Motoneurones are nerve cells in the spinal cord that, when they fire, enable us to make muscle contractions and perform movements. Right now, for example, the motoneurones that control your eye muscles are firing as you move your eyes to read this text.
When we exercise or perform strenuous physical work with the muscles of our limbs, many of our motoneurones fire at high rates. If we exercise at high intensities or for long durations, we eventually feel our muscles become tired or fatigued. The nervous system contributes to this fatigue; however, researchers do not completely understand the extent to which motoneurones contribute to the processes and perceptions associated with fatigue.
One hypothesis is that, during fatigue, motoneurones become less excitable (i.e., more difficult to make fire), and consequently, to continue exercising, we have to try harder (give more effort) to activate them. Our study tested this hypothesis (Finn et al. in press). We examined the excitability of motoneurones before and after fatiguing isometric exercise of the knee extensor muscles (quadriceps). We tested the excitability of motoneurones by activating them with electrical stimulation over the thoracic spine and measuring the response in the muscles. We conducted the experiment on two days, each with a different sized baseline response which allowed us to test different proportions of the quadriceps motoneurones. A small response tested what happened to motoneurones that were mostly active in the fatiguing exercise whereas a large response tested a comparatively less active portion of the motoneurone pool. Comparison of the two responses should tell us if the changes in motoneurone excitability during fatigue exercise are more confined to active motoneurones most involved with the exercise, or if the changes during exercise affect the entire pool of motoneurones.
WHAT DID WE FIND?
During the fatiguing exercise, the excitability of the motoneurones was significantly reduced compared to prior to exercise. Most of this reduction in excitability occurred within the first 2 minutes of exercise. The effort reported by participants to complete the exercise increased from 1.5 to ~7 (on a 10-point scale). This increase confirmed that the participants perceived the exercise as fatiguing. Additionally, we found that the reduction in excitability was greatest when we tested the population of motoneurones that were most active during the exercise. This finding suggests that repetitive activation of the motoneurones during the task was important for the reduction of excitability.
SIGNIFICANCE AND IMPLICATIONS
Our study shows that the motoneurones that control the quadriceps muscles become less excitable during fatiguing contractions. That is, they require more input, such as from the brain, to excite them. This reduction in excitability is likely due to a mechanism which is brought about by the repetitive firing of the motoneurones and which leads to changes of the intrinsic properties of the motoneurones. The increase in effort required during the task despite the same motor output suggests that greater input from the motor cortex is required to maintain firing of motoneurones during fatiguing exercise.
Finn HT, Rouffet DM, Kennedy DS, Green S, and Taylor JL. Motoneuron excitability of the quadriceps decreases during a fatiguing submaximal isometric contraction. J Appl Physiol. doi: 10.1152/japplphysiol.00739.2017.If you cannot access the paper, please click here to request a copy.
Harrison Finn is a PhD student at NeuRA and the University of New South Wales. His research focuses on nervous system fatigue during leg exercise. You can follow him on Twitter @HarrisonFinn or on ResearchGate. To read Harrison’s previous blogs, click here.