Reflex gain of muscle spindle pathways during fatigue

Abstract

There are conflicting observations of the effects of fatigue on the sensitivity of large diameter Ia afferents. Our goal was to characterize any fatigue-related changes in the spinal reflex pathways during fatigue. Manipulation of the Ia afferent response by vibration and tendon tap, in which the motor neuron pool is modulated by both short- and long-loop activation from muscle spindles, were elicited before and after a fatigue task. The fatigue task consisted of intermittent submaximal and maximal voluntary contractions (MVCs). Percent voluntary activation fell from 98.75% MVC to 80.92% MVC following the fatigue task as measured by the twitch interpolation technique. Voluntary contractions of the same force profile as the force produced by 30 s of vibration were produced by having participants (n = 10) follow the trajectory on a computer monitor, before and after the fatigue task. Recruitment thresholds (RTs) of voluntarily activated units showed no change during fatigue; however, units activated via the reflex pathway were recruited approximately 30% sooner during fatigue (P < 0.05). The ratio of the electrical-to-mechanical response of the tendon tap increased significantly with fatigue. Our findings of decreased RTs in response to vibration and increased EMG activity during the tendon tap following the fatigue task indicate that Ia afferent input to the motoneuron pool was increased. The decrease in MVC force indicates that during this time the descending drive was compromised. These results provide evidence that the gain of the gamma loop is increased during fatigue, indicating possible peripheral neural compensation to the motor neuron pool in order to preserve force output.

Hormonal and Neuromuscular Responses to Mechanical Vibration Applied to Upper Extremity Muscles

“…Thus, the GH response in our study appears to be proportional to the recruited muscle volume relative to the exercise intensity, similar to resistance exercise [9]. Additionally, the increase in EMG activity recorded toward the end of each repetition (Delta) could cause an increase in reflex activation [29], [30], which indicates that the “central command” can be intensified via feed-back signaling from the exercising muscles mediated by the afferent nerves as neural reflex mechanisms [9], [28]. Concisely, it may be possible for higher brain centers (i.e., the motor cortex) to play an active role in regulating GH secretion during WBV and this regulatory mechanism may be sensitive to specific peripheral neural mechanisms.…”

“…However, the increase in EMG activity during the treatments (delta) suggests an increase in reflex activation in which the motor units are recruited at the lowest force thresholds (compared with voluntary activation) to compensate for a reduced descending drive in an attempt to maintain the force output during fatigue [29]. Additionally, the selective attenuation of the maximal muscle EMG activity following WBV is dependent on the acceleration load, which suggests that the afferent input could be dependent on the activated muscle fiber types [30], [32], the different muscle spindle responses to muscle vibration, and the specific sensory receptor population [33], [34].…”

Influence of back muscle fatigue on lumbar reflex adaptation during sudden external force perturbations

“…Interestingly, self pre-activation was not significantly affected by fatigue. Consequently, during repetitive, heavy or prolonged lifting tasks the CNS would preferentially pre-activate an antagonist pair of paraspinal muscles (abdominals and erectors) when the perturbation can be anticipated whereas unexpected conditions require modulation of the reflex loop (Biro et al, 2007). Moreover, during fatigue, the CNS would preferentially optimize the minimization of energy expenditure with a decrease in co-activation and consequently in movement accuracy (Missenard et al, 2008).…”

Neuromuscular fatigue following low-intensity dynamic exercise with externally applied vascular restriction

“…The created detectable twitch contractions by using electrical stimuli during MVC contraction and rest have been used to determine the degree of central activation. The twitch interpolation technique has also been used to examine neuromuscular fatigue (Biro et al, 2007), neural activation strategies , neural resistance training adaptations (Jubeau et al, 2006), and clinical neural deficits associated with diseases (Molloy et al, 2006). Therefore, the simultaneous use of EMG and twitch interpolation may help to evaluate the mechanism(s) responsible for neuromuscular fatigue during VR.…”