A comprehensive assessment of shoulder motion

Richards, James G.INTRODUCTION -- Despite the prolonged existence of EMG, current literature still lacks a comprehensive set of muscle activations of commonplace shoulder motions. A complete and detailed understanding of shoulder muscle activation strategies is crucial for clinicians and therapists when making treatment decisions for either typically developed individuals with shoulder injuries or for patients with shoulder dysfunction as a result of conditions such as cerebral palsy or brachial plexus palsy. This descriptive laboratory study is the first of its kind to utilize synchronized EMG and motion capture data to evaluate the timing and amplitude of twenty muscular segments influencing scapulothoracic (ST) and glenohumeral (GH) motion in a group of typically developed individuals performing five ordinary shoulder motions. The data set was evaluated for conserved muscle activation patterns among subjects, synergistic muscular relationships and made available for validation of musculoskeletal shoulder models. This first of five articles discusses the results of the abduction trial. ☐ MATERIALS & Methods -- Five typically developed adult subjects underwent simultaneous EMG and motion capture recording of twenty muscular segments of the shoulder during the five modified-Mallet motions of the shoulder (abduction in the coronal plane, full external rotation at 0° abduction, hand-to-mouth, hand-to-nape of neck, hand-to-base of spine). Dynamic EMG signals were collected for three motion trials, processed, then averaged and normalized to dynamic maximal voluntary contraction trials for each muscle in order to make comparisons between subjects. Normalized EMG signals were visually analyzed in terms of their activation patterns, timing of pattern changes and magnitude of activation. This analysis combined with the resultant observed effects on humerothoracic (HT), GH and ST angular displacement for each motion trial allowed for categorization of muscle segments based on their functional role throughout the motion. Based on muscle activation patterns and their corresponding functional roles in a given motion, strategies of neuromuscular control and synergistic relationships emerged providing a description of how a motion is accomplished. ☐ RESULTS -- The muscle activation patterns that were well conserved across all subjects during abduction include upper trapezius, lower trapezius, levator scapulae, supraspinatus, rhomboid major, anterior deltoid and posterior deltoid. Muscles that appeared to be highly individualized during abduction consisted of middle trapezius, infraspinatus, rhomboid minor, subscapularis, serratus anterior, coracobrachialis and pectoralis minor. Muscles that appeared to play no significant role in abduction were teres major, teres minor, pectoralis major, latissimus dorsi, biceps brachii and triceps brachii. ☐ The muscle activation patterns that were well conserved across all subjects during external rotation include middle trapezius, lower trapezius, levator scapulae, infraspinatus, rhomboid minor, rhomboid major, teres minor, and subscapularis. Muscles that appeared to be highly individualized during external rotation consisted of upper trapezius, supraspinatus, teres major, serratus anterior, coracobrachialis, and pectoralis minor. Muscles that appeared to play no significant role in external rotation were pectoralis major, anterior deltoid, posterior deltoid, latissimus dorsi, biceps brachii and triceps brachii. ☐ CONCLUSIONS -- Based on the consistent waveform patterns observed in conjunction with the motion capture data, muscles were able to be grouped based on their main function during abduction. Humeral primary movers consisted of supraspinatus and anterior deltoid. Scapular primary movers included upper, middle and lower trapezius and levator scapulae. Infraspinatus, teres minor and subscapularis served as the primary GH stabilizers. Rhomboid major, rhomboid minor and pectoralis minor functioned as the primary scapular stabilizers. There were also several muscles that demonstrated activity congruent with the direction of body segment motion that appeared to play a stabilizing role but may have also contributed to motion. The humeral mixed movers/stabilizers were coracobrachialis and posterior deltoid. The sole scapular mixed mover/stabilizer was serratus anterior. The muscles that were essentially inactive during abduction are teres major, pectoralis major, latissimus dorsi, biceps brachii and triceps brachii. Synergistic relationships were also observed between the following muscle groups; supraspinatus-anterior deltoid, upper-middle-lower trapezius, and supraspinatus-levator scapulae. ☐ Based on the consistent waveform patterns observed in conjunction with the motion capture data, muscles were able to be grouped based on their main function during external rotation. Humeral primary movers consisted of infraspinatus and teres minor. Scapular primary movers included middle trapezius, lower trapezius and rhomboid major. Teres major, subscapularis, pectoralis minor, and coracobrachialis served as the primary GH stabilizers. There were also several muscles that demonstrated activity congruent with the direction of body segment motion that appeared to play a stabilizing role but may have also contributed to motion. The ST mixed movers/stabilizers were upper trapezius, levator scapulae, supraspinatus, rhomboid minor, and serratus anterior. The muscles that were essentially inactive during external rotation were pectoralis major, anterior deltoid, posterior deltoid, latissimus dorsi, biceps brachii and triceps brachii. A synergistic relationship was also observed between the humeral and scapular primary movers (middle trapezius, lower trapezius, infraspinatus, rhomboid major and teres minor).Ph.D.University of Delaware, Biomechanics and Movement Science Progra