DIMOULAS, PM; HENRY, H; RUBENSON, J; MARSH, RL; Northeastern University; Northeastern University; Northeastern University; Northeastern University: Homogenous activation patterns of the anatomically compartmentalized semimembranosus muscle in Bufo marinus

Locomotor muscles in vertebrates may be compartmentalized in several ways: anatomically, neuromuscularly, and functionally. We examined whether the anatomically compartmentalized semimembranosus muscle (SM) in the toad Bufo marinus shows any functional specialization in the activation of these compartments during various locomotor tasks. The SM is a superficial, biarticulate, hip extensor that originates on the ventro-lateral third of the ischium and inserts on the ventro-medial surface of the femur and tibiofibula. This muscle is divided into three anatomical compartments. Approximately 75% of the muscle mass consists of two in-series compartments with a tendinous inscription connecting the proximal and distal compartments. The toad SM has a third longitudinal compartment making up ~25% of the muscle mass. The fascicles of the longitudinal compartment do not span from origin to insertion, but instead insert on an aponeurotic tendon onto which the distal in-series compartment also inserts. We measured the electromyograms (EMGs) of all three compartments of the SM in conjunction with simultaneous video recordings during three distinct locomotor activities: “walking”, jumping, and swimming. Our results indicate that during these locomotor activities all compartments are simultaneously activated. Thus, based on EMG�s, we have found no evidence for functional specializations of the anatomically compartmentalized SM of the toad. We conclude that the reasons for the anatomical compartmentalization of the SM must be sought in developmental or evolutionary trajectories. Supported by NIH grant # AR47337.