Recruitment Patterns of the Scapular Rotator Muscles in Freestyle Swimmers with Subacromial Impingement

Abstract

Athletes with shoulder pathology consistently demonstrate abnormalities in scapular rotator activity, suggesting that muscle dysfunction is a factor to consider in the aetiology or recurrence of shoulder pain. However, one important measure of the coordinated activity between the scapular rotators, their timing or temporal recruitment pattern, remains undetermined. The purposes of this study were to 1. provide normative data on the temporal recruitment pattern of the scapular rotators in freestyle swimmers, 2. determine the effect of a unilateral shoulder injury on this pattern, 3. determine whether these effects extend to the non-injured side, and 4. determine the effect of injury on the consistency (variability) of muscle recruitment. Surface EMG data for the upper and lower trapezius and serratus anterior were recorded bilaterally from two groups of competitive freestyle swimmers during controlled bilateral elevation in the plane of the scapula. An injured group comprising nine swimmers with unilateral shoulder pathology and a control group of nine non-injured swimmers were included. Temporal data determined for the onset of muscle activation for each muscle were then compared between groups using an ANOVA and a one-sided F test. The results of the study indicate that in non-injured swimmers, upper trapezius is activated 217 ms prior to shoulder motion, followed by serratus anterior activation 53 ms after motion commences. Lower trapezius was not recruited until 349 ms after shoulder motion, when the arm had attained 15 degrees elevation. In injured swimmers, all three muscles on the injured side displayed significantly increased variability in the timing of activation (p < 0.05), whilst the serratus anterior was significantly delayed in its activation on the non-injured side (p < 0.05). Skill hand preference was shown to have no effect on muscle recruitment. The findings of this study indicate that a relationship does exist between shoulder injury and the temporal recruitment patterns of the scapular rotators, such that injury reduces the consistency of muscle recruitment. They further suggest that injured subjects have muscle function deficits on their unaffected side.

Three-Dimensional Scapular and Clavicular Kinematics and Scapular Muscle Activity During Retraction Exercises

“…Excessive activity of the upper trapezius muscle and decreased activation of the middle and lower trapezius muscles have been associated with abnormal scapular movement in previous studies. 5,6,37,39,55,59 Based on findings of these studies, Cools et al 4 recommended exercises that minimize upper trapezius while preferentially increasing lower and middle trapezius activation for those individuals with abnormal scapular motion. Exercise 6 resulted in a 15.1% of MVIC effort of the upper trapezius and 50.5% effort of the lower trapezius muscle.…”

Influence of shoulder pain on muscle function: implications for the assessment and therapy of shoulder disorders

“…Some kinematic studies have even shown different movement patterns in both the painful and pain-free shoulder (Hebert et al 2002;Lukasiewicz et al 1999), suggesting that an altered neuromuscular strategy might affect both shoulders. Indeed, Wadsworth et al (Wadsworth and Bullock-Saxton 1997) observed a significant delayed recruitment of the serratus anterior muscle on the pain-free side in individuals with shoulder pain compared to control subjects. In addition, they did not register any significant differences between the symptomatic or asymptomatic side within the shoulder pain group, emphasizing that injury at one side of the body could be associated with alterations in muscle function on the unaffected side.…”

Thoracic posture, shoulder muscle activation patterns and isokinetic strength of semi-professional rugby union players

“…Poor posture, scapular dyskinesia, altered scapular muscle recruitment patterns and shoulder-strength weaknesses or imbalances may be associated with shoulder injuries in athletes; [1] however, this has not been proven conclusively for rugby players. Despite the fact that rugby union enjoys increasing worldwide popularity, it has one of the highest reported incidences of injury.…”

“…[1,8] Among swimmers with shoulder injuries, there is significantly increased variability in the timing of activation in the upper and lower part of the trapezius muscle, [1] reflecting inconsistent or poorly co-ordinated muscle activation. [1] With regard to rugby players, in a study to define muscle-activation patterns in selected shoulder girdle muscles during a front-on tackle in asymptomatic subjects, [9] a consistently earlier activation of the serratus anterior muscle was observed prior to impact, compared with the pectoralis major, biceps brachii, latissimus dorsi and infraspinatus. [9] A combination of electromyography (EMG) and isokinetic dynamometry could provide information regarding the function of shoulder musculature in sport.…”

“…[12] Previous research has indicated a possible association between posture, isokinetic strength, scapular muscle recruitment patterns and injury among the general population and overhead athletes. [1] However, there are few available profiles of rugby union players with regard to the aforementioned factors and the possible associations thereof with injury. The compilation of such profiles for uninjured players could, in theory, assist in the identification of rugby union players who are more likely to be at risk of future injury to the shoulder area.…”