Thoracic position effect on shoulder range of motion, strength, and three-dimensional scapular kinematics.

OBJECTIVES: To determine the effect of thoracic posture on scapular movement patterns, active range of motion (ROM) in scapular plane abduction, and isometric scapular plane abduction muscle force. STUDY DESIGN AND METHOD: Repeated measures design. There were 34 healthy subjects (mean age, 30.2 yrs). Each subject was positioned and stabilized while sitting in both erect and slouched trunk postures. In each sitting posture a three-dimensional electromechanical digitizer was used to measure thoracic flexion and scapular position and orientation in three planes. Measurements were taken with the arm (1) at the side, (2) abducted to horizontal in the scapular plane, and (3) at maximum scapular plane abduction. In each posture, isometric abduction muscle force was measured with the arm at the side and abducted to horizontal in the scapular plane. RESULTS: In the slouched posture, the scapula was significantly more elevated in the interval between 0 to 90 degrees abduction. In the interval between 90 degrees and maximum abduction, the slouched posture resulted in significantly less scapular posterior tilting. There was significantly less active shoulder abduction ROM in the slouched posture (mean difference = 23.6 degrees +/- 10.7 degrees). Muscle force was not different between slouched and erect postures with the arm at the side, but with the arm horizontal muscle force was decreased 16.2% in the slouched position. CONCLUSION: Thoracic spine position significantly affects scapular kinematics during scapular plane abduction, and the slouched posture is associated with decreased muscle force.     

Patterns of motion loss in subjects with idiopathic loss of shoulder range of motion

BACKGROUND: Subjects with idiopathic loss of shoulder motion have difficulty sleeping, performing overhead activities, and completing activities of daily living. Treatment has been variable in approach and success. Cyriax's proposed shoulder capsular pattern was external rotation most limited followed by abduction followed by internal rotation. This investigation was performed to determine if a consistent pattern of motion loss was present in subjects with idiopathic loss of shoulder motion. METHODS: Repeated measurement of 25 (22 female) subjects' bilateral shoulder patterns of motion were compared. The three-dimensional position of the scapula and humerus throughout active shoulder abduction, external rotation, flexion, internal rotation, and scapular plane abduction was evaluated with an electromagnetic motion capture system. Patterns of motion loss were determined for both shoulders. The proportions of peak shoulder motion to mean maximum non-involved shoulders abduction, external, and internal rotation were utilized to determine the pattern of motion loss. FINDINGS: There was a significant difference in motion loss patterns between the involved and non-involved shoulders with the arm at the side. The internal rotation less than abduction less than external rotation pattern was demonstrated in 14 of 25 (56%) involved shoulders. No pattern was present in 14 of 21 (67%) non-involved shoulders. With the arm abducted, internal rotation was the most limited motion in 23 of 25 (92%) involved shoulders. INTERPRETATION: The results did not support Cyriax's proposed glenohumeral capsular pattern. Defining the pattern of motion loss in subjects with idiopathic loss of shoulder motion may assist in diagnosis and treatment.     

Stretching and strengthening exercises: their effect on three-dimensional scapular kinematics.

OBJECTIVE: To quantitatively evaluate the effects of commonly used shoulder exercises on shoulder kinematics and resting posture. STUDY DESIGN: A repeated-measures design was used with measurements performed before and after a 6-week exercise program. METHOD: Twenty asymptomatic subjects with forward shoulder posture were recruited. Stretching exercises for the pectoral muscles and resisted strengthening exercises for the scapular retractors and elevators and the glenohumeral abductors and external rotators were performed three times per week for 6 weeks. A three-dimensional electromechanical digitizer was used to measure thoracic inclination and scapular orientation and position. These measurements were taken with the arm (1) at the side, (2) abducted to 90 degrees, and (3) at maximal abduction. The isometric force of glenohumeral external and internal rotation and horizontal abduction and adduction were measured with a hand-held dynamometer. All subjects were tested before and after the 6-week exercise program. Hotelling's T2 and paired t tests were used for data analysis. RESULTS: The strength of horizontal abduction and internal and external rotation increased after exercise (p < .01). The anterior inclination of the thoracic spine decreased, and the glenohumeral contribution to arm elevation increased (p < .01). Resting scapular posture did not change. As the arm was abducted to 90 degrees, the scapula showed less upward rotation and less superior translation after the exercise program (p < .01). CONCLUSION: The exercise program improved muscle strength, produced a more erect upper trunk posture, increased scapular stability, and altered scapulohumeral rhythm.     

Gender effect on the scapular 3D posture and kinematic in healthy subjects

Populations considered for shoulder analysis are often composed of various ratios of men and women. It is consequently hypothesized that gender has no significant effect on the joint kinematic. However, the literature reports, for the shoulder, differences in the range of motion between genders. The specific influence of gender on the scapulo‐thoracic kinematics has not been studied yet. The dominant shoulder of two populations of men and women composed of 11 subjects each were evaluated in three dimensions for three distinct motions: flexion in the sagittal plane, abduction in the frontal plane and gleno‐humeral internal/external rotation with the arm abducted at 90°. Posture, kinematics and range of motion were studied separately. For flexion and abduction and with regard to the scapular kinematic, external rotation was significantly larger for women than men. The differences were of at least 5° at 120° of humeral elevation. Upward rotations were identical. Women also showed larger average active humero‐thoracic range of motion. The mean differences were of 13°, 7°, 12° and 5° for abduction, flexion, internal rotation and external rotation, respectively. No difference was observed between the scapular resting positions of both populations. The observed differences concerning both the scapular and humeral patterns would indicate that the shoulder behaviour of men and women should not be expected to be similar.     

Scapular behavior in shoulder impingement syndrome.

OBJECTIVE: To quantify the contribution of each scapular rotation to the scapular total range of motion (ROM) in both shoulders of persons with a unilateral shoulder impingement syndrome (SIS), to compare 3-dimensional (3D) scapular attitudes of their symptomatic and asymptomatic shoulders in flexion and in abduction, and to characterize the scapular behavior of these subjects by classifying them into subgroups based on scapular tilting differences between their symptomatic and asymptomatic shoulders. DESIGN: Comparisons of 3D scapular attitudes, scapular total ROM, and percentage of contributions of each scapular rotation to the scapular total ROM. SETTING: A motricity laboratory. PARTICIPANTS: Fifty-one subjects, including 41 with a SIS (29 had an asymptomatic contralateral shoulder) and 10 healthy subjects. INTERVENTIONS: The 3D scapular attitudes were calculated with the subjects in a standardized seated position; with the arm at rest; or at 70 degrees, 90 degrees, and 110 degrees of shoulder flexion and abduction. Axial rotation angles were calculated using a fixed set of Cardanic angles. MAIN OUTCOME MEASURES: At 90 degrees of arm elevation, data from 10 shoulders of healthy subjects were used to set up normative values (99% confidence interval of mean 3D scapular attitudes) to compare with 3D scapular attitudes of symptomatic and asymptomatic shoulders of SIS subjects. We analyzed the scapula behavior of subjects with SIS and classified them into subgroups based on scapular anterior tilting asymmetry. RESULTS: In flexion, almost half of the scapular total ROM was provided by anterior tilting (48.2%-51.3%), whereas in abduction, external rotation (40.3%-42.4%) was the main contributor. Scapular total ROM was higher in abduction than in flexion in all arm positions for both shoulder groups (P <.01). Also, 3D scapular attitude patterns of both shoulders of SIS subjects were different from those of healthy subjects. At 90 degrees, scapular asymmetry in anterior tilting allowed us to classify SIS subjects with respect to more (lead) or less (lag) scapular tilting in the affected side (P <.0001) or no difference (P =.11) between the sides (symmetrical). No significant differences (P >.05), except for a small 2 degrees difference in transverse rotation during arm flexion at 110 degrees (P =.002), were observed in 3D scapular attitudes and scapular total ROM between both shoulders of SIS subjects. Patterns of 3D scapular attitudes and scapular total ROM were significantly different between flexion and abduction arm positions (P <.05). CONCLUSIONS: The contribution of rotations and scapular total ROM differed according to the plane of arm elevation in SIS subjects. Group analyses revealed no differences in 3D scapular attitudes between symptomatic and asymptomatic shoulders of subjects with unilateral SIS. This could be caused by the use, in SIS subjects, of inappropriate neuromuscular strategies affecting both shoulders. However, individual analyses revealed scapular asymmetry in the sagittal plane, which suggests that SIS subjects with less anterior tilting in the symptomatic shoulder, as compared with the asymptomatic contralateral one, may be at high risk of developing chronic SIS. This last finding provides scientific evidence to focus rehabilitation protocols toward a restoration of anterior tilting.     

The measurement of three dimensional scapulohumeral kinematics--a study of reliability

OBJECTIVE: To validate a system, using electromagnetic movement sensors, for the simultaneous measurement of scapular and humeral positions. DESIGN: The study was designed to analyse the repeatability and to quantify the inherent measurement errors, using two observers to measure the movements of five normal subjects. BACKGROUND: Following an earlier study of a system to measure three dimensional scapula motion, the method has been developed to incorporate simultaneous measurement of humeral and scapular position using a new design of scapula locator and a two-channel measurement system. METHODS: The system was used to measure the scapula movements accompanying abduction of the shoulder. The scapula locator was applied at 10 degrees intervals of abduction. This was measured initially using a fluid filled goniometer, but, in a second part of the study, using an additional motion sensor. RESULTS: In the single channel study, the 95% confidence intervals were less than 4 degrees and 10 mm. When simultaneous measurement of arm position was performed the confidence intervals were little changed. CONCLUSIONS: A new system for the measurement of scapulohumeral relationships has been shown to have good inter-and intraobserver reliability. This study opens the way for a full investigation of the scapula motion accompanying three dimensional shoulder motion in both normal subjects and in patients having shoulder pathology.     

Kinematics of the contralateral and ipsilateral shoulder: a possible relationship with post-stroke shoulder pain.

OBJECTIVE: Post-stroke shoulder pain is a common phenomenon in hemiplegia and impedes rehabilitation. The aim of this study was to identify a possible relationship between post-stroke shoulder pain, scapula resting position and shoulder motion. METHODS: Shoulder kinematics of 27 patients after stroke (17 men) were compared with 10 healthy age-matched control subjects. Using an electromagnetic tracking device, the kinematics of both the contralateral and ipsilateral (i.e. paretic and non-paretic) arm during active and passive abduction and forward flexion were measured and expressed in Euler angles. RESULTS: Scapular lateral rotation relative to the thorax was increased in patients with post-stroke shoulder pain compared with both patients without post-stroke shoulder pain and control subjects at rest as well as during arm abduction and forward flexion. Additionally, glenohumeral elevation was decreased in patients with post-stroke shoulder pain during passive abduction. No differences were found regarding scapula position (displacement relative to the thorax). CONCLUSION: In patients with post-stroke shoulder pain a particular kinematical shoulder pattern was established, characterized by enhanced scapular lateral rotation and diminished glenohumeral mobility.     

Three-dimensional shoulder kinematics during a pressure relief technique and wheelchair transfer

OBJECTIVE: To assess 3-dimensional scapulothoracic and glenohumeral kinematics in able-bodied subjects during a weight-relief raise and while transferring to and from a wheelchair. DESIGN: Repeated-measures analysis of variance. SETTING: Research laboratory. PARTICIPANTS: Twenty-five able-bodied subjects without spinal cord injury or shoulder symptoms (20 men, 5 women; age range, 20-37y). INTERVENTIONS: Completion of weight-relief raise and transfer tasks. Main Outcome Measures: An electromagnetic motion capture system tracked 3-dimensional position and orientation of the thorax, scapula, and humerus. Absolute angular values assessed included scapular downward and upward rotation, internal and external rotation, and posterior and anterior tipping all relative to the thorax, as well as humeral internal and external rotation relative to the scapula. Data were compared across 3 phases of the weight-relief raise and for transfer direction (leading arm, trailing arm). RESULTS: Key findings included significantly increased anterior tipping and internal rotation of the scapula and decreased scapular upward rotation and external rotation of the humerus during the weight-relief raise. The leading arm showed significantly greater scapular anterior tipping and internal rotation and less scapular upward rotation and humeral external rotation than the trailing arm during the final phase of the transfer. CONCLUSIONS: Both the weight-relief raise and transfer result in scapular and humeral positions and directions of motion that may negatively impact the available subacromial space. This may present increased risk for injury or progression of shoulder pain in persons who must routinely perform these tasks.     

Scapular kinematics during humeral elevation in adults and children

BACKGROUND: Appropriate motion of the scapula is important for dynamic positioning of the glenoid during humeral elevation. A number of studies have described the typical scapular kinematics during humeral elevation in adults. However, children and adults may have differences in scapulothoracic musculature and scapular osteology. To our knowledge, no study has been performed examining scapular kinematics in children with either typical or atypical development. Consequently in children the influence of age and development on scapular motion is currently unknown. The aim of this study was to describe and compare the kinematic patterns of the scapula during humeral elevation in children with typical development and healthy adults. METHODS: Fifteen adults, 7 females, 25-37 years of age, and 14 children, 8 females, 4-9 years of age, participated in this study. Kinematic data were collected using a magnetic tracking device. Subjects were asked to elevate their arm in the scapular plane (40 degrees anterior to the frontal plane) in a sequence of three trials. FINDINGS: Significant differences were seen between the two age groups in the dependent variables. During scapular plane rotation from 25 degrees to 125 degrees , children showed greater upward rotation (43.9 degrees SD 6.39 degrees ) than adults (29.1 degrees SD 10.1 degrees ). The mean glenohumeral to scapulothoracic ratio in the scapular plane was 2.4:1 for adults, 1.3:1 for children. INTERPRETATION: This study demonstrates that there are significant differences in scapular kinematic patterns between children and adults. Children have a greater contribution from the scapulothoracic joint, specifically upward rotation toward humeral elevation. From a clinical perspective, these results can be used to help determine the incorporation of stabilization and mobilization of the scapulothoracic joint during exercises for a child with impairment at the shoulder for improving shoulder function.     

Velocity effects on the scapulo-humeral rhythm

Objective. The objective of this study is to verify the assumption that the three-dimensional (3-D) shoulder motions can be described by means of an interpolation of statically recorded postures and thus, support the application of non-invasive but static techniques for motion analysis of the shoulder.

Background. During shoulder motions the scapula moves underneath the skin. Recording of motions is only possible by means of invasive methods. An alternative for the recording is palpation of skeletal landmarks on the scapula and subsequent digitization. The method is non-invasive and relatively easy, but static. Motions are modelled by means of interpolation of the subsequent position recordings. Validity of this method, however, has never been demonstrated.

Methods. Seven subjects performed an alternating abduction-adduction motion of the arm in a plane 30 ° forward rotated with respect to the frontal plane, at three sub-maximal frequencies: 0.04, 0.25 and 0.50 Hz. The humeral and scapular motions were recorded by means of a two-dimensional (2-D) X-ray video system. The motions of the humerus, the scapular spine and the glenoid ridge were defined by angles, and the sinusoidal motion curves were characterized by means of the offset, the amplitude and the phase of the motions.

Results. By means of Repeated Measurements Multi-Variate Analysis of Variance, a significant effect of arm motion on the phase and the amplitude of the scapular motion was found. However, the magnitude of the effects are negligibly small for the present applications at sub-maximal arm motion velocities.

Conclusions. For normal arm motions in the vertical plane, the kinematics of the shoulder skeleton can be derived by the interpolation of statically recorded positions of the bones.     

Effect of sitting posture on 3-dimensional scapular kinematics measured by skin-mounted electromagnetic tracking sensors

OBJECTIVE: To determine the effect of trunk sitting posture on scapular kinematics during humeral elevation by using skin-mounted electromagnetic tracking sensors. DESIGN: Repeated-measures design contrasting scapular kinematics in 2 different sitting postures. SETTING: A biomechanics laboratory in Hong Kong with a real-time, 3-dimensional electromagnetic tracking device for measuring movements of the scapula. PARTICIPANTS: A sample of 16 healthy adults (12 women, 4 men; age, 21.6+/-3.92y) with full, pain-free shoulder range of motion and no history of shoulder pathology. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Movements of the scapula were measured while each subject performed humeral elevation in an upright seated position and a slouched seated position. RESULTS: In both postures, posterior tip, lateral and upward rotation of the scapula, and lateral rotation of the humerus were observed during humeral elevation. When the slouched posture was adopted, there were significant decreases in the posterior tip and lateral rotation of the scapula, but there was no significant change in the magnitude of the upward rotation of the scapula. CONCLUSION: Increased thoracic kyphosis significantly alters the kinematics of the scapula during humeral elevation.     

Glenohumeral subluxation, scapula resting position, and scapula rotation after stroke: a noninvasive evaluation.

OBJECTIVES: To determine whether scapular downward tilt (ScDT) and dynamic scapular lateral rotation (ScLR) in subjects with and without stroke is associated with subluxation, and to prove the reliability of a Scapula Locator System in an elderly population. DESIGN: Repeated measures of ScLR by 2 observers. SETTING: Outpatient rehabilitation department of a district general hospital. PARTICIPANTS: To test device reliability, 5 healthy men (mean age +/- standard deviation, 72 +/- 5 yr). To test scapula position, 30 stroke patients (19 men, 11 women; mean age, 73 +/- 6 yr) and 15 healthy controls (12 men, 3 women; mean age, 62 +/- 6 yr). INTERVENTIONS: The control subjects' ScDT was compared with stroke subjects' ScDT after stratification according to 3 patterns of ScLR symmetry and the presence of palpable glenohumeral subluxation. MAIN OUTCOME MEASURES: For device reliability, 3-way analysis of variance. For scapula position, triangulated location by Scapula Locator System of acromion, inferior angle, and root of the scapular spine; then measurement of scapula motion to determine symmetry, lag, or lead. RESULTS: The inter- and intraobserver reliability of the Scapula Locator System device was high (1% of variance each). Normal ScDT was positive (left side: 10.94 degrees +/- 2.62 degrees; right side: 9.69 degrees +/- 4.36 degrees ), indicating a downward-facing glenoid fossa. This finding was unchanged by stroke (10.46 degrees +/- 2.42 degrees ). All controls and 16 stroke subjects had symmetry between shoulders for ScLR rate and ScDT. Two other patterns (p <.01) of ScLR were found after stroke: 8 subjects had a slower rate of affected arm ScLR (lag) with a correspondingly greater ScDT on the affected side (2.61 degrees +/- 6.7 degrees ); 6 subjects had a faster rate of affected arm ScLR (lead) but with an upward-facing glenoid fossa on the affected side (ScDT: -11.84 degrees +/- 8.48 degrees ). No significant inter- or intrasubject difference in ScDT existed in the 6 cases of glenohumeral subluxation. CONCLUSIONS: The scapula normally tilts downward with or without stroke. The effect of stroke is similar on tonic (ScDT) and phasic (ScLR) control of scapula position. Subluxation is not linked with a particular scapular resting position after stroke.     

Characteristics of the scapula movement during shoulder elevation depend on posture

[Purpose] This study aimed to clarify the differences in scapular movement during flexion and abduction of the shoulder joint with different postures. [Participants and Methods] This study included 15 male participants. Their shoulder flexion and abduction and angles of the scapular upward rotation, scapular anterior tilt, scapular external rotation, and thoracic spine flexion were measured. Measurements were taken in three positions: the control, thoracic spine flexion, and thoracic spine extension positions using a three-dimensional motion capture system. [Results] In the shoulder flexion, the amount of change in the scapular external rotation was significantly greater in the thoracic flexion than in the thoracic extension. In shoulder abduction, the amount of change in the scapular anterior tilt and external rotation was significantly greater in the thoracic flexion than in the thoracic extension. A comparison of the scapular angles in shoulder flexion and abduction showed that the upward rotation, posterior tilt, and external rotation were significantly greater in abduction than flexion. [Conclusion] To avoid posture-induced incoordination of the scapula and thorax movement during shoulder elevation, postural adjustment of the thoracic spine based on the movements is necessary for the shoulder joint exercises.     

Comparison of scapular kinematics between elevation and lowering of the arm in the scapular plane

Objective. To compare scapular orientation during both the concentric (elevation) and eccentric (lowering) phases of scapular plane abduction in subjects with and without shoulder impingement.

Design. Mixed model analysis of variance with one between-subjects factor (group) and within-subjects factors of phase, humeral angle, and trial.

Background. Abnormal scapular kinematics have been identified in shoulder impingement patients during the concentric phase of arm elevation, and under static conditions. Because abnormal scapular motion is observed clinically during the eccentric phase of arm elevation, analysis of this phase of motion is warranted.

Methods. Twenty-six symptomatic and 26 healthy subjects performed five repetitions of humeral scapular plane abduction. An electromagnetic tracking device described three-dimensional scapular kinematics during arm elevation and lowering. Angular values for scapular anterior/posterior tipping in the sagittal plane, upward/downward rotation in the scapular plane, and internal/external rotation in the transverse plane were calculated. Scapular orientation relative to the thorax at humeral angles of 40°, 60°, 80°, 100°, and 120° was statistically tested for effects of phase and trial, or for interactions of phase with group or humeral angle.

Results. Internal rotation was significantly increased in the eccentric phase for both groups at the 100° angle (P<0.05) and for the symptomatic group only at the 120° angle (P<0.05). Scapular anterior tipping was significantly decreased during the eccentric phase in both groups at the 80° (P<0.001), 100° (P<0.0001), and 120° (P<0.0001) angles.

Conclusions. Small but statistically significant differences in scapular tipping and internal rotation during the eccentric phase of arm elevation were identified at higher humeral angles in both subject groups, while no significant phase differences for scapular upward rotation or for scapular variables at lower humeral angles were found. Averaged across phases, the symptomatic group demonstrated significant reductions in upward rotation at lower humeral elevation angles, and significant increases in anterior tipping at higher elevation angles as compared to the healthy group.

Relevance Normal and abnormal scapular kinematics during varying types of motion need to be understood in order to optimally design rehabilitation programs for individuals with impingement syndrome.     

Normative values of agonist-antagonist shoulder strength ratios of adults aged 20 to 78 years.

OBJECTIVE: To determine normative values for isometric flexion/extension, abduction/adduction, and external/internal rotation strength ratios about the shoulder and to determine if these ratios are affected by age or gender. STUDY DESIGN: A cross-sectional study of 120 healthy volunteers (60 men, 60 women) aged 20 to 78 years. SETTING: Orthopedic research laboratory. METHODS: Flexion and extension strengths were measured isometrically using a Cybex II dynamometer at arm flexion angles of 30 degrees, 60 degrees, and 90 degrees. Abduction and adduction strengths were measured at 30 degrees, 60 degrees, and 90 degrees abduction. Internal and external rotation strengths were measured (1) with the arm abducted 15 degrees and neutral external/internal rotation and (2) with the arm abducted 90 degrees and externally rotated 30 degrees above the transverse plane. OUTCOME MEASURES: Isometric strength ratios for flexion/extension, abduction/adduction, and external/internal rotation. RESULTS: No statistically significant differences in agonist/antagonist strength ratios were found between dominant and nondominant sides or between genders. Age was associated with changes in strength ratios for measurements taken with the arm flexed or abducted 90 degrees. Posture was found to affect strength ratios. CONCLUSIONS: These data can serve as a normative reference for clinical use.     

The scapulo-humeral rhythm: effects of 2-D roentgen projection

OBJECTIVE: The objective of this study is to illustrate the low accuracy of two-dimensional (2-D) X-ray projection methods for the quantification of the three-dimensional (3-D) shoulder motions. BACKGROUND: The traditional method for the quantification of the gleno-humeral motion is by means of 2-D X-ray recording. The motion was characterized by the scapulo-humeral rhythm: the ratio of the nett humeral elevation over nett scapular rotation. The method was based on the quantification of the planar projection of the spatial positions of X-ray dense structures of the scapula. The deformations introduced by the central projection method, a feature of X-ray projection, cannot be compensated for by calibration: the position of the scapula with respect to the camera setting is unknown, and skeletal landmarks of the scapula cannot uniquely be identified. The transformation from 3-D orientations to 2-D angles will, therefore, be inaccurate. METHODS: A 2-D X-ray projection of the scapula during a typical arm abduction was simulated. The 3-D motion was obtained by means of palpation and subsequent digitization of skeletal landmarks of the scapula. The 3-D positions of the recorded landmarks were projected on a plane by a simulation based on the parameters of the X-ray equipment. The scapulo-humeral rhythm was calculated for the different scapular landmarks, and for the orientation of the subject with respect to the projection axis. The results were compared with previous published scapulo-humeral rhythms. RESULTS: The scapulo-humeral rhythm depends both on the choice of the skeletal landmarks, used to quantify the scapular rotations, and on the orientation of the subject in the X-ray setting. The full range of results obtained from earlier published experiments could be obtained from a simulation based on a single 3-D arm abduction. CONCLUSIONS: The 2-D scapulo-humeral rhythm, obtained from planar X-ray projection, is an inaccurate parameter to define the scapular motions. RELEVANCE: The 2-D scapulo-humeral rhythm is an insensitive parameter to identify clinical disorders in the gleno-humeral motions, 3-D motion recording should be applied. Only when stringent precautions are taken with respect to repeatability of positioning of the subjects, can the method be used to study intra-individual effects, e.g., the follow-up of patients during treatment.     

Effects of local muscle fatigue on three-dimensional scapulohumeral rhythm

The relationship of scapulothoracic motion to glenohumeral motion, commonly referred to as the scapulohumeral rhythm, has been the subject of numerous investigations. The purpose of this study was to assess the effects of localized muscular fatigue on three-dimensional scapulothoracic motion and the resulting scapulohumeral rhythm during elevation of the humerus in the plane of the scapula. A six-degree-of-freedom digitizing system (Metrecom(TM)) was used to define scapular and trunk reference frames, and three-dimensional Eulerian angles were determined for scapular motion for 0-135 degrees of humeral elevation before and after fatigue of the upper and lower trapezius muscles. Local muscle fatigue was determined using spectral analysis of electromyographic signals for the upper and lower trapezius muscles. The results demonstrated an average of 22% decrease in the median frequencies of the muscles sampled following resistive exercise, suggesting a state of local muscle fatigue. Concomitant with the fatigue was a selective decrease in scapulothoracic motion about two of the three scapular axes. RELEVANCE: Occupational and recreational conditions often require repetitive overhead elevations of the arms, resulting in muscular fatigue and various pathologies. During repetitive arm elevations the scapulohumeral rhythm or synchronization between the humerus and the scapula is balanced to allow the most efficient elevation of the arm. A better understanding of this much-discussed scapulohumeral rhythm and the changes induced by muscular fatigue may provide insight into the pathomechanics associated with shoulder dysfunction.