End-divergent architecture diversifies within-muscle mechanical action in human gluteus maximus in vivo

A muscle's mechanical action is affected by its architecture. However, less is known about the architecture of muscles with broad attachments: "end-divergent" muscles. Potential regional variation of fascicle orientation in end-divergent muscles suggests that their mechanical action varies by region. Here, we comprehensively examined 3D architecture and potential action of the human gluteus maximus (typical end-divergent muscle) in vivo. The gluteus maximus fascicles were three-dimensionally reconstructed over the whole muscle belly using diffusion tensor imaging and tractography. We calculated the force fraction and moment-arm length about the hip joint for individual muscle fascicles, and their product (specific torque, an estimate of torque-generating capacity for a given cross-sectional area). We found that the specific torque for hip extension and external rotation tended to be greater in the distal than the other regions, whereas that for hip abduction appeared to be greater in the proximal than the other regions. Notably, the distal-lateral region exhibited a negative specific torque for hip abduction, indicating that fascicles in this region act for hip "adduction". These findings indicate that end-divergent architecture diversifies within-muscle mechanical action in terms of directions as well as magnitudes in vivo.