The function of neuromuscular compartments in human shoulder muscles

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.

The Influence of Modeling Separate Neuromuscular Compartments on the Force and Moment Generating Capacities of Muscles of the Feline Hindlimb

Functional electrical stimulation (FES) has the capacity to regenerate motion for individuals with spinal cord injuries. However, it is not straightforward to determine the stimulation parameters to generate a coordinated movement. Musculoskeletal models can provide a noninvasive simulation environment to estimate muscle force and activation timing sequences for a variety of tasks. Therefore, the purpose of this study was to develop a musculoskeletal model of the feline hindlimb for simulations to determine stimulation parameters for intrafascicular multielectrode stimulation (a method of FES). Additionally, we aimed to explore the differences in modeling neuromuscular compartments compared with representing these muscles as a single line of action. When comparing the modeled neuromuscular compartments of biceps femoris, sartorius, and semimembranosus to representations of these muscles as a single line of action, we observed that modeling the neuromuscular compartments of these three muscles generated different force and moment generating capacities when compared with single muscle representations. Differences as large as 4 N m (∼400% in biceps femoris) were computed between the summed moments of the neuromuscular compartments and the single muscle representations. Therefore, modeling neuromuscular compartments may be necessary to represent physiologically reasonable force and moment generating capacities of the feline hindlimb.