Scapular Muscle Recruitment Pattern: Electromyographic Response of the Trapezius Muscle to Sudden Shoulder Movement Before and After a Fatiguing Exercise

Abstract

4Study 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 Ͻ 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 Ͻ 0.05). Sciences and Physiotherapy, De Pintelaan 185, 6K3, B9000 Ghent, 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.

Shoulder External Rotation Fatigue and Scapular Muscle Activation and Kinematics in Overhead Athletes

“…2 The infraspinatus was chosen because it seems to show the greatest change in activation with repeated elevation and external rotation tasks. 3,6 Activation amplitude of these muscles was measured using a telemetry surface EMG system (model T42L-8T0 Telemetry; Konigsberg Instruments, Inc, Pasadena, CA).…”

“…Such repeated motion is thought to create muscle fatigue, thereby increasing the risk of shoulder injury by altering muscle activation patterns, force couples, and kinematics in the shoulder girdle. [2][3][4][5][6] Using external rotation fatigue protocols aimed at the rotator cuff muscles, investigators have demonstrated concurrent decreases in scapular posterior tilt and external rotation together with increased clavicular retraction. 3,6 However, researchers have reported conflicting results of alterations in upward rotation, with some showing decreased scapular upward rotation 5,6 and others showing increased upward rotation.…”

Influence of a Single Swim Training on the Scapular Position and Isometric Muscle Strength in Young Swimmers

“…As an alternative for strength measurements, EMG analysis has proved to be a reliable tool to detect changes as a result of fatigue. In their study, Cools et al (2002) found significant differences in latency times in the trapezius muscle after a fatiguing protocol. The muscle activation pattern was significantly delayed but not altered.…”

“…Weakness or fatigue of periscapular muscles can be a contributing factor in scapular dyskinesis, abnormal movements of the scapula (Cools et al, 2002;Crotty & Smith, 2000;Laudner, Stanek, & Meister, 2008;Szucs, Navalgund, & Borstad, 2009). Several authors discussed the cause-and-effect relationship between scapular dysfunction and several shoulder pathologies (Cools, Witvrouw, & Mahieu, 2005;Ludewig & Cook, 2000;Lukasiewicz & Mcclure, 1999).…”

“…The muscle activation pattern was significantly delayed but not altered. Delayed muscle onset time, due to fatigue, could possibly lead to altered scapular kinematics (Cools et al, 2002;Diederichsen et al, 2008). In further research, this method can be more appropriate to detect subtle fatigue signs in swimmers than the pattern itself.…”