HUMAN MOTOR UNIT-ACTIVITY DURING INDUCED MUSCLE CRAMP

Muscle cramp was induced in the medial head of the gastrocnemius muscl e in four of seven subjects using unloaded maximal voluntary contracti on of the triceps surae in the shortened position Surface electromyogr aphy over the medial and lateral heads of gastrocnemius and the soleus muscles demonstrated that the muscle activity during cramp was locali zed 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 emits during 16 episodes of cramp and 871 units during 26 voluntary contract ions. For the first 30 s, significantly higher motor unit firing rates were recorded during cramp compared with unloaded voluntary contracti ons. Motor unit firing rates were also more variable during cramp. Whe n the cramped muscle was stretched forcibly to break the cramp, motor unit activity increased in all the triceps surae muscles. In some expe riments, the Achilles tendon of five subjects was vibrated for 50 s be fore and after voluntary contraction or cramp. The tonic vibration ref lex (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 ne urons, mediated by changes in presynaptic input, is a possible mechani sm underlying muscle cramp.