Effects of load inversion in cockroach walking.

To examine how walking patterns are adapted to changes in load, we recorded leg movements and muscle activities when cockroaches (Periplaneta americana) walked upright and on an inverted surface. Animals were videotaped to measure the hindleg femoro-tibial joint angle while myograms were taken from the tibial extensor and flexor muscles. The joint is rapidly flexed during swing and extended in stance in upright and inverted walking. When inverted, however, swing is shorter in duration and the joint traverses a range of angles further in extension. In slow upright walking, slow flexor motoneurons fire during swing and the slow extensor in stance, although a period of co-contraction occurs early in stance. In inverted walking, patterns of muscle activities are altered. Fast flexor motoneurons fire both in the swing phase and early in stance to support the body by pulling the animal toward the substrate. Extensor firing occurs late in stance to propel the animal forward. These findings are discussed within the context of a model in which stance is divided into an early support and subsequent propulsion phase. We also discuss how these changes in use of the hindleg may represent adaptations to the reversal of the effects of gravity.