Fig. 4. Activity of the lump receptor during electrical stimulation of the extensor tibiae muscle with a single pulse. The spike of the fast extensor tibiae motor neuron (FETi) is visible as cross-talk in the recording from the lateral nerve; its waveform changes when the muscle contracts and moves. (A) The tibia was held in the fully flexed position and the resulting distortions of the femoro—tibial joint, measured from high speed images, were followed 40-50 ms after the stimulus by a burst of sensory spikes. (B) When the tibia was held in a partially extended position, the stimulus evoked a distortion of the semi-lunar process but no sensory spikes. (C) The tibia was free to move during the stimulus and there was no distortion and no sensory spikes.