1. Three kinds of motor unit have been distinguished in soleus (SOL) and extensor digitorum longus (EDL) muscles of adult rats on the basis of differences in isometric twitch contraction times measured at 35° C.2. SOL contains about thirty motor units of which about twenty-seven are slow and three are intermediate with average contraction times of 38 and 18 msec respectively.3. EDL contains about forty fast motor units with an average contraction time of 11 msec.4. Average values for maximum isometric tetanic tension were 4·7 g for fast and slow units and 5·85 g for intermediate units.5. Average values for twitch-tetanus ratios ranged from 0·2 to 0·227 for EDL muscles, SOL muscles and the three kinds of motor units.
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