Controlled laboratory study.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.Deficiencies in trapezius muscle recruitment have been identified in patients with shoulder pain. Alterations in the trapezius muscle activation level and timing have been identified in previous research. Scapular muscle exercises in which the middle trapezius (MT) and lower trapezius (LT) muscle showed optimal activity with minimal upper trapezius (UT) muscle participation have been recently identified. However, it is currently unknown if these exercises also promote early activation of the scapular stabilizing musculature.The intermuscular and intramuscular timing of muscle activation (based on an activation level of greater than 10% maximum voluntary contraction beyond basic activity) of the 3 portions of the trapezius muscle during 4 exercises were examined by surface EMG in 30 healthy subjects on the dominant side (14 males, 16 females). A 1-sample t test was used to determine which portions of the trapezius muscle were activated significantly earlier or later than the PD (intermuscular timing). An analysis of variance for repeated measures (3 levels) was used for each exercise to determine possible timing differences among the 3 portions of the trapezius muscle (intramuscular timing).Intermuscular and intramuscular differences in timing of the portions of the trapezius muscle were found. The UT was activated significantly later than the PD (P<.01), and the MT was activated significantly earlier than the PD (P<.01), during the prone extension exercise. During the horizontal abduction with external rotation exercise, the MT (P<.01) and the LT (P = .01) 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.