Scapular muscle recruitment patterns: trapezius muscle latency with and without impingement symptoms.

BACKGROUND: Altered muscle activity in the scapular muscles is commonly believed to be a factor contributing to shoulder impingement syndrome. However, one important measure of the muscular coordination in the scapular muscles, the timing of the temporal recruitment pattern, is undetermined.
PURPOSE: To evaluate the timing of trapezius muscle activity in response to an unexpected arm movement in athletes with impingement and in normal control subjects. STUDY
DESIGN: Prospective cohort study.
METHODS: Muscle latency times were measured in all three parts of the trapezius muscle and in the middle deltoid muscle of 39 "overhand athletes" with shoulder impingement and compared with that of 30 overhand athletes with no impingement during a sudden downward falling movement of the arm.
RESULTS: There were significant differences in the relative muscle latency times between the impingement and the control group subjects. Those with impingement showed a delay in muscle activation of the middle and lower trapezius muscle.
CONCLUSION: The results of this study indicate that overhand athletes with impingement symptoms show abnormal muscle recruitment timing in the trapezius muscle. The findings support the theory that impingement of the shoulder may be related to delayed onset of contraction in the middle and lower parts of the trapezius muscle.