Speed-force relations in the motor units of the cat tibialis posterior muscle

The neuro-mechanical properties and speed-force relations of 81 cat tibialis posterior motor units have been studied. Statistically significant correlations were found between α-axonal conduction velocity (CV) and average twitch lesion (log₁₀ TwT; r = 0.459, p < 0.001), between CV and twitch contraction time (CT; r = -0.395, p < 0.001) and between CT and log₁₀ TwT (r= - 0.277, p < 0.02). The present correlations for the intermediate-sized tibialis posterior are stronger than those previously reported for large muscles such as soleus, medial gastrocnemius, plantaris and flexor hallucis longus. However, they are considerably weaker than those reported for the much smaller lumbrical muscles of the cat's foot. These findings support the contention that the spinal mechanisms governing an orderly recruitment of motor units according to the size of their muscle units must be more complex for large than for small muscles, at least in the cat hindlimb.