Short Communications a New Method Measuring Leg Position of Walking Crustaceans Shows That Motor Output during Return Stroke Depends upon Load

The basic movement of a walking leg consists of two parts, the power stroke (stance phase) and the return stroke (swing phase). The movement of a leg during the power stroke is determined not only by the sensory-neural system of the leg itself but also by the movement of the other supporting legs because of their mechanical coupling. In contrast the movement during the return stroke is under the sole control of the sensory-neural system of the leg itself. Therefore observation of the return stroke movement allows a more direct view of the motor output from the centre controlling the movement of the leg. Motor output can be recorded by electrophysiological methods. However, quantitative interpretation of these results is often difficult as the transformation of the recorded spikes (often from several superimposed motor units) to force is usually unknown. Whereas motor output during the power stroke clearly depends on walking speed and on load, some studies have found return stroke duration to be dependent upon walking speed, others found no such dependence (for reviews see Clarac, 1981; Evoy & Ayers, 1982). A solution to this discrepancy has been proposed on the basis of a model which assumes that load is an essential parameter affecting the movement of the individual walking leg (Cruse, 1983). A central assumption in this model is that motor output during both power stroke and return stroke is increased when the animal walks under load. It follows from this assumption that an increase in the load should lead to faster return stroke movements and, for constant leg amplitudes, to shorter return stroke durations. As mentioned above this prediction can be easily tested for the return stroke. Therefore, we developed a simple method to measure return and power stroke durations and step amplitude for a decapod walking under water.