The fast twitch motor units of cat ankle flexors. 2. Speed-force relations and recruitment order

The relationship between axonal conduction velocity (CV), twitch contraction time (CT), peak tetanic tension (TT) and its peak rate of rise (TṪ) of fast twitch (CT ≤ 45 msec) motor units were examined in two cat hind limb muscles, tibialis anterior (TA) and extensor digitorum longus (EDL). Motor unit samples included: 73 TA units from 5 experiments (sample sizes 17, 10, 16, 18 and 12 units); and 66 EDL units from 6 experiments (11, 5, 9, 3, 18, and 20 units). Weak correlations were found between CV and TT for the TA units, between CV and CT for both the TA and EDL units, and between CT and TT for the TA units. Strong correlations were found between TT and TṪ for both populations. Speed-force relations are thus weak in the pretibial flexors but stronger in the parallel fibered TA muscle than in the pinnately structured EDL muscle. The fast twitch samples were also subdivided into FR (fatigue resistant), FI (intermediate fatigability) and FF (highly fatigable) categories. Speed-force correlations were slightly stronger in the FR subgrouping than in the FI and FF categories. These data are compared to a variety of conflicting data in the literature. Speed-force relations appear strong for small muscles of the cat's foot but more complex and controversial for larger muscles of the cat hind limb. None of the data are shown to negate the concept of an orderly recruitment of α-motoneurons according to the size of the muscle units that they innervate. There is suggestion that the spinal mechanisms underlying α-motoneuron recruitment order are more complex than simply recruitment of small motoneurons before large ones.