Cortical and segmental excitability during fatiguing contractions of the soleus muscle in humans

Abstract

Objective The aim of this study was to examine the cortical and segmental excitability changes during fatigue of the soleus muscle. Methods Ten healthy young subjects performed 45 plantar flexion maximal voluntary contractions (MVCs) (7-s on/3-s off) in 9 epochs of five contractions. Motor evoked potentials (MEPs) using transcranial magnetic stimulation and H-reflexes were assessed during the task. Results The torque and the soleus EMG activity both showed the greatest decline during the 1st epoch, followed by a gradual, but significant decrease by the end of the task (~70% pre-fatigue). The H-reflex sampled at rest after each epoch decreased to 66.6 ± 18.3% pre-fatigue after the first epoch, and then showed no further change. The MEP on 10% pre-fatigue MVC after each epoch increased progressively (252.9 ± 124.2% pre-fatigue). There was no change in the MEPs on the 3rd MVC in each epoch. The silent period on the MVC increased (109.0 ± 9.2% pre-fatigue) early with no further changes during the task. Conclusions These findings support that the motor cortex increases excitability during fatigue, but with a concomitant inhibition. Significance These findings are in contrast to upper extremity muscles and may reflect a distinct response specific to postural, fatigue-resistant muscle.

Changes in corticospinal excitability during an acute bout of resistance exercise in the elbow flexors

“…In this context, the reduction in CMEPs was due to reduced excitability of spinal motoneurons during a fatiguing task. This finding is consistent with the study of Iguchi and Shields (2012) who also reported decreased spinal excitability of the soleus muscle during intermittent maximal 14 contractions. In both of these studies, increased MEP size was considered to be a compensatory mechanism of the motor cortex, in an attempt to compensate for the reduction in motoneuron excitability.…”

“…This pathway most likely compensates at least part of the effect of supraspinal fatigue (Tanaka and Watanabe 2012;Taylor et al 1996). In the current experiment, the decreased MEP size during the sets could be due to decreased voluntary drive caused by decreased facilitation or increased inhibition of the cortical output neurons, or inhibition of the motoneurons at the spinal level (Butler et al 2003;Iguchi and Shields 2012). Unfortunately, cervicomedullary stimulation, which is thought to be the only valid measure for spinal sensitivity, could not be conducted in this experiment.…”

“…It has been shown that TMS induced MEPs, which manifests as changes in corticospinal excitability, increases following different types of contractions (submaximal, maximal, repetitive or sustained) (Iguchi and Shields 2012;Sacco et al 1997;Taylor et al 2000). Several studies have observed increases in the MEP size following exerciseinduced muscle fatigue (e.g.…”

The H-reflex as an important indicator in kinesiology

“…Other studies have investigated changes in the amplitude of the H-reflex during various types of contractions of the soleus muscle [56,57]. In one study, Duclay and Martin [58] obtained the lowest value of the Hmax/Mmax ratio during the eccentric contraction; whereas the isometric and concentric contractions showed a similar ratio.…”

“…Phadke et al [83] also observed the effect of a 20-minute walk on the decrease in the amplitude of the H-reflex due to the presynaptic inhibition. Studies also show that the amplitude of the H-reflex may return to its resting values as quickly as a minute following exercises on a cycloergometer [57].…”