LEE, David V.; Harvard University: Elasticity in the joints and whole legs of goats vs. dogs: trading economy for precision?

While dogs (Canis lupus familiaris) are excellent long-distance runners and terrain generalists, goats (Capra hircus) are specialists on rough, mountainous terrain. The disparate mechanical requirements of these two niches suggest that associated structural and functional differences may be apparent in the limbs. Here the joint and whole leg mechanics of goats and dogs of similar size were examined during running and a serial actuator-spring model was applied to determine spring characteristics. Two force platforms were used to measure ground reaction forces and centers of pressure of individual footfalls while a motion capture system recorded joint and foot marker positions. Joint moments and angles as well as leg force and length were computed from these data. The total positive and negative work done by actuators was expressed as a fraction of total positive and negative joint (or leg) work. This was termed �actuation ratio� and yielded values between zero (pure spring-like behavior) and one (pure actuation). The spring constant determined by the model was that which minimized the actuation ratio. The principal joint spring of the goat forelimb was the metacarpo-phalangeal. In contrast, the principal joint spring of the dog forelimb was the wrist. The principal hindlimb joint spring was the ankle in both goats and dogs. Considering the leg force and linear deflection of the leg from shoulder (or hip) to foot revealed similar actuation ratios in goats and dogs but substantially different spring constants. Forelegs and hindlegs of goats were found to be nearly twice as stiff as those of dogs. This substantial difference may reflect the competing requirements of stiffness for precision foot placement in rough terrain and compliance for greater elastic energy storage and enhanced economy.