The objective of this study was to examine the timing of the 3 portions of the trapezius muscle in relation to the posterior deltoid (PD) muscle and in relation to one another during 4 selected shoulder exercises: (1) prone extension, (2) forward flexion in side lying, (3) external rotation in side lying, and (4) prone horizontal abduction with external rotation. The intermuscular and intramuscular timing of muscle activation of the 3 portions of the trapezius muscle during 4 exercises were examined by surface EMG in 30 healthy subjects on the dominant side. Intermuscular and intramuscular differences in timing of the portions of the trapezius muscle were found. The UT was activated significantly later than the PD, and the MT was activated significantly earlier than the PD, during the prone extension exercise. During the horizontal abduction with external rotation exercise, the MTÂ and the LT were activated significantly earlier than the PD. During prone extension, side-lying external rotation, and prone horizontal abduction with external rotation, significant differences were found between the UT and MT, between the UT and LT, but not between the MT and LT. In these exercises the MT and LT were activated significantly earlier than the UT. During forward flexion in side lying, no significant timing differences were found between the activation of the portions of the trapezius.
With the exception of the LT during prone extension, the prone extension exercise and the prone horizontal abduction with external rotation exercise promote early activation of the MT and LT in relation to the scapular and glenohumeral prime mover. Taking into account the limited generalizability of the results due to a narrow age range, these exercises are potentially promising for the treatment of intermuscular and intramuscular timing disorders of the trapezius muscle.