Human Movement Science 42 (2015) 71–80
Contents lists available at ScienceDirect
Human Movement Science journal homepage: www.elsevier.com/locate/humov
Effect of different knee starting angles on intersegmental coordination and performance in vertical jumps Rodrigo G. Gheller a, Juliano Dal Pupo b,⇑, Jonathan Ache-Dias b, Daniele Detanico b, Johnny Padulo c,d, Saray G. dos Santos b a
Federal University of Amazonas, Manaus, Amazonas, Brazil Biomechanics Laboratory, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil c University e-Campus, Novedrate, Italy d Tunisian Research Laboratory ‘‘Sports Performance Optimisation’’ National Center of Medicine and Science in Sports (CNMSS), Tunis, Tunisia b
a r t i c l e
i n f o
PsycINFO classification: 3720 Keywords: Continuous relative phase Kinematic Kinetic Motor control Power output
a b s t r a c t This study aimed to analyze the effect of different knee starting angles on jump performance, kinetic parameters, and intersegmental coupling coordination during a squat jump (SJ) and a countermovement jump (CMJ). Twenty male volleyball and basketball players volunteered to participate in this study. The CMJ was performed with knee flexion at the end of the countermovement phase smaller than 90° (CMJ90), and in a preferred position (CMJPREF), while the SJ was performed from a knee angle of 70° (SJ70), 90° (SJ90), 110° (SJ110), and in a preferred position (SJPREF). The best jump performance was observed in jumps that started from a higher squat depth (CMJ90. Analysis of continuous relative phase showed that thigh–trunk coupling was more in-phase in the jumps (CMJ and SJ) performed with a higher squat depth, while the leg–thigh coupling was more in-phase in the CMJ>90 and SJPREF. Jumping from a position with knees more flexed seems to be the best strategy to achieve the best performance. Intersegmental coordination and jump performance (CMJ and SJ) were affected by different knee starting angles. Ó 2015 Elsevier B.V. All rights reserved.
⇑ Corresponding author at: Universidade Federal de Santa Catarina, Centro de Desportos, Laboratório de Biomecânica, 88040900 Florianópolis, SC, Brazil. Tel./fax: +55 48 3721 8530. E-mail address: [email protected] (J. Dal Pupo). http://dx.doi.org/10.1016/j.humov.2015.04.010 0167-9457/Ó 2015 Elsevier B.V. All rights reserved.
72
R.G. Gheller et al. / Human Movement Science 42 (2015) 71–80
1. Introduction In order to reach maximum jump height, humans often select different initial squat-depth positions. According to Bobbert, Casius, Sijpkens, and Jaspers (2008), when different postures are assumed, the muscle-tendon units of the lower limb are at different lengths, so they will produce different forces and joint moments at a given level of stimulation. The modulation of squat depth may affect some variables related to vertical-jump performance, such as peak power output and impulse (Dowling & Vamos, 1993; Harman, Rosenstein, Frykman, & Rosenstein, 1990), and consequently, the jump height (Moran & Wallace, 2007). Some authors have found that in squat (SJ) and countermovement jumps (CMJs) performed with a higher squat depth, it is possible to reach higher height, net impulse and take-off velocity; however, power output and force decrease in this situation (Domire & Challis, 2007; Kirby, McBride, Haines, & Dayne, 2011; McBride, Kirby, Haines, & Skinner, 2010). On the other hand, Moran and Wallace (2007) state that a smaller range of knee motion combined with an increasing negative phase resulted in a significantly greater countermovement jump height. Therefore, researches have shown inconsistencies in how performance and biomechanical variables are modulated in different initial squat positions or ranges of knee motion during the SJ and CMJ. Additionally, little attention has been given to analysing whether there are changes in movement coordination during jumps performed from different ranges of motion and what the effect of such differences on jump performance might be. Domire and Challis (2007) reported that the SJ performed from a deeper squat depth did not result in larger jump heights, and they suggested that the results were due to non-optimal coordination during the jumps. Movement coordination was previously analyzed by Van Soest and Bobbert (1993), who suggest that when different initial postures are assumed during the jump, irrespective of the force production, different accelerations are generated; hence, the dynamics, and possibly the coordination of movement, could be affected. On the other hand, it has been postulated that subjects may perform movements from a range of initial postures using the same pattern stimulation (Van Soest, Bobbert, & Van Ingen Schenau, 1994), so the dynamic or coordination would not change during jumps performed from different squat depths. Joint coordination has been evaluated by using different methods such as electromyography (Bobbert & Van Ingen Schenau, 1988; Rodacki, Fowler, & Bennett, 2002), timing and sequencing of segmental movements (Rodacki, Fowler, & Bennett, 2001), and continuous relative phase (CRP) (Hamill, Van Emmerik, Heiderscheit, & Li, 1999; Quinzi, Sbriccoli, Alderson, Di Mario, & Camomilla, 2014; Seifert, Leblanc, Chollet, & Delignières, 2010; Van Emmerik & Wagenaar, 1996). The last method is based on a dynamic system approach (Kelso, 1984), providing information on the stability of the coordination patterns by time–angle relationships inter-/intralimbs during an entire cycle of movement (Hamill et al., 1999; Kelso, 1984). The CRP was recently used for analysing the effects of fatigue on coordination during continuous jumps (Dal Pupo, Dias, Gheller, Detanico, & Santos, 2013). This method could also be an interesting tool for investigating the effect of different ranges of knee motion on vertical-jump coordination. To our knowledge, no previous studies have analyzed the coordination of CMJ and SJ with this approach considering different knee-angle positions. The main hypothesis is that the CRP changes according to the level of squat depth. Therefore, this study aimed to analyze the effect of different knee starting angles on jump performance (jump height), kinetic parameters, and intersegmental coupling coordination during SJ and CMJ.
2. Methods 2.1. Participants Twenty male volleyball and basketball players (23.5 ± 3.58 years; 82.38 ± 9.83 kg; 185 ± 6.31 cm; 13.79 ± 3.31% body fat) volunteered to participate in this study. Participants trained on a regular basis
R.G. Gheller et al. / Human Movement Science 42 (2015) 71–80
73
(three sessions per week) during at least the four years that preceded the study, and all were currently competing at college level. All athletes performed vertical jumps in their regular training routines. They did not report injuries or other conditions that prevented them from training or otherwise influenced their maximal physical performance. This study was approved by the Research Ethics Committee of the Federal University of Santa Catarina, Brazil. All participants were informed about the procedures, and signed an informed consent form, in accordance with the Declaration of Helsinki. 2.2. Procedures The participants visited the laboratory on two days. On the first day, a familiarization was performed. On the second day, participants randomly performed the countermovement jumps and static squat jumps from different knee-flexion angles, which consequently determined different levels of squat depth. Participants were requested to avoid training and caffeinated drinks 24 h prior to testing sessions. All athletes were evaluated at the same time of day in a laboratory where the ambient temperature was 24 °C. The SJs were performed from the following knee-angle start positions: preferred position (SJPREF) and with the maximum knee-flexion angle at 70° (SJ70), 90° (SJ90), and 110° (SJ110) (Fig. 1). Each knee-angle position was determined in the familiarization session, in which participants were instructed to squat in a slow and controlled manner until they reached the required knee angle. They were instructed to maintain that squat depth to allow the knee-angle measurement using a goniometer (CarciÒ, Goiânia, Brazil). From this phase, a rigid bar was positioned under the participants’ hips (posterior thigh) and attached to a vertical support graduated every 1 cm. This mechanism allowed the adjustment of the bar height according to the specific knee-flexion angle (see Fig. 1). During the jumps, participants squatted until they could feel the bar on their posterior thigh, and then remained static some seconds before performing the concentric action as fast as possible. In the preferred SJ position, the participants were free to squat until an individual and satisfactory maximum
Fig. 1. Initial position in three different conditions of knee angle during static squat jump and countermovement jump on force platform (FP).
74
R.G. Gheller et al. / Human Movement Science 42 (2015) 71–80
knee-flexion angle. It was verified in a posterior kinematic analysis that the knee angle in the SJPREF was, on average, 96.1 ± 1.1°. Vertical ground reaction force was automatically monitored, and when it dropped below 5% of the body weight prior to the beginning of the jump (push-off force), the squat jump was not considered and a new trial was performed (Hasson, Dugan, Doyle, Humphries, & Newton, 2004). The positions of the feet and the vertical support were identical during both the familiarization and the trials in order to guarantee the correct knee angle. In the CMJs, the following positions were tested: (1) with relative knee flexion at the end of the countermovement phase smaller than 90° (CMJ90); and (3) a preferred position (CMJPREF) (Fig. 1). To determine each knee angle before testing (i.e., familiarization), participants performed the descent portion of the jump in a slow and controlled manner until the specified angle, measured by a goniometer, was reached. In the CMJ90. A light elastic band was positioned under the participants’ hips at each pre-established knee angle and attached to a vertical support, which allowed the participants to control their squat depth before initiating the ascent phase of the countermovement jump. It was verified by kinematic analysis that the knee angle during the transition of negative to positive phase in the CMJPREF was, on average, 84.9 ± 6.8°. During jump trials, participants were instructed to perform the countermovement phase of the squat depth as quickly as possible until reaching the elastic band, which could suffer a little stretch to allow natural movement behavior. The execution order of the jumps (CMJ versus SJ) and the different conditions of knee-flexion angle were randomized. Participants performed three trials in each condition for both the static squat jump and the countermovement jump. In both SJ and CMJ, participants were required to keep their hands on their hips (for controlling arm contribution) and to jump with the trunk as erect as possible to limit or reduce the energy gains associated with trunk action. The vertical jumps were performed on a force platform (Quattro Jump, 9290 AD, Kistler, Winterthur, Switzerland) sampling at 500 Hz. Two-dimensional kinematics were obtained during the test using a calibrated camera (ELPH 500HS Canon) sampling at 120 Hz. The camera was positioned perpendicularly 6 m from where the tests were conducted (Belli, Rey, Bonnefoy, & Lacour, 1992). A set of body landmarks were placed on the right side of the participant’s body at the following sites: fifth metatarsal, lateral malleolus, lateral femoral epicondyle of the knee, the most prominent protuberance of the greater trochanter, and acromial process. The landmarks were semi-automatically digitized (SkillSpector, Video4coach, Denmark), and their coordinates (x and y) were used to calculate the angular kinematics. 2.3. Data analysis Ground reaction force (GRF) data obtained during the jumps were filtered using a low-pass, fourth-order Butterworth filter with a cut-off frequency of 10 Hz, determined from spectral analysis (Kram, Griffin, Donelan, & Chang, 1998). The GRF data were used to define some key instants of the jumps such as: (i) beginning of CMJ – defined as the instant in which the vertical ground reaction force reached body weight minus 5% (Ugrinowitsch, Tricoli, Rodacki, Batista, & Ricard, 2007). The SJ started from the moment that ground reaction force was equal to body weight plus 5%, indicating the point at which the jumper begins to accelerate up; (ii) beginning of concentric phase – in the SJ, this instant coincides with the beginning of the jump, but in the CMJ, it was determined from the moment at which the center of mass velocity becomes positive. The center of mass velocity was obtained by integrating the acceleration curve; (iii) take-off – defined as the instant at which the vertical force reached zero. From the filtered data, the following parameters were obtained: (a) maximum force (FMAX) – both absolute and normalized for body mass; (b) relative net vertical impulse (VI) (Kirby et al., 2011); (c) rate of force development (RFD) – considered as the mean slope of the force–time curve in the time interval of 0–30 ms for countermovement jump and 0–50 ms for squat jump. These times were established based on the interval duration from the beginning of the concentric phase to the peak force of the curve (Dal Pupo, Detanico, & Santos, 2012); (d) jump height – obtained by double integration of the GRF. First, the acceleration was calculated by dividing the GRF by the body mass, as measured on the platform (Eq. (1)):
R.G. Gheller et al. / Human Movement Science 42 (2015) 71–80
aðtÞ ¼
FðtÞ m a0
75
ð1Þ
where a is the acceleration, t is the time, F is the force (GRF), m is the body mass, and a0 is the initial acceleration. Secondly, the velocity was obtained via trapezoid integration of the area under the acceleration curve (Eq. (2)). The integration began at the start of the jump and ended at take-off. Finally, the vertical displacement of center of mass (s) was obtained by integration of the velocity curve (Eq. (3)). The highest value of s was considered to be the jump height.
v ðtÞ ¼ sðtÞ ¼
Z
Z
aðtÞ dt
ð2Þ
v ðtÞ v 0 dt
ð3Þ
where v is the velocity, t is the time, a is the acceleration, dt is the time interval, v0 is the initial velocity, and s is vertical displacement of center of mass; peak power output (PP) – calculated by multiplying the GRF by velocity (Eq. (4)) in the ascent phase of the jump (from when the center of mass velocity becomes positive until take-off). The highest value of the curve was used for analysis:
power output ¼ FðtÞ v ðtÞ
ð4Þ
where F is force, v is the velocity and t is the time. Coordinates of the landmarks were filtered using a low-pass, fourth-order Butterworth filter with a cut-off frequency of 15 Hz, determined from spectral analysis (Kram et al., 1998). Knee-joint angle was determined between the thigh and the leg segments on the back of the body, and hip-joint angle was formed between the trunk and the thigh segments. These joint angles were analyzed in the initial position of the SJ and in the instant of transition between negative to positive phase of the CMJ. The segmental angles of the trunk (angle between the horizontal and the trunk segment), thigh (angle between the horizontal and the thigh segment) and leg (angle between the horizontal and the leg segment) were determined for intersegmental coupling coordination analysis. Continuous relative phase (CRP) was calculated to assess intersegmental coupling coordination between segments as described by Hamill et al. (1999). CRP consisted of plotting the angular position of one segment versus the segment angular velocity, obtaining the phase plot. After normalization procedures (+1 to 1), the phase angle was obtained comprising a range from 0° to 180°, determined as the four-quadrant arc-tangent angle formed between the right-hand horizontal and a line drawn from the origin to a specific data point. The continuous relative phase was the difference between the phase angles of the two segments. The following couplings were calculated: thigh–trunk and leg–thigh, in accordance with Dal Pupo et al. (2013). The root mean square was calculated to represent the time histories of the CRP and within athlete variation of the CRP in the positive and negative phases of jumps. The negative phase (only in the CMJ) was identified from the moment at which the jumper was standing upright and stationary until the lowest point of the countermovement, and the positive phase (of both the SJ and CMJ) was characterized from this moment until the instant of take-off. An algorithm implemented in MATLAB™ software was used to obtain all kinematic data. 2.4. Statistical analysis Data were reported as means and standard deviations. The Shapiro–Wilk test was performed to verify the normality of the residual data. The sphericity of the data was verified according to the Mauchly’s test results (p > .05). Analysis of variance with repeated measures (within-subjects ANOVA) and Bonferroni post hoc tests were used to compare most of the variables among different conditions of knee-flexion angles for both the CMJ and SJ. Additionally, Friedman and Wilcoxon tests were used for the SJ at the leg–thigh coupling. The significance level was set at p < .05. The analyses were performed with the Statistical Package for Social Sciences (SPSS Inc. v.17.0, Chicago, USA).
76
R.G. Gheller et al. / Human Movement Science 42 (2015) 71–80
3. Results Analysis of variance revealed that the jump height and force–time parameters of the CMJ were affected by different knee starting flexion angles (Table 1). According to post hoc analysis, higher jump height and relative net impulse (VI) were verified in the CMJ90. Rate of force development did not show differences among the jump positions. In the SJ (Table 2), differences among the conditions (knee-flexion angles) were verified for all variables except for the VI (p = .11). As observed in the CMJ, higher jump height was observed in the highest squat depth (SJ70) and the SJPREF, while the higher values of PP and FMAX were produced in the lowest squat depth (SJ110) and the SJPREF. RFD was higher in the SJ110 compared to the others positions. Fig. 2 shows CRP of the CMJ tested in different conditions. No differences were found in CRP thigh– trunk (Fig. 2, left panel) among the three conditions (i.e., different knee starting flexion angles) of the CMJ in the descent phase (F = 10.7; p = .35; g2 = 0.05). In the ascent phase, differences were found among the angles analyzed (F = 20.51; p < .01; g2 = 0.48), in which lower CRP values were verified in the CMJ90 (p < .01). In the leg–thigh coupling (Fig. 2, right panel), no differences were found among the three positions of the CMJ in the descent phase (F = 0.39; p = .62; g2 = 0.04). However, in the ascent phase, differences among positions were observed (F = 35.3; p < .01; g2 = 0.62); in this case, the lower CRP was found in the CMJ>90 when compared to the CMJPREF (p < .01) and CMJ90), a lower jump height was observed. This suggests a positive relationship between impulse and jump height for CMJ. Our results are in agreement with those found by Kirby et al. (2011), McBride et al. (2010) that verified higher VI in jumps (both SJ and CMJ) performed from a deeper squat position. Continuous relative phase is a method frequently used by researchers, first and foremost in motor-control research (Kelso, 1984; Scholz, 1990), and later used in running to detect lower extremity running injuries (Hamill et al., 1999; Li, Van den Bogert, Caldwell, Van Emmerik, & Hamill, 1999), as well as to investigate coordination in skilled athletes during cyclic (Seifert et al., 2010) and non-cyclic actions (Quinzi et al., 2014). In vertical jumps, CRP has also been calculated to investigate the effect of fatigue on movement coordination (Dal Pupo et al., 2013). In the present study, this method was used to provide a measure of intersegmental coordination during the jumps performed from different knee starting angles, based on a dynamical systems approach. The changes verified in the CRP among the tested conditions indicated that when different initial postures are assumed in vertical jumps the coordination pattern is altered, confirming the proposition of Van Soest and Bobbert (1993). According to the authors, alterations in initial postures affect the muscle-tendon unit lengths and will produce different forces and joint moments. However, if the muscles were producing the same forces and the same joint moments as a function of time, these postural alterations would generate different accelerations in different postures, and hence the dynamics of movement would be affected. In the CMJ, there was no difference in CRP during the negative phase among the three jump conditions for both couplings analyzed, but the magnitude or range of countermovement affected the CRP during the negative phase. The thigh–trunk coupling was more in-phase in the CMJ90 and CMJPREF. Interestingly, in this condition (CMJ
Contents lists available at ScienceDirect
Human Movement Science journal homepage: www.elsevier.com/locate/humov
Effect of different knee starting angles on intersegmental coordination and performance in vertical jumps Rodrigo G. Gheller a, Juliano Dal Pupo b,⇑, Jonathan Ache-Dias b, Daniele Detanico b, Johnny Padulo c,d, Saray G. dos Santos b a
Federal University of Amazonas, Manaus, Amazonas, Brazil Biomechanics Laboratory, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil c University e-Campus, Novedrate, Italy d Tunisian Research Laboratory ‘‘Sports Performance Optimisation’’ National Center of Medicine and Science in Sports (CNMSS), Tunis, Tunisia b
a r t i c l e
i n f o
PsycINFO classification: 3720 Keywords: Continuous relative phase Kinematic Kinetic Motor control Power output
a b s t r a c t This study aimed to analyze the effect of different knee starting angles on jump performance, kinetic parameters, and intersegmental coupling coordination during a squat jump (SJ) and a countermovement jump (CMJ). Twenty male volleyball and basketball players volunteered to participate in this study. The CMJ was performed with knee flexion at the end of the countermovement phase smaller than 90° (CMJ90), and in a preferred position (CMJPREF), while the SJ was performed from a knee angle of 70° (SJ70), 90° (SJ90), 110° (SJ110), and in a preferred position (SJPREF). The best jump performance was observed in jumps that started from a higher squat depth (CMJ90. Analysis of continuous relative phase showed that thigh–trunk coupling was more in-phase in the jumps (CMJ and SJ) performed with a higher squat depth, while the leg–thigh coupling was more in-phase in the CMJ>90 and SJPREF. Jumping from a position with knees more flexed seems to be the best strategy to achieve the best performance. Intersegmental coordination and jump performance (CMJ and SJ) were affected by different knee starting angles. Ó 2015 Elsevier B.V. All rights reserved.
⇑ Corresponding author at: Universidade Federal de Santa Catarina, Centro de Desportos, Laboratório de Biomecânica, 88040900 Florianópolis, SC, Brazil. Tel./fax: +55 48 3721 8530. E-mail address: [email protected] (J. Dal Pupo). http://dx.doi.org/10.1016/j.humov.2015.04.010 0167-9457/Ó 2015 Elsevier B.V. All rights reserved.
72
R.G. Gheller et al. / Human Movement Science 42 (2015) 71–80
1. Introduction In order to reach maximum jump height, humans often select different initial squat-depth positions. According to Bobbert, Casius, Sijpkens, and Jaspers (2008), when different postures are assumed, the muscle-tendon units of the lower limb are at different lengths, so they will produce different forces and joint moments at a given level of stimulation. The modulation of squat depth may affect some variables related to vertical-jump performance, such as peak power output and impulse (Dowling & Vamos, 1993; Harman, Rosenstein, Frykman, & Rosenstein, 1990), and consequently, the jump height (Moran & Wallace, 2007). Some authors have found that in squat (SJ) and countermovement jumps (CMJs) performed with a higher squat depth, it is possible to reach higher height, net impulse and take-off velocity; however, power output and force decrease in this situation (Domire & Challis, 2007; Kirby, McBride, Haines, & Dayne, 2011; McBride, Kirby, Haines, & Skinner, 2010). On the other hand, Moran and Wallace (2007) state that a smaller range of knee motion combined with an increasing negative phase resulted in a significantly greater countermovement jump height. Therefore, researches have shown inconsistencies in how performance and biomechanical variables are modulated in different initial squat positions or ranges of knee motion during the SJ and CMJ. Additionally, little attention has been given to analysing whether there are changes in movement coordination during jumps performed from different ranges of motion and what the effect of such differences on jump performance might be. Domire and Challis (2007) reported that the SJ performed from a deeper squat depth did not result in larger jump heights, and they suggested that the results were due to non-optimal coordination during the jumps. Movement coordination was previously analyzed by Van Soest and Bobbert (1993), who suggest that when different initial postures are assumed during the jump, irrespective of the force production, different accelerations are generated; hence, the dynamics, and possibly the coordination of movement, could be affected. On the other hand, it has been postulated that subjects may perform movements from a range of initial postures using the same pattern stimulation (Van Soest, Bobbert, & Van Ingen Schenau, 1994), so the dynamic or coordination would not change during jumps performed from different squat depths. Joint coordination has been evaluated by using different methods such as electromyography (Bobbert & Van Ingen Schenau, 1988; Rodacki, Fowler, & Bennett, 2002), timing and sequencing of segmental movements (Rodacki, Fowler, & Bennett, 2001), and continuous relative phase (CRP) (Hamill, Van Emmerik, Heiderscheit, & Li, 1999; Quinzi, Sbriccoli, Alderson, Di Mario, & Camomilla, 2014; Seifert, Leblanc, Chollet, & Delignières, 2010; Van Emmerik & Wagenaar, 1996). The last method is based on a dynamic system approach (Kelso, 1984), providing information on the stability of the coordination patterns by time–angle relationships inter-/intralimbs during an entire cycle of movement (Hamill et al., 1999; Kelso, 1984). The CRP was recently used for analysing the effects of fatigue on coordination during continuous jumps (Dal Pupo, Dias, Gheller, Detanico, & Santos, 2013). This method could also be an interesting tool for investigating the effect of different ranges of knee motion on vertical-jump coordination. To our knowledge, no previous studies have analyzed the coordination of CMJ and SJ with this approach considering different knee-angle positions. The main hypothesis is that the CRP changes according to the level of squat depth. Therefore, this study aimed to analyze the effect of different knee starting angles on jump performance (jump height), kinetic parameters, and intersegmental coupling coordination during SJ and CMJ.
2. Methods 2.1. Participants Twenty male volleyball and basketball players (23.5 ± 3.58 years; 82.38 ± 9.83 kg; 185 ± 6.31 cm; 13.79 ± 3.31% body fat) volunteered to participate in this study. Participants trained on a regular basis
R.G. Gheller et al. / Human Movement Science 42 (2015) 71–80
73
(three sessions per week) during at least the four years that preceded the study, and all were currently competing at college level. All athletes performed vertical jumps in their regular training routines. They did not report injuries or other conditions that prevented them from training or otherwise influenced their maximal physical performance. This study was approved by the Research Ethics Committee of the Federal University of Santa Catarina, Brazil. All participants were informed about the procedures, and signed an informed consent form, in accordance with the Declaration of Helsinki. 2.2. Procedures The participants visited the laboratory on two days. On the first day, a familiarization was performed. On the second day, participants randomly performed the countermovement jumps and static squat jumps from different knee-flexion angles, which consequently determined different levels of squat depth. Participants were requested to avoid training and caffeinated drinks 24 h prior to testing sessions. All athletes were evaluated at the same time of day in a laboratory where the ambient temperature was 24 °C. The SJs were performed from the following knee-angle start positions: preferred position (SJPREF) and with the maximum knee-flexion angle at 70° (SJ70), 90° (SJ90), and 110° (SJ110) (Fig. 1). Each knee-angle position was determined in the familiarization session, in which participants were instructed to squat in a slow and controlled manner until they reached the required knee angle. They were instructed to maintain that squat depth to allow the knee-angle measurement using a goniometer (CarciÒ, Goiânia, Brazil). From this phase, a rigid bar was positioned under the participants’ hips (posterior thigh) and attached to a vertical support graduated every 1 cm. This mechanism allowed the adjustment of the bar height according to the specific knee-flexion angle (see Fig. 1). During the jumps, participants squatted until they could feel the bar on their posterior thigh, and then remained static some seconds before performing the concentric action as fast as possible. In the preferred SJ position, the participants were free to squat until an individual and satisfactory maximum
Fig. 1. Initial position in three different conditions of knee angle during static squat jump and countermovement jump on force platform (FP).
74
R.G. Gheller et al. / Human Movement Science 42 (2015) 71–80
knee-flexion angle. It was verified in a posterior kinematic analysis that the knee angle in the SJPREF was, on average, 96.1 ± 1.1°. Vertical ground reaction force was automatically monitored, and when it dropped below 5% of the body weight prior to the beginning of the jump (push-off force), the squat jump was not considered and a new trial was performed (Hasson, Dugan, Doyle, Humphries, & Newton, 2004). The positions of the feet and the vertical support were identical during both the familiarization and the trials in order to guarantee the correct knee angle. In the CMJs, the following positions were tested: (1) with relative knee flexion at the end of the countermovement phase smaller than 90° (CMJ90); and (3) a preferred position (CMJPREF) (Fig. 1). To determine each knee angle before testing (i.e., familiarization), participants performed the descent portion of the jump in a slow and controlled manner until the specified angle, measured by a goniometer, was reached. In the CMJ90. A light elastic band was positioned under the participants’ hips at each pre-established knee angle and attached to a vertical support, which allowed the participants to control their squat depth before initiating the ascent phase of the countermovement jump. It was verified by kinematic analysis that the knee angle during the transition of negative to positive phase in the CMJPREF was, on average, 84.9 ± 6.8°. During jump trials, participants were instructed to perform the countermovement phase of the squat depth as quickly as possible until reaching the elastic band, which could suffer a little stretch to allow natural movement behavior. The execution order of the jumps (CMJ versus SJ) and the different conditions of knee-flexion angle were randomized. Participants performed three trials in each condition for both the static squat jump and the countermovement jump. In both SJ and CMJ, participants were required to keep their hands on their hips (for controlling arm contribution) and to jump with the trunk as erect as possible to limit or reduce the energy gains associated with trunk action. The vertical jumps were performed on a force platform (Quattro Jump, 9290 AD, Kistler, Winterthur, Switzerland) sampling at 500 Hz. Two-dimensional kinematics were obtained during the test using a calibrated camera (ELPH 500HS Canon) sampling at 120 Hz. The camera was positioned perpendicularly 6 m from where the tests were conducted (Belli, Rey, Bonnefoy, & Lacour, 1992). A set of body landmarks were placed on the right side of the participant’s body at the following sites: fifth metatarsal, lateral malleolus, lateral femoral epicondyle of the knee, the most prominent protuberance of the greater trochanter, and acromial process. The landmarks were semi-automatically digitized (SkillSpector, Video4coach, Denmark), and their coordinates (x and y) were used to calculate the angular kinematics. 2.3. Data analysis Ground reaction force (GRF) data obtained during the jumps were filtered using a low-pass, fourth-order Butterworth filter with a cut-off frequency of 10 Hz, determined from spectral analysis (Kram, Griffin, Donelan, & Chang, 1998). The GRF data were used to define some key instants of the jumps such as: (i) beginning of CMJ – defined as the instant in which the vertical ground reaction force reached body weight minus 5% (Ugrinowitsch, Tricoli, Rodacki, Batista, & Ricard, 2007). The SJ started from the moment that ground reaction force was equal to body weight plus 5%, indicating the point at which the jumper begins to accelerate up; (ii) beginning of concentric phase – in the SJ, this instant coincides with the beginning of the jump, but in the CMJ, it was determined from the moment at which the center of mass velocity becomes positive. The center of mass velocity was obtained by integrating the acceleration curve; (iii) take-off – defined as the instant at which the vertical force reached zero. From the filtered data, the following parameters were obtained: (a) maximum force (FMAX) – both absolute and normalized for body mass; (b) relative net vertical impulse (VI) (Kirby et al., 2011); (c) rate of force development (RFD) – considered as the mean slope of the force–time curve in the time interval of 0–30 ms for countermovement jump and 0–50 ms for squat jump. These times were established based on the interval duration from the beginning of the concentric phase to the peak force of the curve (Dal Pupo, Detanico, & Santos, 2012); (d) jump height – obtained by double integration of the GRF. First, the acceleration was calculated by dividing the GRF by the body mass, as measured on the platform (Eq. (1)):
R.G. Gheller et al. / Human Movement Science 42 (2015) 71–80
aðtÞ ¼
FðtÞ m a0
75
ð1Þ
where a is the acceleration, t is the time, F is the force (GRF), m is the body mass, and a0 is the initial acceleration. Secondly, the velocity was obtained via trapezoid integration of the area under the acceleration curve (Eq. (2)). The integration began at the start of the jump and ended at take-off. Finally, the vertical displacement of center of mass (s) was obtained by integration of the velocity curve (Eq. (3)). The highest value of s was considered to be the jump height.
v ðtÞ ¼ sðtÞ ¼
Z
Z
aðtÞ dt
ð2Þ
v ðtÞ v 0 dt
ð3Þ
where v is the velocity, t is the time, a is the acceleration, dt is the time interval, v0 is the initial velocity, and s is vertical displacement of center of mass; peak power output (PP) – calculated by multiplying the GRF by velocity (Eq. (4)) in the ascent phase of the jump (from when the center of mass velocity becomes positive until take-off). The highest value of the curve was used for analysis:
power output ¼ FðtÞ v ðtÞ
ð4Þ
where F is force, v is the velocity and t is the time. Coordinates of the landmarks were filtered using a low-pass, fourth-order Butterworth filter with a cut-off frequency of 15 Hz, determined from spectral analysis (Kram et al., 1998). Knee-joint angle was determined between the thigh and the leg segments on the back of the body, and hip-joint angle was formed between the trunk and the thigh segments. These joint angles were analyzed in the initial position of the SJ and in the instant of transition between negative to positive phase of the CMJ. The segmental angles of the trunk (angle between the horizontal and the trunk segment), thigh (angle between the horizontal and the thigh segment) and leg (angle between the horizontal and the leg segment) were determined for intersegmental coupling coordination analysis. Continuous relative phase (CRP) was calculated to assess intersegmental coupling coordination between segments as described by Hamill et al. (1999). CRP consisted of plotting the angular position of one segment versus the segment angular velocity, obtaining the phase plot. After normalization procedures (+1 to 1), the phase angle was obtained comprising a range from 0° to 180°, determined as the four-quadrant arc-tangent angle formed between the right-hand horizontal and a line drawn from the origin to a specific data point. The continuous relative phase was the difference between the phase angles of the two segments. The following couplings were calculated: thigh–trunk and leg–thigh, in accordance with Dal Pupo et al. (2013). The root mean square was calculated to represent the time histories of the CRP and within athlete variation of the CRP in the positive and negative phases of jumps. The negative phase (only in the CMJ) was identified from the moment at which the jumper was standing upright and stationary until the lowest point of the countermovement, and the positive phase (of both the SJ and CMJ) was characterized from this moment until the instant of take-off. An algorithm implemented in MATLAB™ software was used to obtain all kinematic data. 2.4. Statistical analysis Data were reported as means and standard deviations. The Shapiro–Wilk test was performed to verify the normality of the residual data. The sphericity of the data was verified according to the Mauchly’s test results (p > .05). Analysis of variance with repeated measures (within-subjects ANOVA) and Bonferroni post hoc tests were used to compare most of the variables among different conditions of knee-flexion angles for both the CMJ and SJ. Additionally, Friedman and Wilcoxon tests were used for the SJ at the leg–thigh coupling. The significance level was set at p < .05. The analyses were performed with the Statistical Package for Social Sciences (SPSS Inc. v.17.0, Chicago, USA).
76
R.G. Gheller et al. / Human Movement Science 42 (2015) 71–80
3. Results Analysis of variance revealed that the jump height and force–time parameters of the CMJ were affected by different knee starting flexion angles (Table 1). According to post hoc analysis, higher jump height and relative net impulse (VI) were verified in the CMJ90. Rate of force development did not show differences among the jump positions. In the SJ (Table 2), differences among the conditions (knee-flexion angles) were verified for all variables except for the VI (p = .11). As observed in the CMJ, higher jump height was observed in the highest squat depth (SJ70) and the SJPREF, while the higher values of PP and FMAX were produced in the lowest squat depth (SJ110) and the SJPREF. RFD was higher in the SJ110 compared to the others positions. Fig. 2 shows CRP of the CMJ tested in different conditions. No differences were found in CRP thigh– trunk (Fig. 2, left panel) among the three conditions (i.e., different knee starting flexion angles) of the CMJ in the descent phase (F = 10.7; p = .35; g2 = 0.05). In the ascent phase, differences were found among the angles analyzed (F = 20.51; p < .01; g2 = 0.48), in which lower CRP values were verified in the CMJ90 (p < .01). In the leg–thigh coupling (Fig. 2, right panel), no differences were found among the three positions of the CMJ in the descent phase (F = 0.39; p = .62; g2 = 0.04). However, in the ascent phase, differences among positions were observed (F = 35.3; p < .01; g2 = 0.62); in this case, the lower CRP was found in the CMJ>90 when compared to the CMJPREF (p < .01) and CMJ90), a lower jump height was observed. This suggests a positive relationship between impulse and jump height for CMJ. Our results are in agreement with those found by Kirby et al. (2011), McBride et al. (2010) that verified higher VI in jumps (both SJ and CMJ) performed from a deeper squat position. Continuous relative phase is a method frequently used by researchers, first and foremost in motor-control research (Kelso, 1984; Scholz, 1990), and later used in running to detect lower extremity running injuries (Hamill et al., 1999; Li, Van den Bogert, Caldwell, Van Emmerik, & Hamill, 1999), as well as to investigate coordination in skilled athletes during cyclic (Seifert et al., 2010) and non-cyclic actions (Quinzi et al., 2014). In vertical jumps, CRP has also been calculated to investigate the effect of fatigue on movement coordination (Dal Pupo et al., 2013). In the present study, this method was used to provide a measure of intersegmental coordination during the jumps performed from different knee starting angles, based on a dynamical systems approach. The changes verified in the CRP among the tested conditions indicated that when different initial postures are assumed in vertical jumps the coordination pattern is altered, confirming the proposition of Van Soest and Bobbert (1993). According to the authors, alterations in initial postures affect the muscle-tendon unit lengths and will produce different forces and joint moments. However, if the muscles were producing the same forces and the same joint moments as a function of time, these postural alterations would generate different accelerations in different postures, and hence the dynamics of movement would be affected. In the CMJ, there was no difference in CRP during the negative phase among the three jump conditions for both couplings analyzed, but the magnitude or range of countermovement affected the CRP during the negative phase. The thigh–trunk coupling was more in-phase in the CMJ90 and CMJPREF. Interestingly, in this condition (CMJ
