Maximum Tension Predicts Relative Endurance of Fast-Twitch Motor Units in the Cat

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1. The relationships between maximum tetanic tension (P0), endurance time, and axonal conduction velocity (CV) were investigated in fast-twitch motor units of the cat flexor carpi radialis (FCR) and medial gastrocnemius (MG) muscles, and in one flexor digitorum longus (FDL) muscle. Endurance time was the length of time that a unit could maintain 25% of its maximum tetanic tension during a sustained contraction. Motor unit tension was “clamped” at 25% of maximum by altering the stimulation rate of a unit’s motor axon through computer feedback control.

2. In individual experiments, including the one investigated FDL muscle, an inverse relation was consistently found between maximum tension and endurance time. Pooled data from the FCR and MG muscles also resulted in significant correlations between maximum tetanic tension and endurance time.

3. Following the force-clamp contraction, some motor units were subjected to the standard fatigue test of Burke and colleagues (6). Motor units were classified as type FR (fast twitch, fatigue resistant) or type FF* (fast twitch, fast fatiguing after the force-clamp contraction). For both type FR and FF* units, maximum tetanic tension and endurance time were found to be inversely related. However, no correlation was found between maximum tetanic tension and fatigue index for type FR units. Only when all type F (FR + FF*) units were considered as a population was there a significant correlation between these two properties.

4. Other investigators have shown that maximum tetanic tension and axonal conduction velocity are highly correlated with the recruitment order of motoneurons (e.g., Refs. 2, 26). Endurance time was found to be more tightly coupled with contraction strength than with conduction velocity. In 12 of 14 experiments, significant Spearman rank correlation coefficients were found between endurance time and tension, whereas significant correlations were found in only 3 of 14 experiments for endurance time and conduction velocity.

5. Pairs of motor units isolated from the same muscle were formed to see if the unit with the smaller tension had the slower conduction velocity and the longer endurance time. Across all muscles, the probability that the unit with the smallest tension had the greatest endurance time was 0.91 (441 of 487 pairs). By contrast, the probability that the least forceful unit of the pair had the slowest conduction velocity was 0.61.

6. In four experiments, pairs of type-identified units were examined. Among FR-FR pairs, the least forceful unit had the greatest endurance time in 88% of 43 pairs. For FF*-FF* pairs, the percentage was somewhat lower, 72% of 29 pairs. The highest percentage was found for FR-FF* pairs, 99% of 88 pairs. Only among the FR-FR pairs did the probability for the least forceful unit to have the slower conduction velocity exceed 50% (i.e., random), ~65% for 43 pairs.

7. Among type F units, the consequences of two different recruitment schemes are discussed with regard to the force output of the muscle. By using endurance time as a measure of muscle-unit performance, it is concluded that fewer units would be required to maintain a submaximal contraction if recruitment proceeded by increasing contraction strength rather than by increasing axonal conduction velocity. For sustained contractions involving the type F population, then, strict adherence to a recruitment sequence that is based on motoneurons having progressively faster axonal conduction velocities would have the effect to hasten fatigue.