Meeting Abstract
Adaptive and stable walking requires that a leg’s stance-to-swing transition is coordinated with the step cycles of the other leg(s). The properties of load sensors in both mammals and insects suggest a particulary elegant, leg-local mechanism to do so: A leg in stance lifts off the ground when sensing a sudden reduction of its own load induced by another leg’s touchdown due to mechanical coupling. Here we test the plausibility of this mechanism in freely walking stick insects (Carausius morosus) using simultaneous recordings of leg kinematics, ground reaction forces, and electromyographic activity. First, we show that the insect load sensors (campaniform sensilla) can reliably encode the unloading of a leg in stance during walking. Based on joint torques, we predict that two sensor groups on the trochanter change their activity upon the onset of unloading. This could trigger a motor effect promoting the leg’s stance-to-swing transition. Second, we show that the onset of unloading is directly linked to the transfer of mechanical load among legs: (i) unloading is closely preceded by the touchdown of the neighboring posterior leg; (ii) according to a static simulation, this leg takes on load with highest efficacy; (iii) the levator muscle of the leg in stance is activated only after the onset of unloading. Taken together, our results demonstrate that leg-local load signals can contribute significantly to inter-leg coordination by exploiting the mechanical coupling between legs. We predict that this mechanism is used to similar advantage across insect species, in analogy to load-based mechanisms proposed in mammals.
