Neuromuscular adaptations to muscle fatigue during submaximal isometric contractions in man

Abstract: Neuromuscular Adaptations to Muscle Fatigue during Submaximal Isometric Contractions in Man Wolfgang N. Löscher Doctoral thesis from the Department of Neuroscience, Karolinska Institute,Stockholm,Sweden . During a sustained contraction, the force producing capacity of a muscle gradually decreases. Inorder to maintain a constant force output, the neuromuscular system has to adapt to the contractile fatigueprocess. The aim of the present work was to study these adaptations during short- and long-lastingcontractions by analysing voluntary and reflex myoelectric activities, muscle twitch responses and forcetremor. During brief isometric handgrips, a linear relationship between the amplitude of the surfaceelectromyogram (EMG) and the force output was found, while tremor increased initially and then decreasedat high contraction levels. Both EMG and tremor changes reflected effort related modulations of motor unitrecruitment and motor unit firing rates. The maximal voluntary and involuntary EMG amplitude was reduced at decreased gastrocnemiusmuscle lengths. This reduction originated from either an impairment of neuromuscular transmission orchanges in the geometry of muscle fibres, the latter resulting in a reduced number of fibres located withinthe pick-up volume of the recording electrode. During isometric contractions of the triceps surae, sustained at 30% of a maximum voluntarycontraction until endurance limit, the net excitatory drive to the a-motoneuron pool increased, as assessedby twitch occlusion and H-reflexes. In parallel, EMG and tremor amplitude increased, which appeared topredominantly result from recruitment of unfatigued motor units and bursting EMG activity. The increasedexcitatory drive was found to additionally modulate Renshaw interneurons, as recurrent inhibition of soleusa-motoneurons, evaluated by paired H-reflexes, was reduced after ten minutes of a fatiguing plantar flexion. Toward endurance limit of a long-lasting submaximal plantar flexion, tremor amplitude increased ina non-linear fashion. When la-afferent input to the a-motoneuron pool was reduced by either tendonvibration or an ischemic block, the tremor increase was significantly diminished. This demonstrates thesignificance of stretch reflex activity for the enhancement of tremor. It appears that la-afferent input triggersoscillations in the stretch reflex arc during a fatiguing contraction, which result in bursting EMG activityand augmented tremor. At endurance limit, EMG and twitch occlusion results indicated that central fatigue had occurred, ascomplete muscle activation was not achievable. However, after one minute of electrical stimulation to thesame torque level, subjects were able to voluntarily continue the contraction and finally achieve full muscleactivation. As the electrical stimulation allowed for muscle spindle, supraspinal and motoneuronal recovery,while metabolic stress and contractile fatigue continued, it appeared that central fatigue was not caused bymetabo-receptor or nociceptor mediated peripheral reflex inhibition of the a-motoneuron pool. In conclusion, during a long-lasting submaximal constant-force contraction of the triceps surae, theneuromuscular system adapts to contractile fatigue mainly by recruitment of new motor units, facilitated byan increased excitatory drive to the a-motoneuron pool and by a reduction of Renshaw inhibition. Theincrease in excitatory drive seems to additionally increase the probability of oscillations in the stretch reflexloop, resulting in enhancement of tremor during fatigue. Despite this increase, excitatory drive fails to reachits maximum at endurance limit. Thus, central fatigue occurs, which appears to be independent of peripheralreflex inhibition, but due to fatigue at the muscle spindle, the motoneuronal and/or the supraspinal level.Key words. isometric, electromyography, tremor, H-reflex, twitch, motoneuron, recruitment, la-afferents,recurrent inhibition, central fatigue, excitatory drive, gastrocnemius, soleus, triceps surae, tendon vibration,ischemia, Renshaw cellStockholm 1995 ISsN: 91-628-1757-4