novice group were also specified as follows: (a) 2 years to 5 years of ballet training; (b) 1.5 hr and less than 3 hr of weekly routine training; and (c) a single-leg support for one complete revolution of turning but the inability to perform two complete revolutions. The exclusion criteria of the participants were a history of a vestibular or balance problem and previous injuries of the lower extremity and back. This information was obtained from participants’ self-reported history. Each participant read and signed an informed consent approved by the hospital institutional review board. Note that experienced dancers are generally older than novice dancers. This is mainly because dancers start to learn dance at the preschool age, the experienced dancers often have more years of training than the novice dancers, and the balance ability is also positively correlated with age (Kilby & Newell, 2012). Instrumentations An acquisition system with eight Eagle high-speed cameras (Motion Analysis Corporation, Santa Rosa, CA) was
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sampled at 200 Hz to collect 3-D trajectories of makers placed on each participant. A modified Helen Hayes marker set with 36 markers was used and placed on the specific anatomical locations of participants who wore leotards and individual ballet shoes (see Figure 1). The locations of each segment are listed as the following anatomical landmarks: forehead, top head, rear head, sternal notch, xiphoid process, umbilicus, seventh cervical spinal process (C7), sacrum, bilateral acromion process, midpoints of the arm and forearm, lateral epicondyles of the humerus, radial styloid processes, third metacarpal heads, anterior superior iliac spines (ASIS), midpoints of the lateral sides of the thigh and shank, greater trochanters, lateral knee joint lines, lateral malleoli of ankles, midpoints between the first and fifth metatarsal heads, and the posterior heels. Two static standing trials were collected before performing a turn. Eight additional markers were placed on both limbs of the medial humeral epicondyles, ulnar styloid processes, medial knee, and ankle joints, and they were removed during dynamic trials. Two force plates (Kistler Instrument Corporation, Winterhur, Switzerland) were mounted on the ground to record 3-D ground reaction force with sampling at 1,000 Hz and synchronized with the motion acquisition system. To detect the ground reaction force correctly, the performance area was expanded with a customized wooden floor that was 800 mm £ 600 mm £ 40 mm (length £ width £ height), and the floor was covered with vinyl to create a floor surface that the participants were familiar with during their routine practices. Procedures All female participants performed single-revolution pirouette en dehors by spinning the gesture leg outward and backward with support by the dominant leg. The participant’s dominant leg was the leg chosen for kicking an object without a second thought, and all of the
participants were tested with right-leg as the dominant leg in this study. Note that five successful trials of pirouette en dehors were acquired in each dancer, and a successful trial means that participants need to perform pirouette en dehors on the force plates and land on the force plates as well. The kinematic and kinetic parameters were recorded and analyzed during the pirouette en dehors, and the task was divided into five phases (see Figure 2): (a) the preparatory (PRE) phase, (b) turning with double-leg support (TDS), (c) turning with single-leg support in preswing (TSSp), (d) turning with single-leg support in mid-swing (TSSm), and (e) the ending phase (END). At the beginning, an initial ballet fourth foot position was requested by placing a gesture leg (left leg) behind the dominant leg (right leg) as preparation. Meanwhile, an upright trunk and extended knees were positioned prior to turning. An audible cue was delivered to start turning, and a preparatory movement was executed by flexing the knees and shifting body weight to the supporting leg in front of the body for rotation. Then, the TDS phase started, and both knees extended to initiate counterclockwise rotation by raising the gesture leg and pushing off the ground. When the gesture leg was initially pushed off and spun, the TSSp phase began with the singleleg supporting and ended at a ballet retire position with the gesture leg flexed and foot pointed close to the medial knee joint line of the supporting leg. Then, the dancer moved into the TSSm phase. Prior to the end of the rotation, the gesture leg was descending down back to the ground and placed behind the supporting leg. Both legs were flexed and the trunk was upright with arms curled, and the movement was finished when the participants fully extended their knees to the ballet fifth foot position. Data Analysis Five phases of the pirouette en dehors and the duration of each phase were analyzed. At the beginning of the TDS phase to initiate push-off for turning, the peak 3-D ground
FIGURE 1 Marker set. Note. ASIS ¼ anterior superior iliac spines; C7 ¼ seventh cervical spinal process.
CONTROL STRATEGY OF BALLET TURNS IN DANCERS
between the global vertical axis and the instantaneous rotation axis during the TSSp phase. In performing the analysis, the minimum, maximum, average, range, and standard deviation of the ANGLErotation axis were also considered. In addition, the trunk inclination angle (ANGLEtrunk) was measured as the angle between the global vertical axis and the vector from the sacral marker to the C7 marker in the sagittal plane during the END phase. During the single-leg support phase, the joint angles of the hip, knee, and ankle were resampled into 100% with 101 frames, respectively, from the beginning of the TSSp phase to the end of the TSSm phase. The joint angle and time (in percentage) reached to maximum extension angle of the hip, knee, and ankle were identified, respectively. Meanwhile,
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reaction force (i.e., vertical, anterior-posterior, and mediallateral directions) of both the supporting and gesture legs placed on two separate force plates were recorded. The peak resultant ground reaction force was also calculated in the last two phases (i.e., TSSm and END). All the ground reaction forces were normalized to the corresponding body weight of each individual. The instantaneous rotation axis of the torso was calculated using the weighted least squares method reported by Sommer (1992) based on the motion of the landmarks on the torso (namely the C7, xiphoid process, sternal notch, umbilicus, sacrum, and bilateral ASIS markers). Meanwhile, the inclination angle of the instantaneous rotation axis (ANGLErotation axis) was also calculated as the angle
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FIGURE 2 Five phases in pirouette en dehors. Note. SL ¼ supporting leg, GL ¼ gesture leg.
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a proximal-to-distal pattern of the maximum segmental extension angle of the hip, knee, and ankle joints was identified in each trial. Furthermore, an instantaneous angular velocity of the torso was also calculated at the time of the retire position. During each phase of the pirouette en dehors, the AI for the head (AIhead) with respect to trunk was calculated as a ratio of standard deviation differences between relative and absolute angles being divided by a summation of both standard deviations of relative and absolute angles (Equation 1).
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AI head
s gHr 2 s gHa ¼ s gHr þ s gHa
statistical significance was set at p , .05 for the temporalspatial parameters, maximum extension angles, AIhead, peak 3-D ground reaction forces, ANGLEtrunk, and the minimum, maximum, average, and ranges of the ANGLErotation axis. Note the adjusted mean values by ANCOVA were presented. The eta 2 was used to represent the effect size (small, eta 2 ¼ .01; medium, eta 2 ¼ .06; large, eta 2 ¼ .14).
RESULTS Temporal-Spatial Parameters
ð1Þ
where s gH is a standard deviation of the relative angles r of the head with respect to the shoulder, and s gH is a a standard deviation of the absolute angles of the head with respect to a global anterior-posterior axis. Prior to calculating the index, the absolute angles and the H relative angles of the head gH and g , respectively, a r were computed first (see Figure 3a). The absolute angle gH a was defined as the angle between a global anterior-posterior axis and a line of the head formed by the forehead and rear head markers. The relative angle gH was defined as an angle r between two adjacent segments based on the head line and the shoulder line between markers placed on the right and left acromion processes of the shoulders. All variables were analyzed using the Statistical Package for the Social Sciences Version 17.0 (SPSS for Windows, Chicago, IL). Significant differences between the pirouette en dehors performed by novice and experienced dancers were detected by means of an analysis of covariance (ANCOVA) with body height as the covariance to eliminate their confounding effects on the outcome measures. The
The novice group was found to take a longer time (0.80 ^ 0.06 s) during the PRE phase compared with the experienced group (0.42 ^ 0.05 s), F(1, 66) ¼ 22.667, p , .001, eta 2 ¼ .250. However, the novice group spent less time during the TDS phase (novice ¼ 0.52 ^ 0.02 s, experienced ¼ 0.55 ^ 0.02 s), F(1, 66) ¼ 5.72, p ¼ .020, eta 2 ¼ .078, and the TSSp phase (novice ¼ 0.37 ^ 0.02 s, experienced ¼ 0.50 ^ 0.02 s), F(1, 66) ¼ 47.21, p , .001, eta 2 ¼ .410. There was no significant difference in the TSSm phase (novice ¼ 0.44 ^ 0.02 s, experienced ¼ 0.43 ^ 0.02 s), F(1, 66) ¼ 0.14, p ¼ .906, eta 2 , .001, or the END phase (novice ¼ 1.28 ^ 0.06 s, experienced ¼ 1.23 ^ 0.06 s), F(1, 66) ¼ 2.16, p ¼ .156, eta 2 ¼ .031. On the other hand, novice dancers had faster angular velocity of torso rotation at the retire position (novice ¼ 594.33 ^ 26.23 degrees/s, experienced ¼ 549.46 ^ 23.92 degrees/ sec), F(1, 66) ¼ 4.13, p ¼ .046, eta 2 ¼ .057. Joint Angle and Moving Sequence of the Lower Extremity No significant group differences were found in the maximum angles of hip extension (novice ¼ 5.99 ^ 1.54
FIGURE 3 (a) A schematic anchoring index of the head (AIhead), and (b) the index value changes in each phase of one-revolution pirouette en dehors. *Statistically significant difference between groups ( p , .05). rHa ¼ an absolute angle respective to an external anterior-posterior axis and head line; rHr ¼ a relative angle between the head and shoulder lines; PRE ¼ preparatory; TDS ¼ turning with double-leg support; TSSp ¼ single-leg support in preswing; TSSm ¼ single-leg support in mid-swing; END ¼ ending phase.
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CONTROL STRATEGY OF BALLET TURNS IN DANCERS TABLE 2 Ground Reaction Force During the TDS Phase (Normalized to Body Weight)
Supporting leg Peak posterior force Peak medial-lateral force Peak vertical force Gesture leg Peak anterior force Peak medial-lateral force Peak vertical force
F(1, 66)
p Value
Effect Size (Eta2)
Experienced
Novice
0.06 (0.01) 0.12 (0.01) 0.81 (0.03)
0.08 (0.01) 0.15 (0.01) 0.91 (0.03)
6.35 2.09 2.46
.014* .153 .121
.084 .029 .034
0.08 (0.01) 0.21 (0.01) 1.02 (0.03)
0.12 (0.01) 0.22 (0.01) 1.05 (0.04)
13.03 ,0.01 2.45
,.001* .983 .122
.159 ,.001 .034
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Note. TDS ¼ turning with double-leg support. *Statistically significant difference between groups ( p , .05).
FIGURE 4 Extension (þ ) joint angles in the hip, knee, and ankle joints during turning with the single-leg support phase represented by one experienced dancer (top) and one novice dancer (bottom); the arrows indicate the occurrence of the maximum join angle.
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C.-W. LIN ET AL. TABLE 3 Inclination Angle of the Instantaneous Rotation Axis During the TSSp Phase (Degrees)
Average Minimum Maximum Range Standard deviation
Experienced
Novice
F(1, 66)
p Value
Effect Size (Eta2)
3.33 (0.20) 1.43 (0.18) 4.95 (0.36) 3.52 (0.37) 1.04 (0.12)
4.74 (0.22) 2.25 (0.20) 7.24 (0.39) 4.96 (0.41) 1.63 (0.13)
23.10 14.46 17.03 4.48 7.36
,.001* ,.001* ,.001* .038* .008*
.254 .175 .200 .062 .098
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TSSp ¼ turning with single-leg support in preswing. * Statistically significant difference between groups ( p , .05).
degrees, experienced ¼ 8.01 ^ 1.41 degrees), F(1, 66) ¼ 2.13, p ¼ .149, eta 2 ¼ .030, knee extension (novice ¼ 6.98 ^ 1.13 degrees, experienced ¼ 6.02 ^ 1.03 degrees), F(1, 66) ¼ 3.14, p ¼ .081, eta 2 ¼ .044, or ankle plantarflexion (novice ¼ 5.61 ^ 2.27 degrees, experienced ¼ 10.58 ^ 2.07 degrees), F(1, 66) ¼ 3.23, p ¼ .077, eta 2 ¼ .045. In addition, 63.89% of experienced dancers had maximum extension angles in a sequential extension pattern from proximal to distal (hip ! knee !
ankle), whereas only 41.67% of novice dancers exhibited such a sequential pattern (see Figure 4). Ground Reaction Force During the TDS phase, the novice group was found to exert a significantly greater peak posterior force than the experienced group (see Table 2). However, no significant group differences were found in the peak medial-lateral
FIGURE 5 Ground reaction force (GRF) during turning with the double-leg support phase on the supporting leg (left) and gesture leg (right) represented by one experienced dancer and one novice dancer.
CONTROL STRATEGY OF BALLET TURNS IN DANCERS
force and the vertical force. For the gesture leg in the TDS phase, the novice group was found to exert a greater peak anterior propulsion force than that of the experienced group, but no significance was found between groups in the mediallateral and vertical directions (see Figure 5). On the other hand, no significant group difference for the peak landing resultant force was found (novice ¼ 1.57 ^ 0.03 body weight, experienced ¼ 1.39 ^ 0.03 body weight), F(1, 66) ¼ 23.24, p , .001, eta 2 ¼ .252, during the TSSm and END phases.
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Anchoring Index and Inclination Angles The novice group was shown to have a smaller AIhead than that of the experienced group for all five phases of turning (see Figure 3b). A similar trend of decreasing AIhead from the PRE, F(1, 66) ¼ 8.08, p ¼ .006, eta 2 ¼ .106, TDS, F(1, 66) ¼ 30.98, p , .001, eta 2 ¼ .312, and TSSp, F(1, 66) ¼ 28.36, p , .001, eta 2 ¼ .294, phases was observed for both groups, and then the AIhead was increased from the TSSp, TSSm, F(1, 66) ¼ 34.50, p , .001, eta 2 ¼ .337, to the END phases, F(1, 66) ¼ 4.40, p ¼ .040, eta 2 ¼ .061. The positive values of the AIhead across the five phases of turning were shown more in the advanced dancer group than in the novice group. The average, minimum, maximum, range, and standard deviation of ANGLErotation axis were all found to be greater for the novice group in the single-leg support phase compared with the experienced group (see Table 3). The novice dancers were also found to show a greater range of ANGLEtrunk during the END phase compared with the experienced dancers (novice ¼ 7.83 ^ 0.51 degrees, experienced ¼ 4.68 ^ 0.57degrees), F(1, 66) ¼ 28.00, p , .001, eta 2 ¼ .292.
DISCUSSION To fully understand ballet pirouettes from a biomechanical perspective, we investigated performance and strategies of ballet turns in novice and experienced dancers. The findings indicated that the experienced ballet dancers had the fewest changes of the ANGLErotation axis in the preswing phase. The experienced dancers also had greater positive AIhead in all five phases accompanied with a decreased peak ground reaction force in both the supporting leg and gesture leg during the initial push-off compared with the novice dancers. In addition, the experienced group also showed the fewest changes in the range of ANGLEtrunk during the END phase of turning. Temporal-Spatial Parameters The longer preparatory duration exhibited by the novice dancers was consistent with that of a previous study (Laws,
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1978). Their study revealed that the professional dancers take less time in the preparation phase of pirouette en dehors than do the less skilled dancers. Thus, the novice dancers spending more time during the preparation phase prior to turning initiation may suggest that they need more time to adjust their posture before turning. They might also spend more time in the preparation phase for cognitive thinking because they were in the early stages of skill acquisition (Newell, 1991). A greater angular velocity on the retire position but shorter duration in the TDS and TSSp phases were found in the novice group. Because the effect size of the angular parameter was small, we should interpret this finding with caution. Given that the same degrees of angle should be completed in both groups, the novice group spent less time to achieve the same degrees of angle and thus had greater angular velocity compared with the experienced group. Ground Reaction Force To better quantify the necessary forces to initiate turning in ballet dancers, the 3-D ground reaction forces showed that only the anterior-posterior force was found to be different and was greater for the novice dancers during the TDS phase. In this phase, the dancers’ supporting legs were changing from double-leg support to single-leg support, and each leg also revealed different directional forces in either the anterior or posterior direction. The smaller anterior-posterior ground reaction force needed for the turning demonstrated that the experienced dancers exerted smaller peak forces in the anterior-posterior direction for both legs. By contrast, the greater anteriorposterior forces in the novice dancers indicated that they put more effort into maintain their postures and dynamic balance. Therefore, adopting the proper magnitude of ground reaction force in the supporting and gesture legs is essential for performing a poised turn. This also suggests that too much force leads to the body deviating from the rotational axis, while too little force makes it incapable to initiate a turn (Laws, 2002). Furthermore, a proper landing is important for the sake of aesthetic requirement and injury prevention. Although the impulses of landing from ballet turns were not as large as that from jump landing, a greater amount of angular velocity decreased within a short period was also a risk for injury when landing from turns. The foot and ankle joints thus have to absorb the whole body weight as well as the force generated from the angular velocity, and thus, great demands of the lower extremities are noted (Krasnow & Wyon, 2011). In the current study, although no difference in landing force was observed between the groups, the different strategies may be utilized in the experienced and novice dancers, respectively. The novice dancers inclined their trunk forward to compensate
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instability compared with the experienced dancers, who landed with their trunk slightly forward, and this further revealed that the novice dancers had poorer postural control ability.
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Kinematics Even though the two groups did not differ in the maximum extension angles of the hip, knee, and ankle joints during the single-leg support phase, different sequences of time to maximum extension angles in these joints did exist between groups. The majority of experienced dancers used the sequence of the proximal-to-distal pattern (hip ! knee ! ankle joints), but few novice dancers used this sequential extension pattern. When performing pirouette en dehors, the sequential pattern helps dancers to centralize their segmental mass and to align the center of mass (COM) to the center of pressure (COP) efficiently. Furthermore, the sequential pattern was also observed in gymnasts who performed fast backward trunk skill (i.e., bending head and trunk backward as fast as possible) compared with untrained participants (Pedotti, Crenna, Deat, Frigo, & Massion, 1989). The untrained participants had synchronized muscle activation in the back, thighs, and legs, whereas the gymnast adopted the distal-to-proximal sequential pattern when performing the backward trunk skill. Based on this, we suggest that both gymnasts and experienced ballet dancers adapted to their training to perform the movement efficiently. Furthermore, the synchronized muscle activation froze the degrees of freedom and is common in early learning (Vorro & Hobart, 1981), and this condition could also contribute to the lack of sequential pattern in the novice dancers. Anchoring Index The positive AIhead indicates that the head moves with fewer variations relative to the global axis. A positive AIhead also demonstrates that the participants use the global anteriorposterior axis as a stabilization reference (space reference) more often than the adjacent segment (local reference) to control posture of the head segment. However, the negative value shows the participants take local reference, instead of the space reference, as the stabilization reference. This is the so-called “en bloc” strategy, which means moving with adjacent segments together (Golomer, Toussaint, Bouillette, & Keller, 2009). During the TDS phase, the positive AIhead in the experienced group but negative AIhead in the novice group indicated that the experienced dancers utilized visual input as a stabilization strategy during turns. The utilization of vision and gazing at space may have a positive effect on turning, because gazing on a specific target in space prevents vertigo or dizziness (Amblard et al., 2001). It is the so-called “spotting technique” in ballet (Kirstein & Stuart,
2004), which is to focus on the target as long as possible then whip the head around and refocus on the target. This technique decreases head motion and improves sensory processing from the head to minimize body oscillations (Chatfield, Krasnow, Herman, & Blessing, 2007). This also further explains why the dancers depend on vision and constantly use mirrors for postural stabilization (Kilby & Newell, 2012). Inclination Angles The experienced dancers had a smaller ANGLErotation axis during the TSSp phase, indicating a smaller deviation and a better performance. This is consistent with a previous investigation on the differences between dancers and untrained participants during single-leg support with gesture leg raising 45 degrees laterally (Mouchnino, Aurenty, Massion, & Pedotti, 1992). In that study, the dancers used the trunk “translation strategy” (i.e., maintaining the trunk axis vertically by using hip adduction/abduction motion), whereas the untrained participants utilized the “inclination strategy” (i.e., inclining the trunk for postural stability) when performing the gesture-leg raising. The inclination strategy used by novice dancers may be one of reasons for greater ANGLErotation axis during ballet turns in the current study. The novice dancers inclined their trunk to compensate for the rising of the gesture leg, but this inclined trunk would likely abate the visual sensory inputs to postural control (Clark & Iltis, 2008). Given that the inclination strategy leads the COM and COP apart on the transverse plane, imbalance would also occur. By contrast, the translation strategy ensures that the visual sensory inputs are not disturbed and make certain continuous sensory inputs. When utilizing translation strategy, the COM and COP separation on the transverse plane can be limited because the major adjustment was performed by the hip joints. As a result, the translation strategy was suggested to be superior to the inclination strategy while turning in the view of balance control. The higher push force at propulsion along with greater angular velocity resulted in a larger ANGLEtrunk and difficulty in control of postural stability. Given less verticality in novice dancers, aligning rotation axis and gravity reference would be a challenge for novice dancers while maintaining the balance within a small base of support. Therefore, the novice dancers inclined their trunk forward to regain their balance in the END phase. Implication According to current findings, some suggestions are made to novice dancers and dance educators. During training, novice dancers are suggested to reduce the push
CONTROL STRATEGY OF BALLET TURNS IN DANCERS
force of the gesture leg during the TDS phase and to adapt more visual input to maintain their stability, especially during the TDS phase. While turning with single-leg support, extending the hip joint first then the knee joint and later the ankle joint to obtain better postural stability is also preferred. Finally, excessive trunk-forward inclination should be avoided during the END phase.
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Limitations Three limitations of the present study need to be acknowledged. First, the age difference between groups was a concern; experienced dancers are generally older than novice dancers. However, this was reasonable because they start learning dance around the same age, and the ability of balance as positively correlated to age due to different training experiences has been evidenced (Kilby & Newell, 2012). Second, this study considered only the trunk control during pirouette en dehors. Movement of the extremities logically affects performance of ballet turns. Therefore, investigation into the influence of the extremities on ballet turns should be the next step for future study. Third, body height- and body weightmatched controls should be recruited for future study to eliminate their potential effect on movement performance and execution strategy.
Conclusion Experienced dancers had a smaller inclination angle of rotational axis by adapting the “translation strategy,” which implies that adjustment of biomechanical factors was efficient for performing better movements and maintaining postural stability. By contrast, the novice dancers maintained their postural stability through the “inclination strategy” so that the visual sensory inputs were distorted. Furthermore, the novice dancers performed more poorly than experienced dancers because of higher push forces, lack of a “proximal-to-distal extension sequence” pattern in the supporting leg, and insufficient visual spotting stabilization for postural stability. Finally, the novice dancers inclined their trunk forward to regain balance at the END phase.
WHAT DOES THIS ARTICLE ADD? The strategy used to perform ballet turns differed between experienced and novice dancers. Large push force and an insufficient proximal-to-distal extension sequence pattern were potential factors for inferior performance in novice dancers. Moreover, novice dancers utilized trunk-forward inclination to regain balance in the END phase. Note that the
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lack of visual spotting stabilization affected postural stability. REFERENCES Amblard, B., Assaiante, C., Vaugoyeau, M., Baroni, G., Ferrigno, G., & Pedotti, A. (2001). Voluntary head stabilisation in space during oscillatory trunk movements in the frontal plane performed before, during and after a prolonged period of weightlessness. Experimental Brain Research, 137, 170– 179. Charalambous, L., Irwin, G., Bezodis, I. N., & Kerwin, D. (2012). Lower limb joint kinetics and ankle joint stiffness in the sprint start push-off. Journal of Sports Sciences, 30, 1–9. Chatfield, S., Krasnow, D., Herman, A., & Blessing, G. (2007). A descriptive analysis of kinematic and electromyographic relationships of the core during forward stepping in beginning and expert dancers. Journal of Dance Medicine and Science, 11, 76 –84. Clark, S., & Iltis, P. W. (2008). Effects of dynamic head tilts on sensory organization test performance: A comparison between college-age athletes and nonathletes. Journal of Orthopaedic & Sports Physical Therapy, 38, 262–268. Golomer, E., Rosey, F., Dizac, H., Mertz, C., & Fagard, J. (2008). The influence of classical dance training on preferred supporting leg and whole body turning bias. Laterality, 14, 165–177. Golomer, E., Toussaint, Y., Bouillette, A., & Keller, J. (2009). Spontaneous whole body rotations and classical dance expertise: How shoulder–hip coordination influences supporting leg displacements. Journal of Electromyography and Kinesiology, 19, 314 –321. Iida, Y., Kanehisa, H., Inaba, Y., & Nakazawa, K. (2011). Activity modulations of trunk and lower limb muscles during impact-absorbing landing. Journal of Electromyography and Kinesiology, 21, 602– 609. Kilby, M. C., & Newell, K. M. (2012). Intra- and inter-foot coordination in quiet standing: Footwear and posture effects. Gait & Posture, 35, 511–516. Kirstein, L., & Stuart, M. (2004). The classic ballet: Basic technique and terminology. New York, NY: Knopf. Krasnow, D., & Wyon, M. (2011). Biomechanical research in dance: A literature review. Medical Problems of Performing Artists, 26, 3 –23. Laws, K. (1978). An analysis of turns in dance. Dance Research Journal, 11, 12 –19. Laws, K. (2002). Physics and the art of dance: Understanding movement. Oxford, England: Oxford University Press. Mesure, S., Azulay, J. P., Pouget, J., & Amblard, B. (1999). Strategies of segmental stabilization during gait in Parkinson’s disease. Experimental Brain Research, 129, 573– 581. Mouchnino, L., Aurenty, R., Massion, J., & Pedotti, A. (1992). Coordination between equilibrium and head-trunk orientation during leg movement: A new strategy build up by training. Journal of Neurophysiology, 67, 1587–1598. Naito, K., Fukui, Y., & Maruyama, T. (2012). Energy redistribution analysis of dynamic mechanisms of multi-body, multi-joint kinetic chain movement during soccer instep kicks. Human Movement Science, 31, 161–181. Newell, K. M. (1991). Motor skill acquisition. Annual Review of Psychology, 42, 213–237. Pedotti, A., Crenna, P., Deat, A., Frigo, C., & Massion, J. (1989). Postural synergies in axial movements: Short and long-term adaptation. Experimental Brain Research, 74, 3–10. Robert, G., Blouin, J., Ruget, H., & Mouchnino, L. (2007). Coordination between postural and movement controls: Effect of changes in body mass distribution on postural and focal component characteristics. Experimental Brain Research, 181, 159 –171. Salvia, P., Woestyn, L., David, J. H., Feipel, V., Van, S., Jan, S., . . . Rooze, M. (2000). Analysis of helical axes, pivot and envelope in active wrist circumduction. Clinical Biomechanics, 15, 103–111.
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Sienko, K. H., Balkwill, M. D., Oddsson, L. I., & Wall, C. (2008). Effects of multi-directional vibrotactile feedback on vestibular-deficient postural performance during continuous multi-directional support surface perturbations. Journal of Vestibular Research, 18, 273 –285. Sommer, H. (1992). Determination of first and second order instant screw parameters from landmark trajectories. Journal of Mechanical Design, 114, 274 –281. Sveistrup, H., Schneiberg, S., McKinley, P. A., McFadyen, B. J., & Levin, M. F. (2008). Head, arm and trunk coordination during reaching in children. Experimental Brain Research, 188, 237–247.
Vorro, J., & Hobart, D. (1981). Kinematic and myoelectric analysis of skill acquisition: I. 90 cm subject group. Archives of Physical Medicine and Rehabilitation, 62, 575–582. Woltring, H. J. (1976). Calibration and measurement in 3-dimensional monitoring of human motion by optoelectronic means. II. Experimental results and discussion. Biotelemetry, 3, 65–97. Woltring, H. J. (1980). Planar control in multi-camera calibration for 3-D gait studies. Journal of Biomechanics, 13, 39– 48. Woltring, H. J. (1994). 3-D attitude representation of human joints: A standardization proposal. Journal of Biomechanics, 27, 1399–1414.
