Master of Kinesiology (MKin)
Faculty of Science
Dr. Jayne Kalmar
Fatigue during sustained or repeated muscular contractions can be from contractile failure within the muscle or reduced excitability in the supraspinal and spinal motor neurons. However, spinal motor neurons can also compensate for fatigue. We speculate that one way the spinal motor neuron compensates for fatigue is through an increase in excitability via the activation of persistent inward currents (PIC). In other conditions where there is a reduction in descending drive, such as during spinal cord injury and aging, there are adaptions of the spinal motor neurons to have elevated PIC amplitudes or towards a greater prevalence of PIC, respectively. Although this increase may not necessarily be beneficial, it indicates that there is a compensatory increase in PIC during persistent reductions of descending drive. The purpose of this study was to explore how the motor neuron responds to fatigue by examining firing rates, PIC, and motor neuron pool excitability. Twelve participants attended two testing sessions. During one session the participants performed five three-minute sets of intermittent isometric plantarflexion contractions, during the other session the participants sat quietly. In both sessions participants performed maximal voluntary contractions and submaximal contractions for estimating PIC at the start, after each set, and again after fifteen minute of recovery. Using peripheral nerve stimulation, H-reflex and M-wave recruitment curves were elicited at the start, after the final set, and after fifteen minutes of recovery. Motor unit firing rates were assessed during the intermittent fatiguing contractions. Additional analyses were performed to assess whether the activation of the antagonist muscle group during fatigue influenced PIC, whether level of physical activity affected the initial PIC amplitude, and whether there were any sex-differences. There was no significant change in maximal voluntary force over the fatigue protocol, however there was a significant increase in muscle EMG during the fatiguing contractions. There were no significant changes in motor unit firing rates, spinal motor neuron pool excitability, or PIC. Although there was no change in PIC, a secondary analysis was performed to assess whether there was a sex-difference in PIC over the fatigue protocol. There was a significant interaction, indicating that females had a significant increase in PIC during the fatigue protocol, but males did not. There were no other interactions between session, time, and sex for the other measures of fatigue and motor neuron behaviour. Therefore, from these results we conclude that PIC is modulated during fatigue in women, but not men. This finding may contribute to our understanding of the mechanisms leading to greater fatigue-resistance in females. Further research should examine the stability of this relationship during more vigorous fatigue protocols.
Cecire, Nathalie, "Spinal Motor Neuron Excitability During Fatigue" (2019). Theses and Dissertations (Comprehensive). 2142.
Available for download on Monday, March 07, 2022