A neuro-mechanical model containing fast and slow muscle fibres applied to mimic stop and re-start of a stepping leg

Abstract

By means of our model, we first demonstrated that locomotion (stepping) is indeed primarily determined by the activity of the fast muscle fibres, even though the slow ones are co-active. We also showed that in static positions the residual stiffness of the slow muscles is capable of maintaining the posture. We then applied our model to elucidate intra-leg coordination of the muscle activity when the animal stops and when it re-starts. According to our model, intra-leg coordination at stopping is brought about by inhibiting the individual motoneurons via inhibitory interneurons, rather than switching off the central pattern generators associated with the antagonistic muscle pairs of the leg. The simulations showed that the leg completes a stance phase after the touch-down of the tarsus until the femur reaches its static horizontal position, and the tibia its static flexion. Depending on the steady-state horizontal position (angle) of the femur, the stepping commenced with a swing or a stance phase. The above simulation results are in good agreement with the relevant experimental findings. The model thus provides a putative intra-leg coordination mechanism that can account for the processes that take place at stop and at re-start of stepping of a multi-joint leg, like that of the stick insect.