A small branch of the nerve to soleus muscle in normal human subjects was stimulated intramuscularly with a needle electrode while the subjects were maintaining a steady voluntary contraction. The EMG and force fluctuations produced by these stimuli were recorded and averaged. 2. In addition to the M-wave produced directly by stimulating motoneurons and the resultant twitch contraction, one or more EMG waves were seen with a latency greater than 100 ms. These later waves produced further contractions, and when there were several later waves, the EMG and force fluctuations appeared as a damped oscillation with a frequency between 5.5 and 8 Hz. 3. By varying the angle of the ankle and hence the time course of the twitch contraction, the timing of the latter waves was shown to be closely related to the contraction time. Thus, the later waves appeared to be produced reflexly by the tension fluctuations, rather than directly by the stimulus. 4. The frequency response function between the tension fluctuations and the reflux EMG responses was computed. The gain agreed with that of primary muscle spindle afferents, but the phase data showed extra lags consistent with a time delay which was too long to be spinal in origin. This reflex probably involves supraspinal centers.
