The function of neuromuscular compartments in human shoulder muscles

Abstract

The aim of this study was to use a surface electromyographic (sEMG) technique with a ballistic isotonic shoulder joint adduction movement to determine the function of the neuromuscular compartments (NMCs) within the pectoralis major, deltoid, and latissimus dorsi muscles. Sixteen male subjects (mean age 22 yr) with no known history of shoulder pathologies volunteered to participate. Timing and intensity of muscle contraction, recorded with 15 pairs of bipolar sEMG electrodes, were compared during performance of 40° coronal-plane ballistic [movement time (MT) < 400 ms] shoulder joint adduction movements. The results suggested that heterogeneous sEMG was present across the breadth of all three muscles, indicating the presence of individual NMCs with significant (P < 0.05) differences observed within the three muscles in NMC onset, duration, timing of peak NMC intensity, or relative intensity of NMC activation. For example, within the deltoid NMC activation was closely related to moment arm (MA) length with the NMC, with the largest antagonist MA deltoid NMC3 having a late period of activation [antagonist (Ant)] to slow glenohumeral joint (GHJ) rotation and maintain its final joint position [with agonist 2 burst (Ag2)]. The most obvious triphasic EMG patterns (e.g., Ag1-Ant-Ag2) were observed between the first NMCs activated in the two agonist muscles and the last NMC activated in the antagonist deltoid muscle. In conclusion, our findings suggest the presence of in-parallel NMCs within the superficial muscles of the GHJ and show that biomechanical parameters, such as the MA at end-point movement position, influence the function of each NMC and its contribution to alternating patterns of agonist and antagonist muscle activity typical of ballistic movement.