Motor unit activity in human biceps brachii during sustained contractions
To sustain the force exerted during a voluntary contraction, the nervous system adjusts the output from the spinal cord that is delivered to the muscle. The adjustments involve changing motor unit activity, both the rate at which action potentials are discharged and the recruitment of more motor units. These adjustments can be influenced by the input received by the motor neuron pool from spinal reflex pathways. These mechanisms are responsible for variation in the duration a submaximal contraction can be sustained. This dissertation examined the recruitment and discharge patterns during sustained contractions and the role of one reflex pathway on the discharge of motor units in the biceps brachii muscle. The first study compared the transient recruitment of single motor units recorded with subcutaneous electrodes to the bursts of activity observed in the surface electromyogram. The findings validated previous suggestions that transient recruitment can be detected in the surface electromyogram. The second study examined the mechanisms underlying the transient recruitment of motor units and the associated discharge characteristics. It was demonstrated that the difference between recruitment threshold for a motor unit and the target force during the contraction influences the discharge characteristics of the motor unit when it is recruited during a sustained contraction. The third study described a reflex pathway from brachioradialis afferents that inhibits the discharge of single motor units in biceps brachii during sustained contractions. This study also revealed that the strength of the reflex inhibition varies with forearm position. The fourth study examined the influence of practice on the association between changes in the duration that a fatiguing contraction can be sustained and the strength of the inhibitory reflex. Practice reduced the reflex inhibition of motor units in the short head of biceps brachii, slowed the rate of increase of electromyographic activity in the same muscle, and prolonged the time to failure for the submaximal contraction. The results obtained in this dissertation indicate that the ability to sustain a submaximal contraction depends critically on the adjustments that are made by the nervous system.