1. The endurance of slow-twitch motor units from the soleus (SOL) and medial gastrocnemius (MG) muscles of the cat were tested by determining the length of time (endurance time, E(t)) that a unit could maintain its tension output at 85% of maximum. Motor-unit tension was clamped at the target level by altering the stimulation rate of a unit's motor axon through computer feedback control. Tested in this way, units of both muscles displayed a wide range of E(t)s, ~40- to 50-fold. 2. Electromyographic (EMG) waveforms of motor units subjected to force-clamp contractions were analyzed to access whether any activity-dependent changes in their waveform shape might predict E(t). Three measurements of waveform shape were determined: baseline-to- baseline duration, peak-to-peak amplitude, and area. Typically, amplitude decreased and duration increased as a contraction proceeded, whereas area remained fairly constant. Because changes in each measure were very similar for units of widely different E(t)s, it was concluded that neuromuscular junction failure and changes in the excitability of the sarcolemma (excluding the t-tubule system) play a minor role in determining E(t). 3. E(t) was highly correlated with the mean stimulation rate (E(t)/number of stimuli) used during the force-clamp contractions. Mean rate was seen to progressively decrease with increasing E(t). This correlation could not be explained by measures of isometric contractile speed or relaxation (e.g., twitch contraction time or half-relaxation time) measured before the force-clamp contractions. Both contraction time and half-relaxation time were found to be unrelated to both E(t) and the rate used to stimulate the unit during the force-clamp contraction. 4. Among type S units of SOL and MG, maximum tetanic tension and E(t) were not related. A significant relation (r = -0.49) was found between axonal conduction velocity and E(t) for SOL units (n = 38). In addition, a significant correlation (r = 0.47) was found between conduction velocity and tetanic tension for SOL units. Perhaps because of the small sample of type S units from MG (n = 10), conduction velocity was found not to be related to either E(t) or tetanic tension. 5. Others have shown that a motor unit's maximum tetanic tension and axonal conduction velocity are correlated with its order of recruitment among motoneurons innervating a muscle. Recent work has further shown that among type F units the order in which a motoneuron is recruited is highly correlated with the fatigue resistance of its muscle unit. In contrast, the present study suggests that among type S units a much weaker relationship exists between a unit's recruitment order and its fatigue resistance, if the interrelationships among conduction velocity, tetanic tension, and E(t) can be used as a guide. The functional significance of E(t) in relation to the recruitment order of type S units is discussed.
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