The properties of motor units in a fast and a slow twitch muscle during post-natal development in the kitten.

1. Motor units were functionally isolated from a fast and a slow twitch muscle of kittens: flexor digitorum longus (FDL) and soleus. Both muscles were examined in two groups: one aged about 2 weeks and the other 6 weeks.2. A clear division into alpha and gamma axons could be made. The mean conduction velocity of alpha axons from FDL was higher than that of soleus axons at both ages.3. Tetanic tensions were expressed as a percentage of whole muscle tension. The distributions of motor unit tensions were symmetrical in soleus muscles but were skewed towards large values in FDL. In both types of muscle the scatter of motor unit tensions was greater at 2 weeks than at 6, the mean percentage tension also fell with age.4. The motor unit twitch times to peak were symmetrically distributed in soleus but skewed towards large values in FDL. The variance was less than in adults.5. Motor unit axonal conduction velocity was related to tetanic tension and twitch speed. The relationship between twitch time to peak and the ratio of twitch to tetanic tension varied with age but was not seen so clearly.