CRUSE, H.*; SCHUMM, M.: The influence of changing substrate conditions on swing movement trajectories

Trajectories of swing movements of stick insects walking on different forms of substrates were studied. When walking on an inclined surface, the body is rotated around its long axis such that the vertical axis of the body is held more vertical than the direction perpendicular to the walking plane. A simulation shows that sense organs measuring leg joints are sufficient to explain this behavior. As a result of body rotation, swing movement trajectories have to be changed. In the downhill legs, the coxa-trochanter joint is more elevated than in the uphill legs. In the uphill legs, the femur-tibia joint is extended more which compensates for the smaller lift in the coxa-trochanter joint, leading to a higher extreme position in the uphill legs. By experimentally changing the starting position of a swing movement in vertical direction and in the direction parallel to body long axis, the trajectories cannot be predicted by an earlier hypothesis. A modified network allows for a description of the behavioral results. Swing movements depend on the width of the substrate. Walking on a wide treadwheel leads to different swing trajectories walking on a more narrow one. The contribution of the various leg joints is different depending on the substrate, suggesting an economical solution. If one joint is near its upper limit, the parallel neighboring joint may be activated to meet the goal of leg movement (motor equivalence). The form of the swing trajectory depends on the placement of the tarsus in the preceding stance. If the tarsus is placed flat on the ground, swing height is larger than when the tarsus grasps around an edge. Transition between different swing trajectories is not gradual. The form of the trajectory is determined by the current sensory input on a step by step basis.