Journal of Occupational Rehabilitation, Vol. 6, No. 3, 1996
The Effect of Head Position on Scapular Orientation and Muscle Activity During Shoulder Elevation Paula M. Ludewig 1,3 and T h o m a s M. Cook 2
The purposes of this study were to compare the effects of flexed head positions on scapular orientation and muscle activity during humeral elevation, and to determine any association of kinematic and electromyographic (EMG) responses. Twenty-five subjects, 19-37years old and without any recent history of shoulder or neck symptoms, were evaluated. Three-dimensional scapular coordinate data and surface EMG signals from the trapezius (upper and lower), levator scapulae, and serratus anterior were collected at static positions (0O, 90~ and 140 ~ of humeral elevation in the scapular plane and head positions (0O, 25 ~ and 50~ of sagittal plane flexion. Scapular upward rotation and tipping were significantly decreased in flexed head positions. The effect of head position on scapular tipping increased as humeral elevation increased. Mean EMG activity of the levator scapula, upper trapezius, and serratus anterior was unchanged across head positions. The lower trapezius demonstrated small but statistically significant increases in mean activity at the 0O and 140 ~ arm positions when the head was flexed to 50 ~ Significant correlations were found between some E M G and kinematic responses in flexed head positions, however, no consistent patterns were apparent across muscles or positions. KEY WORDS: head position; shoulder joint; scapula; electromyography.
INTRODUCTION Frequent use of the shoulder to perform overhead activities results in a high occurrence o'f general shoulder symptoms (1-3). Vague syndromes of pain in the shoulder region, including the parascapular musculature and posterior neck, are particularly common in industrial workers (3-6). In certain occupations, the prevalence of these symptoms can reach 30-60% or more, and combined shoulder and neck complaints are frequent (3, 4). Hypothesized risk factors include work at or IPhysical Therapy Graduate Program, The Universityof Iowa, Iowa City, Iowa. 2Department of Preventive Medicine and PhysicalTherapy Graduate Program, The Universityof Iowa, Iowa City, Iowa. 3Correspondence should be directed to Paula Ludewig, Physical Therapy Graduate Program, 2600 Steindler Building, The Universityof Iowa, Iowa City, Iowa 52242. 147 1053-0487/96/0900-0147509.50/0 9 1996 Plenum Publishing Corporation
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above shoulder level, flexed cervical postures, repetitive contractions of the shoulder muscles, and/or prolonged static contraction of the neck and shoulder muscles (4). When considering more specific diagnoses, a majority of epidemiologic studies identify rotator cuff tendonitis, specifically involving the supraspinatus tendon, as the primary shoulder disorder (2, 7, 8). Incidence and prevalence rates average 20% or more in occupations involving considerable upper extremity use (4, 7, 8). A decrease in the suprahumeral space during arm elevation, causing irritation of the soft tissue structures (rotator cuff tendon and musculature, subacromial bursae and biceps tendon) attempting to pass under the acromion or coracoacromial arch or ligament, has been termed impingement (9,10). Repetitive impingement frequently results in rotator cuff tendonitis and if not corrected can progress to a rotator cuff tear (11). The etiology of mechanical impingement includes structural abnormalities of the anterior acromion as well as functional instability of the glenohumeral or scapulothoracic joints (9, 11, 12). Lack of appropriate motion of either the glenohumeral or scapulothoracic component during elevation is believed to cause a functional, vs. a structural, decrease in the size of the suprahumeral space (9,13). Normal kinematics of humeral elevation requires scapular upward rotation and posterior tipping (14-16). Decreases in these rotations may contribute to a decrease in the suprahumeral space and subsequent impingement of rotator cuff structures. Many of the muscles responsible for shoulder motion are multijoint muscles having the potential to change their function at the shoulder with changes in position of the other joints they cross. Therefore, attachments of muscles between the cervical spine, head and scapula, particularly the upper trapezius and levator scapulae muscles, allow cervical and head positions to be possible contributors to alterations in scapular mechanics. If changes in scapular orientation occur secondary to changes in head position, the shoulder may be predisposed to potential impingement. Alternatively, adaptations in muscle activation of scapular musculature may occur with changes in head and cervical spine positions. Increased muscle activation of the scapulo-thoracic muscles would minimize scapular kinematic changes and decrease the potential for impingement. However, a strategy of increasing activation of the scapular rotators may not be ideal, as these muscles would become more prone to fatigue if required to attain or maintain higher activation levels repetitively or for prolonged periods (17). The importance of changes in head and cervical position on scapular mechanics and muscle activation is dependent upon the functional use of the arm in elevated positions while the head and cervical spine are flexed. Ergonomic studies have indicated the occurrence of flexed cervical postures combined with shoulder movements in certain work settings (18, 19). Specific occupations such as dentistry, "bench work," or assembly line work in particular industries involve repetitive or prolonged postural tasks in these positions (3, 4, 17). The effects of arm position and/or head and cervical position on muscle activity, muscle fatigue, and shoulder and neck symptoms are a continuing focus in ergonomics (3, 17, 19, 20). To date, however, no studies have assessed the specific effects of differing head and shoulder
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position combinations on scapular kinematics or EMG activity of the scapular rotators. The purposes of this study were (1) to compare the effects of three different head positions (neutral, partially flexed, and fully flexed) on scapular rotation angles and muscle activity in selected shoulder elevation positions, and (2) to determine if any correlations exist between changes (across head position) in kinematics and changes in EMG activity.
METHODS Subjects
Twenty-five subjects between the ages of 19 and 37 who had no recent history of cervical or shoulder pain, pathology, or range of motion (ROM) restriction were recruited for the study. Subjects included 13 women with a mean height of 1.66 (+/-.08) meters, a mean weight of 58.2 (+/-7.7) kg, and a mean age of 24.2 (+/-3.8) years, and 12 men with a mean height of 1.79 (+/-.08) meters, a mean weight of 75.9 (+/-6.3) kg, and a mean age of 27.8 (+/-6.1) years. Twenty-three of the 25 subjects were righthand dominant. All subjects received a verbal and written information summary describing the study and signed an institutionally approved consent form prior to participation. Instrumentation
A visual feedback system used in this study allowed the subjects to maintain the desired head and arm positions within and between trials. Gravity referenced pendulum potentiometers were attached to a headband and a cuff on the arm. Voltage outputs from the potentiometers were displayed on analog meters placed in front of the subject (Fig. 1A). Voltage outputs consistent with the desired head positions and arm elevation angles were marked on the meters for the subject to target. Three-dimensional coordinate locations for the scapula and trunk were collected using an electromechanical linkage digitizer. Customized data acquisition software and a 12 bit A/D board (Dash 16F, Metrabyte Corporation, Stoughton, MA) were used for online data collection to a microcomputer and for calculation of x, y, z Cartesian coordinates. The digitizer and related software were determined to have a linear accuracy of better than 3.8 mm and angular accuracy of better than 2~ (21). Preamplified silver-silver chloride surface electrode assemblies with an interelectrode distance of 20 mm and 8 mm diameter active electrodes (Therapeutics Unlimited, Iowa City, IA) were used to collect EMG activity simultaneously as voltages were collected from the digitizer. Signal amplification was accomplished with a GCS 67 amplifier with adjustable gain settings, high input impedance (greater than 15 megohms at 100 Hz), a common mode rejection ratio of 87dB at 60 Hz,
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B
C
VIIr, v
. . . . .
Fig. 1. Diagram of a subject positioned for data collection. (A) Neutral head and arm positions, 1) pendulum potentiometerattached to headband, 2) upper thoracicstabilization, 3) lumbar support and stabilization,4) anterior trunk stabilization,5) pendulum potentiometerattached to arm cuff, 6) meter system for visual feedbackof head and arm position; (B) 25* flexed head position, 90* arm position; (C) 50* flexed head position, 140"arm position. Note arm elevation is midway between the frontal and sagittal planes (inset).
a bandwidth (-3 dB) of 40--4000 Hz, and a maximum achievable signal-to-noise ratio (SNR) of 202,000:1. Based on the minimum amplitude of E M G signals obtained in this study, the effective SNR was always greater than 6500:1. A 55 msec time constant was used to root mean square (rms) process the raw signals. This rms processing provided a moving average of the E M G signal lowpass filtered at 2.9 Hz (22). E M G signals from each muscle were monitored on an oscilloscope (Tektronix 7313, Beaverton, OR) throughout data collection in order to verify signal quality. Procedures
Surface electrodes were used to collect data from the trapezius (upper and lower), levator scapulae, and serratus anterior muscles. The levator scapulae electrode was placed between the posterior margin of the sternocleidomastoid muscle and the anterior margin of the upper trapezius where this muscle is superficially located (19, 23). The upper trapezius electrode was placed one-third of the distance between C7 and the acromion process; the lower trapezius electrode one-half the distance between the inferior angle of the scapula and the thoracic spine; and the serratus anterior electrode over the muscle fibers just lateral to the inferior angle of the scapula (24). All electrodes were aligned parallel to muscle fiber direction as described anatomically. A ground electrode was placed on the distal ulna of the left wrist. Verification of signal quality and gain adjustments were completed for each muscle. The E M G activity during maximum voluntary isometric contractions (MVICs) was collected for the lower trapezius and serratus anterior in traditional manual muscle test positions (25). For the upper trapezius and levator scapulae muscles,
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MVICs were performed in two head positions, neutral, and 25-35 ~ of sagittal plane flexion. This allowed adjustment for length-induced EMG changes or changes in electrode position over the muscle when the head was flexed. The subject was seated and upper trapezius MVICs were collected with maximum resistance to arm abduction at a position of 90~ degrees of humeral abduction in the scapular plane (26). Levator MVICs were collected with maximum resistance to right lateral flexion of the head while the arm was actively held by the subject at 90~ of abduction in the scapular plane (26). Subjects were stabilized in sitting with the trunk aligned vertically to a trunk reference frame on the chair (Fig. 1A). Shoulder elevation in the plane of the scapula (27) was controlled by having the subject elevate the right arm along a flat planar surface angled 30~ anterior to the coronal plane. The subject actively obtained each head and arm position combination in a preselected random order and data was collected while subjects maintained each position. Three trials were completed in each position combination. The head positions tested included 0~ (neutral), 25 ~ and 50 ~ sagittal plane flexion. The arm positions included 0~ 90 ~ and 140~ of humeral elevation relative to the trunk (Fig. 1). Thirty seconds rest was allowed between each trial with the option of increased rest if subjects felt any symptoms of fatigue. However, no subjects elected any greater than the standard 30 second rest period between trials. Three points on the scapula, including the inferior angle, the medial inferior edge of the spine of the scapula and the posterolateral tip of the acromion, were palpated and digitized (Fig. 2). The three
AT
Fig. 2. The three points located and digitized on the scapula and subsequent axis orientation. X s, Ys, and Z s defined the anatomical coordinate system embedded in the scapula, with the positive X s axis directed medial to lateral along the spine of the scapula, the positive Ys axis directed anteriorly perpendicular to the plane of the scapula, and the positive Z s axis directed superiorly. UR/DR: Upward/Downward rotation; AT/PT: Anterior/Posterior Tipping; IR/ER: Internal/External Rotation. Orientation of the scapula was described relative to the cardinal planes of the trunk.
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points used to describe the trunk orientation were digitized from the trunk reference frame at the initiation of the session. Data collection was triggered with a footswitch and sampled at 300 Hz for 0.5 seconds at each point. The rms processed EMG signals were simultaneously sampled at 300 Hz (28). Data was collected on the right shoulder only for all subjects. Data Reduction and Analysis The three points digitized for each segment were used to establish local reference frames (29). The subsequent axis orientation for the scapula is illustrated in Fig. 2. The trunk reference frame was established coincident with the cardinal planes. The three-dimensional angular orientation of the scapula at each arm position was described relative to the trunk using a Z, Y', X" ordered Cardan angle rotation sequence (29, 30). Rotations about Z s defined scapular rotation relative to the coronal plane, thus describing the orientation of the scapular plane. This rotation was not of interest in this study. Rotations about Y~ defined upward/downward rotation of the scapula about an axis perpendicular to the plane of the scapula, and rotations about X s defined anterior/posterior tipping of the scapula about a mediallateral axis (Fig. 2). The amplitude of the EMG signal for each muscle was determined as the calculated average of the rms signal for each position combination and was normalized to the respective MVIC for each muscle and head position. For each dependent variable, the mean of the three trials at each position combination was input to Statistical Analysis System software (SAS Institute, Cary, NC). A two-factor repeated measures design was used to determine the effects of head position and arm position on scapular orientation and muscle activity. The dependent variables of interest were scapular upward rotation angle and tipping angle all relative to the trunk and normalized EMG values from the previously stated muscles. A significance level of .05 was used to test for statistical differences in the overall model. If a significant interaction of head and arm position was present, simple effects of head position at each level of arm position were tested. Tukey followup tests were performed where appropriate to adjust the significance level for multiple comparisons. Additionally, the difference between values for each dependent variable at the 25~ and 50~ head positions, as conipared to the neutral head position at the same arm position, was calculated for each subject and defined as the response for that head position. This was necessary because different subjects may have adopted differing strategies of response to flexed head positions. For example, some subjects might respond with a decrease in scapular upward rotation angle in flexed head positions, while others might maintain scapular position but demonstrate increases in EMG activity of the scapular upward rotator musculature. If these differing strategies of response existed, a group effect for any given variable would be diminished and the ANOVA model might not detect effects of differing head positions. However, those subjects demonstrating the smallest changes in kinematics across head positions should demonstrate the greatest changes in EMG activity across head positions and vice versa. These differing strategies of response could
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subsequently be detected in correlational analysis between E M G and kinematic variables, and therefore correlational analysis (Pearson r) was completed. Eight pairs of dependent variable responses (4 E M G variables x 2 kinematic variables) were assessed for an association at the six position combinations involving head flexion. This resulted in 48 (8 x 6) correlation values.
RESULTS The independent effect of arm position was not of primary interest in this study and it is reported elsewhere (15). The focus of the present study was on the effect of head position and the interactive effect of head position and arm position. The results of the analysis for the kinematic variables are presented in Fig. 3. The scapular upward rotation angle demonstrated no significant interaction effect of head position and arm position, however, a significant main effect was found for head position (p < .01). A small but progressive decrease in the mean values across head position (19.9 ~ at a neutral head position and 18.5 ~ at a 50 ~ head position) was demonstrated. Tukey followup tests resulted in statistically significant differences for all pairwise comparisons. A significant interaction effect of head position and arm position was present in the analysis of scapular tipping angle and, therefore, simple effects at each arm position were tested separately. No significant effect for head position was found at a 0 ~ humeral position, however, significant effects for head position were found at 90 ~ and 140~ humeral elevation angles (p < .01). Again, differences were small but progressively increased across head angles and humeral angles. At a 90 ~ arm position a 2.1 ~ mean difference was demonstrated between the neutral and 50 ~ flexed head positions, while a 3.5 ~ mean difference was noted between these same head positions at a 140~ arm position. Followup testing revealed significant differences for all pairwise comparisons across head positions at both the 90 ~ and 140~ humeral positions. The analyses of results for the EMG variables demonstrated no significant interaction effects and no significant effect of head position on the upper trapezius, levator scapulae, and serratus anterior. The lower trapezius analysis revealed a significant interaction of head position and arm position and subsequently simple effects were tested at each arm position. A significant effect of head position was found at the 0~ and 140~ arm positions, but not the 90~ arm position. Followup tests showed the 50 ~ head.position to be significantly different from the 0 ~ head position at both the 0 ~ and 140~ arm positions. The magnitudes of these differences, however, were quite small. Lower trapezius E M G increased only 0.5% MVIC at the 50~ head position as compared to the neutral head position when the arm was at 0 ~ elevation and 2.3% MVIC between these same head positions when the arm was at 140~ of elevation. Figure 4 summarizes the results of the lower trapezius analyses. Of the 48 correlational analyses, only four resulted in associations significantly different from zero (p < .05; Table I) and the significant associations were not consistent across muscles or positions. As multiple comparisons (six position combinations) were made for each variable pair, the possible increased risk of finding
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Fig. 3. Mean scapular orientation (and standard error) across head positions. (A) Upward rotation angle; (B) Tipping angle.
chance associations should be noted. Additionally, based on visual analyses of the plotted response variables, no nonlinear associations between kinematic and EMG responses were apparent.
DISCUSSION This study addresses a possible mechanism by which flexed head and cervical postures could contribute to shoulder impingement during occupational tasks. Although the statistically significant kinematic changes are small, they are consistent in direction and progressive in magnitude as the angle of head flexion is increased. Additionally, the changes observed are consistent with changes that would be hypothesized from an increase in length or change in line of action of the levator scapulae muscle across head positions. Forces produced by the levator act to oppose upward rotation of the scapula (31). If this muscle is stretched in flexed head positions, it may contribute to the subtle decreases in scapular upward rotation that were observed. In addition, the line of action of the levator scapulae would have an increased tendency to tip the scapula anteriorly as head flexion increases, assuming an axis of rotation at the AC joint (Fig. 5). Therefore, in flexed head positions, levator tension or tightness might interfere with scapular movement toward a posterior tipped orientation which normally occurs as the humerus elevates. The rehabilitation professional should consider the possible influence of this muscle when assessing and outlining a treatment plan for a worker with shoulder impingement symptoms who is frequently exposed to flexed head or cervical postures. Also persons with pain or spasm in the levator scapulae are likely to demonstrate increased magnitudes of response to flexed head positions. Further study is necessary to determine if such a trend exists.
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Fig. 4. Mean lower trapezius EMG activity (and standard error) across head and arm positions.
Table I. Significant Correlations of Kinematic and EMG Responses Head position/arm position 25o/90 ~ 25*/90 ~ 50o/0* 50~ ~
Variable pair Tipping Tipping Tipping Upward
and upper trapezius EMG and serratus anterior EMG and upper trapezius EMG rotation and serratus anterior EMG
Pearson coefficient
R2
.46 -.46 -.54 -.48
.21 .21 .29 .23
As previously indicated, functional decreases in the subacromial space due to changes in scapular orientation could contribute to impingement of the rotator cuff structures (9, 13). However, the clinical significance of the small magnitudes of kinematic changes detected in the present study is uncertain. The subacromial space is believed to be only 7-14 mm with the arm in neutral rotation at the side of the body (32), and may be less when the rotator cuff structures are passing under the acromion and coracoacromial ligament during humeral elevation. If an otherwise healthy worker assumes the most extreme position combination (50~ head flexion, 140~ arm elevation) repetitively or for prolonged periods, the magnitude of kinematic responses demonstrated in this study could predispose them to the development of shoulder impingement symptoms, especially when the decrease in upward rotation is combined with a decrease in posterior tipping. In the 25~ flexed head posture, the small magnitude of kinematic and EMG changes suggests substantial adaptability in young asymptomatic individuals. Subsequently, this head posture is unlikely to create risk to the shoulder in these subjects. This adaptability may not be present in persons with past histories of shoulder impingement or shoulder and
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A
B
! Fig. 5. Line of action of the levator scapulae muscle in the sagittal plane. (A) Neutral head position; (B) flexed head position.
neck complaints. Also, older workers, or workers assuming these postures during conditions of dynamic shoulder motions, muscle fatigue, or hand loads may experience greater effects. Ergonomic recommendations should consider possible influences of these additional factors. The progressive increases in activity of the lower trapezius seen with head flexion positions are consistent with a hypothesis of increased activity to minimize kinematic changes. However, the largest mean difference of only 2.3% MVIC is unlikely to be of any practical significance toward fatigue production. This lack of effect of head position on scapular muscle activity cannot be explained by differing strategies of response among individuals. Although significant correlations were present at some position combinations, the EMG responses were not consistently related to kinematic responses. Additionally, the strongest correlation explains only 29% of the variance in the EMG responses across subjects (Table I). Furthermore, the lack of a head position effect on the EMG variables is not due to a lack of statistical power. Greater than 80% power was available to detect differences as small as 3% of MVIC for the muscle activity comparisons made.
SUMMARY AND CONCLUSIONS In a group of subjects with no recent history of shoulder or neck complaints, flexed head postures contributed to a slight decrease in mean upward rotation and tipping angles of the scapula. This effect increased with increasing arm elevation for the scapular tipping variable. The magnitude of kinematic responses at the extreme position combination could predispose an otherwise healthy worker to develop shoulder impingement symptoms. Mean EMG activity of the scapular muscles studied was unchanged or increased less than 3% MVIC with flexed head positions.
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The lack of an effect of head position on muscle activity variables cannot be explained by differing strategies of response among individuals. Occupational-related shoulder pain and combined shoulder and neck complaints are prevalent. Prolonged or repetitive shoulder elevation and awkward cervical and head postures are believed to be contributory factors. Further research is needed to identify specific mechanisms of injury and enhance prevention and rehabilitation programs addressing these problems.
REFERENCES 1. Bjelle A, Hagberg M, Michaelson G. Occupational and individual factors in acute shoulderneck disorders among industrial workers. Br J lnd Med 1981; 38: 356-363. 2. Herberts P, Kadefors R. A study of painful shoulder in welders. Acta Orthop Scand 1976; 47: 381-387. 3. Luck JV, Andersson GBJ. Occupational shoulder disorders. In: Rockwood CA, Matsen FA, eds. The shoulder. Philadelphia: W.B. Saunders Company, 1990, pp. 1088-1108. 4. Hagberg M, Wegman DH. Prevalence rates and odds ratios of shoulderneck diseases in different occupational groups. Br J Ind Med 1987; 44: 602-610. 5. ViikariJuntura E. Neck and upper limb disorders among slaughterhouse workers: An epidemiologic and clinical study. Scand J Work Environ Health 1983; 9: 283-290. 6. Westerling D, Jonsson BG. Pain from the neckshoulder region and sick leave. Scand J Soc Med 1980; 8: 131-136. 7. Herberts P, Kadefors R, Andersson G, Petersen I. Shoulder pain in industry: An epidemiologic study on welders. Acta Orthop Scand 1981; 52: 299-306. 8. Herberts P, Kadefors R, Hogfors C, Sigholm G. Shoulder pain and heavy manual labor. Clin Orthop 1984; 191: 166-178. 9. Fu FH, Harner CD, Klein AH. Shoulder impingement syndrome: A critical review. Clin Orthop 1991; 269: 162-173. 10. Seeger LL, Gold RH, Bassett LW, Ellman H. Shoulder impingement syndrome: MR findings in 53 shoulders. Am J Roentgenol 1988; 150: 343-347. 11. Neer CS. Impingement lesions. Clin Orthop 1983; 173: 70-77. 12. Neer CS. Anterior acromioplasty for the chronic impingement syndrome in the shoulder. J Bone Joint Surg 1972; 54A: 41-50. 13. Kibler WB. Role of the scapula in the overhead throwing motion. Contemp Orthop 1991; 22: 525-532. 14. Laumann U. Kinesiology of the shoulder joint. In: Kolbel R, Helbig, Blauth, eds. Shoulder replacement. Berlin: SpringerVerlag, 1987, pp. 23-31. 15. Ludewig PM, Cook TM, Nawoczenski DA. Threedimensional scapular orientation and muscle activity at selected positions of humeral elevation. J Orthop Sports Phys Ther 1996 (in press). 16. Van Der Helm FCT, Pronk GM. Threedimensional recording and description of motions of the shoulder mechanism. J Biomech Eng 1995; 117: 27-40. 17. Chaffin DB. Localized muscle fatigue--Definition and measurement. J Occup Med 1973; 15: 346-354. 18. HarmsRingdahl K, Eckholm J, Schuldt K, Nemeth G, Arborelius UP. Load moments and myoelectric-activity when the cervical spine is held in full flexion and extension. Ergonomics 1986; 29: 1539-1552. 19. Schuldt K. On neck muscle activity and load reduction in sitting postures. An electromyographic and bioimechanical study with applications in ergonomics and rehabilitation. Scand J Rehab Med 1988; 19 (Suppl): 149. 20. Hagberg M. Electromyographic signs of shoulder muscle fatigue in two elevated arm positions. Am J Phys Med 1981; 60: 111-121. 21. Ludewig PM. The Effect of Head Position on Scapular Rotation and Muscle Activity During Shoulder Elevation. Iowa City, IA: University of Iowa Master's thesis, 1994. 22. Soderberg GL, ed. Selected Topics in Surface Electromyography for Use in the Occupational Setting: Expert Perspectives. U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health Publication No. 91100, 1992.
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23. Sundelin G, Hagberg M. The effects of different pause types on neck and shoulder EMG activity during VDU work. Ergonomics 1989; 32: 527-537. 24. Delagi EF, Iazetti J, Perotta A, Morrison D. Anatomic guide for the electromyographer (2nd Ed). Springfield: Charles C. Thomas, 1980. 25. Kendall HO, Kendall FP. Muscles testing and function. Baltimore: Williams & Wilkins, 1949. 26. Schuldt K, HarmsRingdahl IC Activity levels during isometric test contractions of neck and shoulder muscles. Scand J Rehab Med 1988; 20: 117-127. 27. Johnston TB. The movements of the shoulderjoint: A plea for the use of the "plane of the scapula" as the plane of reference for movements occurring at the humeroscapular joint. Br J SurE 1937; 25: 252-260. 28. Soderberg GL, Cook TM. Electromyography in biomechanics. Phys Ther 1984; 64: 1813-1820. 29. Wei SH, McQuade KJ, Smidt GL. Threedimensional joint range of motion measurements from skeletal coordinate data. J Orthop Sports Phys Ther 1993; 18: 687-691. 30. Craig JJ. Introduction to robotics: Mechanics and control (2nd Ed). New York: AddisonWesley Company, 1989, pp. 1967. 31. Brunnstrom S. Clinical kinesiology (2nd Ed). Philadelphia: F.A. Davis Company, 1966. 32. Weiner DS, Macnab I. Superior migration of the humeral head: A radiological aid in the diagnosis of tears of the rotator cuff. J Bone Joint Surg 1970; 52B: 524-527.
The Effect of Head Position on Scapular Orientation and Muscle Activity During Shoulder Elevation Paula M. Ludewig 1,3 and T h o m a s M. Cook 2
The purposes of this study were to compare the effects of flexed head positions on scapular orientation and muscle activity during humeral elevation, and to determine any association of kinematic and electromyographic (EMG) responses. Twenty-five subjects, 19-37years old and without any recent history of shoulder or neck symptoms, were evaluated. Three-dimensional scapular coordinate data and surface EMG signals from the trapezius (upper and lower), levator scapulae, and serratus anterior were collected at static positions (0O, 90~ and 140 ~ of humeral elevation in the scapular plane and head positions (0O, 25 ~ and 50~ of sagittal plane flexion. Scapular upward rotation and tipping were significantly decreased in flexed head positions. The effect of head position on scapular tipping increased as humeral elevation increased. Mean EMG activity of the levator scapula, upper trapezius, and serratus anterior was unchanged across head positions. The lower trapezius demonstrated small but statistically significant increases in mean activity at the 0O and 140 ~ arm positions when the head was flexed to 50 ~ Significant correlations were found between some E M G and kinematic responses in flexed head positions, however, no consistent patterns were apparent across muscles or positions. KEY WORDS: head position; shoulder joint; scapula; electromyography.
INTRODUCTION Frequent use of the shoulder to perform overhead activities results in a high occurrence o'f general shoulder symptoms (1-3). Vague syndromes of pain in the shoulder region, including the parascapular musculature and posterior neck, are particularly common in industrial workers (3-6). In certain occupations, the prevalence of these symptoms can reach 30-60% or more, and combined shoulder and neck complaints are frequent (3, 4). Hypothesized risk factors include work at or IPhysical Therapy Graduate Program, The Universityof Iowa, Iowa City, Iowa. 2Department of Preventive Medicine and PhysicalTherapy Graduate Program, The Universityof Iowa, Iowa City, Iowa. 3Correspondence should be directed to Paula Ludewig, Physical Therapy Graduate Program, 2600 Steindler Building, The Universityof Iowa, Iowa City, Iowa 52242. 147 1053-0487/96/0900-0147509.50/0 9 1996 Plenum Publishing Corporation
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above shoulder level, flexed cervical postures, repetitive contractions of the shoulder muscles, and/or prolonged static contraction of the neck and shoulder muscles (4). When considering more specific diagnoses, a majority of epidemiologic studies identify rotator cuff tendonitis, specifically involving the supraspinatus tendon, as the primary shoulder disorder (2, 7, 8). Incidence and prevalence rates average 20% or more in occupations involving considerable upper extremity use (4, 7, 8). A decrease in the suprahumeral space during arm elevation, causing irritation of the soft tissue structures (rotator cuff tendon and musculature, subacromial bursae and biceps tendon) attempting to pass under the acromion or coracoacromial arch or ligament, has been termed impingement (9,10). Repetitive impingement frequently results in rotator cuff tendonitis and if not corrected can progress to a rotator cuff tear (11). The etiology of mechanical impingement includes structural abnormalities of the anterior acromion as well as functional instability of the glenohumeral or scapulothoracic joints (9, 11, 12). Lack of appropriate motion of either the glenohumeral or scapulothoracic component during elevation is believed to cause a functional, vs. a structural, decrease in the size of the suprahumeral space (9,13). Normal kinematics of humeral elevation requires scapular upward rotation and posterior tipping (14-16). Decreases in these rotations may contribute to a decrease in the suprahumeral space and subsequent impingement of rotator cuff structures. Many of the muscles responsible for shoulder motion are multijoint muscles having the potential to change their function at the shoulder with changes in position of the other joints they cross. Therefore, attachments of muscles between the cervical spine, head and scapula, particularly the upper trapezius and levator scapulae muscles, allow cervical and head positions to be possible contributors to alterations in scapular mechanics. If changes in scapular orientation occur secondary to changes in head position, the shoulder may be predisposed to potential impingement. Alternatively, adaptations in muscle activation of scapular musculature may occur with changes in head and cervical spine positions. Increased muscle activation of the scapulo-thoracic muscles would minimize scapular kinematic changes and decrease the potential for impingement. However, a strategy of increasing activation of the scapular rotators may not be ideal, as these muscles would become more prone to fatigue if required to attain or maintain higher activation levels repetitively or for prolonged periods (17). The importance of changes in head and cervical position on scapular mechanics and muscle activation is dependent upon the functional use of the arm in elevated positions while the head and cervical spine are flexed. Ergonomic studies have indicated the occurrence of flexed cervical postures combined with shoulder movements in certain work settings (18, 19). Specific occupations such as dentistry, "bench work," or assembly line work in particular industries involve repetitive or prolonged postural tasks in these positions (3, 4, 17). The effects of arm position and/or head and cervical position on muscle activity, muscle fatigue, and shoulder and neck symptoms are a continuing focus in ergonomics (3, 17, 19, 20). To date, however, no studies have assessed the specific effects of differing head and shoulder
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position combinations on scapular kinematics or EMG activity of the scapular rotators. The purposes of this study were (1) to compare the effects of three different head positions (neutral, partially flexed, and fully flexed) on scapular rotation angles and muscle activity in selected shoulder elevation positions, and (2) to determine if any correlations exist between changes (across head position) in kinematics and changes in EMG activity.
METHODS Subjects
Twenty-five subjects between the ages of 19 and 37 who had no recent history of cervical or shoulder pain, pathology, or range of motion (ROM) restriction were recruited for the study. Subjects included 13 women with a mean height of 1.66 (+/-.08) meters, a mean weight of 58.2 (+/-7.7) kg, and a mean age of 24.2 (+/-3.8) years, and 12 men with a mean height of 1.79 (+/-.08) meters, a mean weight of 75.9 (+/-6.3) kg, and a mean age of 27.8 (+/-6.1) years. Twenty-three of the 25 subjects were righthand dominant. All subjects received a verbal and written information summary describing the study and signed an institutionally approved consent form prior to participation. Instrumentation
A visual feedback system used in this study allowed the subjects to maintain the desired head and arm positions within and between trials. Gravity referenced pendulum potentiometers were attached to a headband and a cuff on the arm. Voltage outputs from the potentiometers were displayed on analog meters placed in front of the subject (Fig. 1A). Voltage outputs consistent with the desired head positions and arm elevation angles were marked on the meters for the subject to target. Three-dimensional coordinate locations for the scapula and trunk were collected using an electromechanical linkage digitizer. Customized data acquisition software and a 12 bit A/D board (Dash 16F, Metrabyte Corporation, Stoughton, MA) were used for online data collection to a microcomputer and for calculation of x, y, z Cartesian coordinates. The digitizer and related software were determined to have a linear accuracy of better than 3.8 mm and angular accuracy of better than 2~ (21). Preamplified silver-silver chloride surface electrode assemblies with an interelectrode distance of 20 mm and 8 mm diameter active electrodes (Therapeutics Unlimited, Iowa City, IA) were used to collect EMG activity simultaneously as voltages were collected from the digitizer. Signal amplification was accomplished with a GCS 67 amplifier with adjustable gain settings, high input impedance (greater than 15 megohms at 100 Hz), a common mode rejection ratio of 87dB at 60 Hz,
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Fig. 1. Diagram of a subject positioned for data collection. (A) Neutral head and arm positions, 1) pendulum potentiometerattached to headband, 2) upper thoracicstabilization, 3) lumbar support and stabilization,4) anterior trunk stabilization,5) pendulum potentiometerattached to arm cuff, 6) meter system for visual feedbackof head and arm position; (B) 25* flexed head position, 90* arm position; (C) 50* flexed head position, 140"arm position. Note arm elevation is midway between the frontal and sagittal planes (inset).
a bandwidth (-3 dB) of 40--4000 Hz, and a maximum achievable signal-to-noise ratio (SNR) of 202,000:1. Based on the minimum amplitude of E M G signals obtained in this study, the effective SNR was always greater than 6500:1. A 55 msec time constant was used to root mean square (rms) process the raw signals. This rms processing provided a moving average of the E M G signal lowpass filtered at 2.9 Hz (22). E M G signals from each muscle were monitored on an oscilloscope (Tektronix 7313, Beaverton, OR) throughout data collection in order to verify signal quality. Procedures
Surface electrodes were used to collect data from the trapezius (upper and lower), levator scapulae, and serratus anterior muscles. The levator scapulae electrode was placed between the posterior margin of the sternocleidomastoid muscle and the anterior margin of the upper trapezius where this muscle is superficially located (19, 23). The upper trapezius electrode was placed one-third of the distance between C7 and the acromion process; the lower trapezius electrode one-half the distance between the inferior angle of the scapula and the thoracic spine; and the serratus anterior electrode over the muscle fibers just lateral to the inferior angle of the scapula (24). All electrodes were aligned parallel to muscle fiber direction as described anatomically. A ground electrode was placed on the distal ulna of the left wrist. Verification of signal quality and gain adjustments were completed for each muscle. The E M G activity during maximum voluntary isometric contractions (MVICs) was collected for the lower trapezius and serratus anterior in traditional manual muscle test positions (25). For the upper trapezius and levator scapulae muscles,
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MVICs were performed in two head positions, neutral, and 25-35 ~ of sagittal plane flexion. This allowed adjustment for length-induced EMG changes or changes in electrode position over the muscle when the head was flexed. The subject was seated and upper trapezius MVICs were collected with maximum resistance to arm abduction at a position of 90~ degrees of humeral abduction in the scapular plane (26). Levator MVICs were collected with maximum resistance to right lateral flexion of the head while the arm was actively held by the subject at 90~ of abduction in the scapular plane (26). Subjects were stabilized in sitting with the trunk aligned vertically to a trunk reference frame on the chair (Fig. 1A). Shoulder elevation in the plane of the scapula (27) was controlled by having the subject elevate the right arm along a flat planar surface angled 30~ anterior to the coronal plane. The subject actively obtained each head and arm position combination in a preselected random order and data was collected while subjects maintained each position. Three trials were completed in each position combination. The head positions tested included 0~ (neutral), 25 ~ and 50 ~ sagittal plane flexion. The arm positions included 0~ 90 ~ and 140~ of humeral elevation relative to the trunk (Fig. 1). Thirty seconds rest was allowed between each trial with the option of increased rest if subjects felt any symptoms of fatigue. However, no subjects elected any greater than the standard 30 second rest period between trials. Three points on the scapula, including the inferior angle, the medial inferior edge of the spine of the scapula and the posterolateral tip of the acromion, were palpated and digitized (Fig. 2). The three
AT
Fig. 2. The three points located and digitized on the scapula and subsequent axis orientation. X s, Ys, and Z s defined the anatomical coordinate system embedded in the scapula, with the positive X s axis directed medial to lateral along the spine of the scapula, the positive Ys axis directed anteriorly perpendicular to the plane of the scapula, and the positive Z s axis directed superiorly. UR/DR: Upward/Downward rotation; AT/PT: Anterior/Posterior Tipping; IR/ER: Internal/External Rotation. Orientation of the scapula was described relative to the cardinal planes of the trunk.
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points used to describe the trunk orientation were digitized from the trunk reference frame at the initiation of the session. Data collection was triggered with a footswitch and sampled at 300 Hz for 0.5 seconds at each point. The rms processed EMG signals were simultaneously sampled at 300 Hz (28). Data was collected on the right shoulder only for all subjects. Data Reduction and Analysis The three points digitized for each segment were used to establish local reference frames (29). The subsequent axis orientation for the scapula is illustrated in Fig. 2. The trunk reference frame was established coincident with the cardinal planes. The three-dimensional angular orientation of the scapula at each arm position was described relative to the trunk using a Z, Y', X" ordered Cardan angle rotation sequence (29, 30). Rotations about Z s defined scapular rotation relative to the coronal plane, thus describing the orientation of the scapular plane. This rotation was not of interest in this study. Rotations about Y~ defined upward/downward rotation of the scapula about an axis perpendicular to the plane of the scapula, and rotations about X s defined anterior/posterior tipping of the scapula about a mediallateral axis (Fig. 2). The amplitude of the EMG signal for each muscle was determined as the calculated average of the rms signal for each position combination and was normalized to the respective MVIC for each muscle and head position. For each dependent variable, the mean of the three trials at each position combination was input to Statistical Analysis System software (SAS Institute, Cary, NC). A two-factor repeated measures design was used to determine the effects of head position and arm position on scapular orientation and muscle activity. The dependent variables of interest were scapular upward rotation angle and tipping angle all relative to the trunk and normalized EMG values from the previously stated muscles. A significance level of .05 was used to test for statistical differences in the overall model. If a significant interaction of head and arm position was present, simple effects of head position at each level of arm position were tested. Tukey followup tests were performed where appropriate to adjust the significance level for multiple comparisons. Additionally, the difference between values for each dependent variable at the 25~ and 50~ head positions, as conipared to the neutral head position at the same arm position, was calculated for each subject and defined as the response for that head position. This was necessary because different subjects may have adopted differing strategies of response to flexed head positions. For example, some subjects might respond with a decrease in scapular upward rotation angle in flexed head positions, while others might maintain scapular position but demonstrate increases in EMG activity of the scapular upward rotator musculature. If these differing strategies of response existed, a group effect for any given variable would be diminished and the ANOVA model might not detect effects of differing head positions. However, those subjects demonstrating the smallest changes in kinematics across head positions should demonstrate the greatest changes in EMG activity across head positions and vice versa. These differing strategies of response could
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subsequently be detected in correlational analysis between E M G and kinematic variables, and therefore correlational analysis (Pearson r) was completed. Eight pairs of dependent variable responses (4 E M G variables x 2 kinematic variables) were assessed for an association at the six position combinations involving head flexion. This resulted in 48 (8 x 6) correlation values.
RESULTS The independent effect of arm position was not of primary interest in this study and it is reported elsewhere (15). The focus of the present study was on the effect of head position and the interactive effect of head position and arm position. The results of the analysis for the kinematic variables are presented in Fig. 3. The scapular upward rotation angle demonstrated no significant interaction effect of head position and arm position, however, a significant main effect was found for head position (p < .01). A small but progressive decrease in the mean values across head position (19.9 ~ at a neutral head position and 18.5 ~ at a 50 ~ head position) was demonstrated. Tukey followup tests resulted in statistically significant differences for all pairwise comparisons. A significant interaction effect of head position and arm position was present in the analysis of scapular tipping angle and, therefore, simple effects at each arm position were tested separately. No significant effect for head position was found at a 0 ~ humeral position, however, significant effects for head position were found at 90 ~ and 140~ humeral elevation angles (p < .01). Again, differences were small but progressively increased across head angles and humeral angles. At a 90 ~ arm position a 2.1 ~ mean difference was demonstrated between the neutral and 50 ~ flexed head positions, while a 3.5 ~ mean difference was noted between these same head positions at a 140~ arm position. Followup testing revealed significant differences for all pairwise comparisons across head positions at both the 90 ~ and 140~ humeral positions. The analyses of results for the EMG variables demonstrated no significant interaction effects and no significant effect of head position on the upper trapezius, levator scapulae, and serratus anterior. The lower trapezius analysis revealed a significant interaction of head position and arm position and subsequently simple effects were tested at each arm position. A significant effect of head position was found at the 0~ and 140~ arm positions, but not the 90~ arm position. Followup tests showed the 50 ~ head.position to be significantly different from the 0 ~ head position at both the 0 ~ and 140~ arm positions. The magnitudes of these differences, however, were quite small. Lower trapezius E M G increased only 0.5% MVIC at the 50~ head position as compared to the neutral head position when the arm was at 0 ~ elevation and 2.3% MVIC between these same head positions when the arm was at 140~ of elevation. Figure 4 summarizes the results of the lower trapezius analyses. Of the 48 correlational analyses, only four resulted in associations significantly different from zero (p < .05; Table I) and the significant associations were not consistent across muscles or positions. As multiple comparisons (six position combinations) were made for each variable pair, the possible increased risk of finding
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Fig. 3. Mean scapular orientation (and standard error) across head positions. (A) Upward rotation angle; (B) Tipping angle.
chance associations should be noted. Additionally, based on visual analyses of the plotted response variables, no nonlinear associations between kinematic and EMG responses were apparent.
DISCUSSION This study addresses a possible mechanism by which flexed head and cervical postures could contribute to shoulder impingement during occupational tasks. Although the statistically significant kinematic changes are small, they are consistent in direction and progressive in magnitude as the angle of head flexion is increased. Additionally, the changes observed are consistent with changes that would be hypothesized from an increase in length or change in line of action of the levator scapulae muscle across head positions. Forces produced by the levator act to oppose upward rotation of the scapula (31). If this muscle is stretched in flexed head positions, it may contribute to the subtle decreases in scapular upward rotation that were observed. In addition, the line of action of the levator scapulae would have an increased tendency to tip the scapula anteriorly as head flexion increases, assuming an axis of rotation at the AC joint (Fig. 5). Therefore, in flexed head positions, levator tension or tightness might interfere with scapular movement toward a posterior tipped orientation which normally occurs as the humerus elevates. The rehabilitation professional should consider the possible influence of this muscle when assessing and outlining a treatment plan for a worker with shoulder impingement symptoms who is frequently exposed to flexed head or cervical postures. Also persons with pain or spasm in the levator scapulae are likely to demonstrate increased magnitudes of response to flexed head positions. Further study is necessary to determine if such a trend exists.
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Fig. 4. Mean lower trapezius EMG activity (and standard error) across head and arm positions.
Table I. Significant Correlations of Kinematic and EMG Responses Head position/arm position 25o/90 ~ 25*/90 ~ 50o/0* 50~ ~
Variable pair Tipping Tipping Tipping Upward
and upper trapezius EMG and serratus anterior EMG and upper trapezius EMG rotation and serratus anterior EMG
Pearson coefficient
R2
.46 -.46 -.54 -.48
.21 .21 .29 .23
As previously indicated, functional decreases in the subacromial space due to changes in scapular orientation could contribute to impingement of the rotator cuff structures (9, 13). However, the clinical significance of the small magnitudes of kinematic changes detected in the present study is uncertain. The subacromial space is believed to be only 7-14 mm with the arm in neutral rotation at the side of the body (32), and may be less when the rotator cuff structures are passing under the acromion and coracoacromial ligament during humeral elevation. If an otherwise healthy worker assumes the most extreme position combination (50~ head flexion, 140~ arm elevation) repetitively or for prolonged periods, the magnitude of kinematic responses demonstrated in this study could predispose them to the development of shoulder impingement symptoms, especially when the decrease in upward rotation is combined with a decrease in posterior tipping. In the 25~ flexed head posture, the small magnitude of kinematic and EMG changes suggests substantial adaptability in young asymptomatic individuals. Subsequently, this head posture is unlikely to create risk to the shoulder in these subjects. This adaptability may not be present in persons with past histories of shoulder impingement or shoulder and
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A
B
! Fig. 5. Line of action of the levator scapulae muscle in the sagittal plane. (A) Neutral head position; (B) flexed head position.
neck complaints. Also, older workers, or workers assuming these postures during conditions of dynamic shoulder motions, muscle fatigue, or hand loads may experience greater effects. Ergonomic recommendations should consider possible influences of these additional factors. The progressive increases in activity of the lower trapezius seen with head flexion positions are consistent with a hypothesis of increased activity to minimize kinematic changes. However, the largest mean difference of only 2.3% MVIC is unlikely to be of any practical significance toward fatigue production. This lack of effect of head position on scapular muscle activity cannot be explained by differing strategies of response among individuals. Although significant correlations were present at some position combinations, the EMG responses were not consistently related to kinematic responses. Additionally, the strongest correlation explains only 29% of the variance in the EMG responses across subjects (Table I). Furthermore, the lack of a head position effect on the EMG variables is not due to a lack of statistical power. Greater than 80% power was available to detect differences as small as 3% of MVIC for the muscle activity comparisons made.
SUMMARY AND CONCLUSIONS In a group of subjects with no recent history of shoulder or neck complaints, flexed head postures contributed to a slight decrease in mean upward rotation and tipping angles of the scapula. This effect increased with increasing arm elevation for the scapular tipping variable. The magnitude of kinematic responses at the extreme position combination could predispose an otherwise healthy worker to develop shoulder impingement symptoms. Mean EMG activity of the scapular muscles studied was unchanged or increased less than 3% MVIC with flexed head positions.
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The lack of an effect of head position on muscle activity variables cannot be explained by differing strategies of response among individuals. Occupational-related shoulder pain and combined shoulder and neck complaints are prevalent. Prolonged or repetitive shoulder elevation and awkward cervical and head postures are believed to be contributory factors. Further research is needed to identify specific mechanisms of injury and enhance prevention and rehabilitation programs addressing these problems.
REFERENCES 1. Bjelle A, Hagberg M, Michaelson G. Occupational and individual factors in acute shoulderneck disorders among industrial workers. Br J lnd Med 1981; 38: 356-363. 2. Herberts P, Kadefors R. A study of painful shoulder in welders. Acta Orthop Scand 1976; 47: 381-387. 3. Luck JV, Andersson GBJ. Occupational shoulder disorders. In: Rockwood CA, Matsen FA, eds. The shoulder. Philadelphia: W.B. Saunders Company, 1990, pp. 1088-1108. 4. Hagberg M, Wegman DH. Prevalence rates and odds ratios of shoulderneck diseases in different occupational groups. Br J Ind Med 1987; 44: 602-610. 5. ViikariJuntura E. Neck and upper limb disorders among slaughterhouse workers: An epidemiologic and clinical study. Scand J Work Environ Health 1983; 9: 283-290. 6. Westerling D, Jonsson BG. Pain from the neckshoulder region and sick leave. Scand J Soc Med 1980; 8: 131-136. 7. Herberts P, Kadefors R, Andersson G, Petersen I. Shoulder pain in industry: An epidemiologic study on welders. Acta Orthop Scand 1981; 52: 299-306. 8. Herberts P, Kadefors R, Hogfors C, Sigholm G. Shoulder pain and heavy manual labor. Clin Orthop 1984; 191: 166-178. 9. Fu FH, Harner CD, Klein AH. Shoulder impingement syndrome: A critical review. Clin Orthop 1991; 269: 162-173. 10. Seeger LL, Gold RH, Bassett LW, Ellman H. Shoulder impingement syndrome: MR findings in 53 shoulders. Am J Roentgenol 1988; 150: 343-347. 11. Neer CS. Impingement lesions. Clin Orthop 1983; 173: 70-77. 12. Neer CS. Anterior acromioplasty for the chronic impingement syndrome in the shoulder. J Bone Joint Surg 1972; 54A: 41-50. 13. Kibler WB. Role of the scapula in the overhead throwing motion. Contemp Orthop 1991; 22: 525-532. 14. Laumann U. Kinesiology of the shoulder joint. In: Kolbel R, Helbig, Blauth, eds. Shoulder replacement. Berlin: SpringerVerlag, 1987, pp. 23-31. 15. Ludewig PM, Cook TM, Nawoczenski DA. Threedimensional scapular orientation and muscle activity at selected positions of humeral elevation. J Orthop Sports Phys Ther 1996 (in press). 16. Van Der Helm FCT, Pronk GM. Threedimensional recording and description of motions of the shoulder mechanism. J Biomech Eng 1995; 117: 27-40. 17. Chaffin DB. Localized muscle fatigue--Definition and measurement. J Occup Med 1973; 15: 346-354. 18. HarmsRingdahl K, Eckholm J, Schuldt K, Nemeth G, Arborelius UP. Load moments and myoelectric-activity when the cervical spine is held in full flexion and extension. Ergonomics 1986; 29: 1539-1552. 19. Schuldt K. On neck muscle activity and load reduction in sitting postures. An electromyographic and bioimechanical study with applications in ergonomics and rehabilitation. Scand J Rehab Med 1988; 19 (Suppl): 149. 20. Hagberg M. Electromyographic signs of shoulder muscle fatigue in two elevated arm positions. Am J Phys Med 1981; 60: 111-121. 21. Ludewig PM. The Effect of Head Position on Scapular Rotation and Muscle Activity During Shoulder Elevation. Iowa City, IA: University of Iowa Master's thesis, 1994. 22. Soderberg GL, ed. Selected Topics in Surface Electromyography for Use in the Occupational Setting: Expert Perspectives. U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health Publication No. 91100, 1992.
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23. Sundelin G, Hagberg M. The effects of different pause types on neck and shoulder EMG activity during VDU work. Ergonomics 1989; 32: 527-537. 24. Delagi EF, Iazetti J, Perotta A, Morrison D. Anatomic guide for the electromyographer (2nd Ed). Springfield: Charles C. Thomas, 1980. 25. Kendall HO, Kendall FP. Muscles testing and function. Baltimore: Williams & Wilkins, 1949. 26. Schuldt K, HarmsRingdahl IC Activity levels during isometric test contractions of neck and shoulder muscles. Scand J Rehab Med 1988; 20: 117-127. 27. Johnston TB. The movements of the shoulderjoint: A plea for the use of the "plane of the scapula" as the plane of reference for movements occurring at the humeroscapular joint. Br J SurE 1937; 25: 252-260. 28. Soderberg GL, Cook TM. Electromyography in biomechanics. Phys Ther 1984; 64: 1813-1820. 29. Wei SH, McQuade KJ, Smidt GL. Threedimensional joint range of motion measurements from skeletal coordinate data. J Orthop Sports Phys Ther 1993; 18: 687-691. 30. Craig JJ. Introduction to robotics: Mechanics and control (2nd Ed). New York: AddisonWesley Company, 1989, pp. 1967. 31. Brunnstrom S. Clinical kinesiology (2nd Ed). Philadelphia: F.A. Davis Company, 1966. 32. Weiner DS, Macnab I. Superior migration of the humeral head: A radiological aid in the diagnosis of tears of the rotator cuff. J Bone Joint Surg 1970; 52B: 524-527.
