STUDY DESIGN: Controlled laboratory study. OBJECTIVES: 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. BACKGROUND: 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. METHODS: 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). RESULTS: 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. CONCLUSIONS: 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.

J Orthop Sports Phys Ther 2009;39(10):743-752. doi:10.2519/jospt.2009.3089

KEY WORDS: lower trapezius, middle trapezius, posterior deltoid, scapula, upper trapezius

Objectives: To evaluate the muscle latency times of the 3 portions of the trapezius muscle to a sudden arm movement in normal shoulders and to determine if this recruitment pattern is altered as a result of fatigue.

Background: It has been suggested that shoulder impingement may be related to altered muscle activity and muscle fatigue in the scapular stabilizers. Fatigue-induced changes in latency times of the trapezius might influence scapular stability.

Methods and Measures: Muscle latency times were investigated in 30 healthy shoulders with surface electromyography. Muscle activity was measured in all 3 sections of the trapezius and the middle deltoid muscle during a sudden downward falling movement of the arm. Subsequently the shoulder was fatigued on an isokinetic dynamometer, after which muscle latency time measurement was repeated.

Results: ANOVA for repeated measures revealed significant differences in latency times (P < 0.05) among the 4 muscles of interest. Although there were no significant differences among the 3 sections of the trapezius muscle, they all were recruited after the initialization of the deltoid muscle. The recruitment order of the shoulder muscles did not change with muscle fatigue. However, after fatigue, muscle responses were significantly slower in all muscles except for the lower trapezius (P < 0.05).

Conclusions: There is a specific recruitment sequence in the shoulder muscles in response to a sudden arm movement characterized by initial activation of the middle deltoid muscle and followed by simultaneous contraction of all 3 sections of the trapezius. This muscle activation pattern is delayed but not altered with fatigue.

J Orthop Sports Phys Ther. 2002; 32(5):221–229.

Key Words: muscle latency times, muscular balance, neuromuscular properties, scapular stabilizers