Fig. 2. Sensory signals and distortion of a semi-lunar process at the femoro—tibial joint in kicks with only short periods of co-contraction. (A) An initial spike in the fast extensor tibiae motor neuron (FETi) accompanied the movement of the tibia into a flexed position and was signalled by a burst of spikes in sensory neurons from the lump receptor. The tibia then remained flexed for 500 ms before a kick was generated by an 80 ms long co-contraction involving 3 FETi spikes. Only a small distortion of the semi-lunar process resulted and spikes from the lump receptor occurred at low frequency. (B) A kick following a 150 ms long co-contraction with 5 FETi spikes. The tibia was fully flexed about the femur before the displayed recording. The bending of the semi-lunar process was now twice as large and was accompanied by more sensory spikes during the co-contraction phase.