DUDEK, D.M.*; FULL, R.J.: Spring-like behavior of the legs of running insects

The dynamics of many terrestrial organisms during running are similar to those of a bouncing spring-mass system. This observation has led to the search for spring-like behavior in the tendons of mammals. The dynamics of running cockroaches have been modeled as a spring-mass system, but the location of spring-like elements, if they exist, remains a mystery. Cockroaches have sprawled postures with joint axes oriented vertically resulting in the possibility of a passive, exoskeletal leg spring. Therefore, we determined the mechanical impedance (the time-dependant resistance of a material to deformation) and resilience of the metathoracic limb of a cockroach (Blaberus discoidalis) using dynamic oscillations. The hind limb was removed and secured in an orientation consistent with that of a running animal. The leg was oscillated in the dorso-ventral direction (orthogonal to the plane of rotation for the joints) and induced forces recorded. The resulting force-displacement relationships were non-linear and similar in many respects to the stress-strain curve for spider legs (Blickhan, 1986). As much as 50% of the energy used to displace the leg was stored as elastic strain energy and returned. While the relative importance of viscous damping decreased with increasing oscillation frequency, the impedance of the leg increased up to a frequency of 12 Hz. At frequencies above 12 Hz, the impedance of the leg remained constant. Interestingly, the stride frequency of running cockroaches increases with speed up to its preferred stride frequency, which is between 10-14 Hz. At faster speeds, stride frequency remains constant. Taken together, these results suggest that the effects of passive structures are significant and could allow the leg to act as a damped spring during locomotion. Supported by ONR N00014-98-1-0669.