Muscle cramp was induced in the medial head of the gastrocnemius muscle in four of seven subjects using unloaded maximal voluntary contraction of the triceps surae in the shortened position. Surface electromyography over the medial and lateral heads of gastrocnemius and the soleus muscles demonstrated that the muscle activity during cramp was localized to part or all of the medial head of the gastrocnemii. In the same muscle, a tungsten electrode was used to record from 200 motor units during 16 episodes of cramp and 871 units during 26 voluntary contractions. For the first 30 s, significantly higher motor unit firing rates were recorded during cramp compared with unloaded voluntary contractions. Motor unit firing rates were also more variable during cramp. When the cramped muscle was stretched forcibly to break the cramp, motor unit activity increased in all the triceps surae muscles. In some experiments, the Achilles tendon of five subjects was vibrated for 50 s before and after voluntary contraction or cramp. The tonic vibration reflex (TVR) was depressed or absent after four episodes of cramp but it was unchanged after voluntary contraction. These data are interpreted to indicate that motor units are involved in ordinary muscle cramp. A positive feedback loop between peripheral afferents and alpha motor neurons, mediated by changes in presynaptic input, is a possible mechanism underlying muscle cramp.