A spinal pathway between synergists can modulate activity in human elbow flexor muscles

Abstract

Electrical stimulation of the brachioradialis branch of the radial nerve has been shown to inhibit the discharge of voluntarily activated motor units in biceps brachii during weak contractions with the elbow flexor muscles. The purpose of the present study was to characterise the inhibitory reflex by comparing its strength in the short and long heads of the biceps brachii and examining the influence of forearm position on the strength of the reflex. Spike-triggered stimulation was used to assess the influence of radial nerve stimulation on the discharge of single motor units in the biceps brachii of 15 subjects. Stimulation of the radial nerve prolonged the interspike interval (P < 0.001) of motor units in the long (n = 31, 4.8 ± 5.6 ms) and short heads (n = 26, 8.1 ± 12.3 ms) of biceps brachii with no difference between the two heads (P = 0.11). The strength of inhibition varied with forearm position for motor units in both heads (n = 18, P < 0.05). The amount of inhibition was greatest in pronation (7.9 ± 8.9 ms), intermediate in neutral (5.8 ± 7.1 ms), and least in supination (2.8 ± 3.4 ms). These findings indicate that the inhibition evoked by afferent feedback from brachioradialis to low-threshold motor units (mean force 3-5% MVC) in biceps brachii varied with forearm posture yet was similar for the two heads of biceps brachii. This reflex pathway provides a mechanism to adjust the activation of biceps brachii with changes in forearm position, and represents a spinal basis for a muscle synergy in humans.

Reflex Inhibition in Human Biceps Brachii Decreases With Practice of a Fatiguing Contraction

“…As observed in a previous study (Barry et al 2008), the strength of the inhibitory inputs from brachioradialis afferents was similar for the two heads of the biceps brachii, at least before the three practice sessions. However, there was a greater reduction in the strength of reflex inhibition to the short head of biceps brachii after practice.…”

“…The decrease in the strength of reflex inhibition presumably slowed the decline in discharge rate and delayed the recruitment of additional motor units (Carpentier et al 2001;Mottram et al 2005). Consistent with this interpretation, the time to failure for a sustained submaximal contraction is longer when the forearm is supinated and the strength of inhibition from afferents in the radial nerve innervating brachioradialis is lower (Barry et al 2008;Rudroff et al 2005). The spinal mechanisms responsible for the decrease in motor-unit activity in biceps brachii most likely involves an oligosynaptic pathway that produces an inhibitory postsynaptic potential in the motor neurons (Naito et al 1996).…”

“…The task involved isolating a single motor unit that could be discriminated on-line (Time-Amplitude Window Discriminator, Bak Electronics, Mount Airy, MD) and used to trigger a stimulus pulse or control pulse randomly within an interval of 1.5-2 s. The stimulation was delivered 30 ms after a selected motor-unit discharge (Barry et al 2008) at an intensity of 0.9 times brachioradialis MT, and an average of 77 Ϯ 26 triggers were recorded for each motor-unit trial. Control pulses (no stimulation) were recorded to demonstrate that there was no effect due to differences in discharge rate.…”

“…One pathway that might contribute to the prolongation of time to failure is an inhibitory reflex from the branch of the radial nerve that innervates brachioradialis onto biceps brachii motor neurons (Naito et al 1996;Barry et al 2008). Activation of this inhibitory pathway decreases the discharge rate of biceps brachii motor units during isometric contractions with the elbow flexor muscles.…”

“…Activation of this inhibitory pathway decreases the discharge rate of biceps brachii motor units during isometric contractions with the elbow flexor muscles. The strength of the inhibitory reflex onto biceps brachii motor neurons depends on the position of the forearm with greater inhibition when the forearm is neutral compared with supinated (Barry et al 2008), which might explain why the time to failure for a sustained elbow flexor contraction is longer when the forearm is in a supinated position than when it is in a neutral position (Rudroff et al 2005(Rudroff et al , 2007. A suppression of this inhibitory pathway with practice might attenuate the decline in the discharge rate of motor units, delay the recruitment of new motor units, and prolong the time to task failure.…”