Dynamic Walking 2010. Richard Marsh. Coordinated action of muscle-tendon systems in the avian hindlimb during walking, running, and jumping
The extreme variation in hindlimb morphology that has occurred in the evolution of birds adapted to varying types of locomotion provides the opportunity to examine those features of muscle and limb architecture that appear to be associated with a particular locomotor habit, as well as features conserved across many avian taxa. Two examples of muscles systems are presented that provide insight into the coordinated actions across multiple joints and the potential for self-stabilizing behavior in birds adapted for terrestrial legged locomotion. First, the hamstring-like flexor cruris lateralis pars posterior (FCLP) has been found to act in concert with the intermediate head of the gastrocnemius (IG), and with an accessory head attached to the femur (FCLA). In vivo length and EMG measurements in these three muscles suggest the hypothesis that the FCLP acts through the tendon of insertion of the active IG in early stance to provide an extensor moment at the ankle, and switches to a pure hip extensor in late stance when EMG activity in the IG is reduced and FCLA becomes active. The FCLA system also provides the potential for automatic compensation of moments across three joints simultaneously in early stance. Second, the fibularis longus (FL) can alter its function across two joints due to the anatomy of its distal tendon. The FL tendon of insertion splits above the ankle, with one branch extending the ankle and the other branch producing digital flexion, particularly at the tarsometatarsal-phalangeal joint (TMP). Our data indicate that the effective moment arm of the FL at each joint is determined in large part by the angle of the second joint. Because of this interaction, the predicted function of the FL varies greatly between jumping and running. At the angles used in running the FL is predicted to act mostly as a toe flexor, but during jumping the FL switches almost completely from an ankle extensor early in the jump to a toe flexor just before takeoff. This interacting system also provides a potential compensatory mechanism for altering joint moments in both the ankle and TMP in response to variation in angles at these joints. Supported by NIH AR47337 and NSF IOB-0542795.