Fig. 1. Sensory signals from the lump receptor and distortion of the femoro—tibial joint during a kick. Electrical activity of the lateral nerve containing the axons of sensory neurons from the lump receptor, and of the extensor tibiae muscle was recorded at the same time as images of the movements of the femoro—tibial joint. The distortion of the lateral semi-lunar process was plotted from these images, three of which are shown at the times indicated. Full extension of the tibia in the kick occurred at time 0 ms. The arrows show the changing position of the distal tip of the semi-lunar process. The distortion of the dorsal femur and the bending of the semi-lunar process are also shown in tracings from frames at -15, -4 and 0 ms. The movements of the semi-lunar process in the graph appear jerky because of the intermittent sampling. Initial flexion of the tibia was accompanied by spikes in the lateral nerve. During the co-contraction phase (horizontal grey bar), the sensory spikes of the lump receptor occurred at high frequency as the semi-lunar process was bent progressively. The extended position of the tibia following the kick was signalled by sensory spikes in joint receptors. The vertical dashed lines indicate the different phases of the kick. The large spikes in the muscle recording are from the fast extensor tibiae motor neuron (FETi) and the smaller ones from flexor motor neurons.