EXCITATORY DRIVE TO THE ALPHA-MOTONEURON POOL DURING A FATIGUING SUBMAXIMAL CONTRACTION IN MAN

1. This study was undertaken to examine changes of excitatory drive to the triceps surae alpha-motoneuron pool during fatiguing submaximal i sometric contractions in man. Eight healthy subjects maintained isomet ric plantar flexions at 30% of maximum voluntary contraction (MVC) unt il the limit of endurance (range, 6-9 min). 2. Excitability of the alp ha-motoneuron pool to Ia afferent stimulation (H reflex), electromyogr ams (EMG) and maximum compound motor unit action potentials (M(max)) f rom the lateral (LG) and medial heads (MG) of the gastrocnemius as wel l as from the soleus muscle (Sol) were recorded throughout the contrac tion. Superimposed maximum twitch torques (twitch occlusion) and isome tric torque fluctuations (tremor) were also recorded as indirect measu res of excitatory drive. 3. H reflexes were studied at different level s of underlying voluntary contraction to assess the relationship betwe en H reflex amplitude and excitatory drive. With increasing levels of underlying contraction up to MVC, superimposed H reflex amplitude incr eased for LG in six subjects, for MG in all eight and for Sol in five. In the remaining cases, H reflex amplitude first increased and then p lateaued between 30-50% of MVC. 4. H/M(max) ratios increased during fa tigue in those muscles that showed an H reflex amplitude increase with high levels of underlying contraction. In these cases, LG and MG H/M( max) increased significantly after about 50 and 20% of endurance time onward, respectively, whereas Sol H/M(max) demonstrated a significant increase up to 40% of endurance time. 5. EMG root mean square (r.m.s.) increased linearly throughout the contraction for all three muscles, while tremor r.m.s. increased in a non-linear way, with a steeper incr ease from 60% of endurance time onward. Superimposed twitch amplitude decreased significantly front 25% of endurance time onward. 6. It is c oncluded that during fatiguing isometric contractions at 30% of MVC, t he excitatory drive to the triceps surae alpha-motoneuron pool increas es. This is thought to be a compensatory mechanism to facilitate recru itment of new, unfatigued motor units (MUs), and/or to increase MU fir ing rates. The facts that the twitch is not abolished at endurance lim it and that the EMG does not attain its unfatigued MVC level are stron g indications that central fatigue occurred during the sustained subma ximal contraction.