S1-3.9 Jan. 5 Passive Muscle Facilitates Rapid Perturbation Recovery in an Insect Leg LIBBY, T.*; FULL, R.J.; Univ of California, Berkeley firstname.lastname@example.org
Isolated and intact legs of cockroaches begin to recover from vertically directed impulse perturbations within 5 ms and return to within 99% of their original position within the swing phase duration. This response is due to the leg’s passive exoskeletal properties because of their vertically oriented joint axes. To examine recovery from horizontal plane leg perturbations involving passive muscle, we used a small servomotor and material testing techniques including sinusoidal oscillations and impulses to classify the passive dynamic system of the metathoracic leg of Blaberus discoidalis. We focused on motion of the coxa-femur joint that is responsible for fore-aft motion of the foot. Passive viscoelastic forces from muscle and exoskeleton were nonlinear and history dependent. For low speed ramp perturbations, recovery was slow and highly overdamped. For rapid perturbations, legs returned quickly and recovery was slightly underdamped. Applying sinusoidal forces to legs elicited periodic displacement as in running. Adding ramps and impulses confirmed that the passive system alone rejected perturbations to a periodic trajectory in less than 40 ms. Using this system as a model for the swing phase of running, we conclude that passive forces from the exoskeleton and inactive muscle allow the leg to completely recover from large impulse perturbations before footfall at stride frequencies over 8 Hz. Further, the swing phase can act as a stabilizer for leg position, rejecting perturbations to leg displacement that occur during stance phase. Stance phase perturbations that create an error in leg position at the onset of swing do not affect footfall position.