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
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provide normative data on the temporal recruitment pattern of the scapular rotators
in freestyle swimmers,
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determine the effect of a unilateral shoulder injury on this pattern,
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determine whether these effects extend to the non-injured side, and
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determine the effect of injury on the consistency (variability) of muscle recruitment.
Surface EMC 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 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.
Key words
EMC - swimming - upper trapezius - lower trapezius - serratus anterior - subacromial
impingement - motor control
