LIBBY, T.; MEIJER, K.; FULL, R.J.: Partioning Preflex And Reflex Contributions Toward Cockroach Leg Stability: A Simulation Study

It has been hypothesized that animals use neural reflexes as well as the intrinsic stiffness and damping of the musculo-skeletal system (preflexes) to manage perturbations during rapid locomotion. Previously, we imposed perturbations or sinusoidal oscillations in the plane of leg movement to the hind limb of live, tethered cockroaches (Blaberus discoidalis). Leg impedance remained relatively constant as oscillation frequency was increased. EMGs revealed reflex activity in two femoral extensors, muscles 177d and 177e. Activity decreased as a function of frequency, ceasing altogether at about 10 Hz. Above this frequency, the response was almost entirely passive. We hypothesized that the constant impedance results from musculo-skeletal preflexes compensating for the decline in reflex activity as frequency increased. To test this hypothesis, we developed a two-dimensional musculo-skeletal model of the leg based on direct measurements. Simulations confirmed that muscle impedance increases substantially with an increase in frequency. Muscles 177d/e, when partially active, could be responsible for up to half of the overall leg impedance at low frequencies where the reflex contributed most. Perhaps most interesting is the fact that passive muscle force in simulation is only 10-20% of the total measured impedance. Passive leg force may be due more to the exoskeleton than muscles when legs respond to small perturbations. In general, the simulation data point to the hypothesis that the response to low frequency perturbations could be more reflexive in nature, whereas at higher frequencies the leg acts as an over-damped or near-critically damped spring to manage energy so as to ensure stability. Supported by ONR N00014-98-1-0669.