Scapular muscle recruitment pattern: Electromyographic response of the trapezius muscle to sudden shoulder movement before and after a fatiguing exercise

Study Design: Test-retest reliability study and single-group repeated measures design. 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, them all were recruited after the initialization of the deltoid muscle. The recruitment order of the shoulder muscles did not c hange 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.