Changes in the nervous system during fatiguing exercise of the lower limb

During exercise there is a decline in muscular force and performance. Some of this decline is due to exercise-induced changes in the nervous system that reduce the ability to drive the muscle; this is termed central fatigue. The extent to which these changes occur is less known for the lower than the upper limbs. In particular, changes in motoneurone excitability during fatiguing exercise are not well described. This thesis aimed to investigate two possible contributors to reduced excitability of motoneurones innervating the quadriceps muscles, repetitive activation (Chapter 2) and group III/IV afferent feedback (Chapter 3). Motoneurone excitability was assessed during ongoing voluntary drive, and in the silent period that follows a single pulse of transcranial magnetic stimulation (TMS). Chapter 2 showed that during a sustained submaximal contraction, motoneurone excitability of the quadriceps decreased. This decrease was only seen when motoneurone excitability was tested in the silent period. Also, the decrease was greater when the pool of motoneurones tested were proportionally more active during the contraction. This suggests that repetitive motoneurone activation contributes to reduced excitability. Chapter 3 showed that the firing of group III/IV muscle afferents arising from the knee extensors reduced quadriceps motoneurone excitability, as did feedback from the knee flexors of the same limb. Again, these decreases were only seen when tested in the silent period after TMS. When tested during ongoing drive, motoneurone excitability was increased and this increase was postulated to be due to compression of motoneurone recruitment. Chapter 4 assessed the ability to maximally drive the quadriceps motoneurones during group III/IV afferent feedback from the plantarflexors. The firing of these afferents caused a reduction in quadriceps force and voluntary activation. Thus, fatigue of the calf muscles can impair the ability to drive quadriceps motoneurones voluntarily. In conclusion, this thesis shows that human quadriceps motoneurones become less excitable during fatiguing exercise, likely through intrinsic and afferent mechanisms. Impaired voluntary drive to quadriceps motoneurones can also reduce muscle output. Of note, changes in excitability were clearly shown only when tested in the silent period after TMS suggesting that ongoing drive may confound measures of excitability during contractions