Scapular Muscle Recruitment Patterns: Trapezius Muscle Latency with and without Impingement Symptoms

TLDR

Altered muscle activity in scapular muscles is believed to contribute to shoulder impingement syndrome, yet the timing of recruitment patterns remains undetermined. The study aimed to evaluate the timing of trapezius muscle activity during an unexpected arm movement in athletes with and without impingement. A prospective cohort design measured muscle latency times in the trapezius and middle deltoid of 39 impingement athletes and 30 controls during a sudden downward arm fall. Impingement athletes exhibited delayed activation of the middle and lower trapezius, indicating abnormal recruitment timing that may underlie shoulder impingement.

Abstract

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.