Master of Kinesiology (MKin)
Kinesiology and Physical Education
Faculty of Science
Dr. Jayne Kalmar
Neuromuscular fatigue is associated with reduced supraspinal drive. Similarly, intracortical facilitation and muscle activation are reduced following concussion. Both fatigue and mild traumatic brain injury are associated with increased noradrenergic and serotonergic activity in animal models. Given that monoaminergic-dependent persistent inward currents (PIC) set spinal motor neuron (MN) gain, we speculate that PIC will increase during fatigue to compensate for supraspinal hypoexcitability and that this will be more pronounced in people with concussion. Therefore, the purpose of this experiment was to assess spinal MN excitability during fatigue in people with concussion and healthy controls. 20 participants (10 concussion, average age 22.05 ± 2.25) completed two experimental sessions on two separate days (fatigue and rest, randomized and counterbalanced). On the fatigue day, paired motor unit analysis was used to estimate soleus motor neuron PIC, before, during, and after an isometric plantarflexion fatigue protocol (5 sets of 40, 3s ankle plantarflexion contractions at 50% of maximal voluntary contraction). Excitability of the soleus motor neuron pool was assessed at rest using slopes of the H reflex recruitment curve before and after the protocol. On the rest day, estimates of PIC and H reflexes were made at the same time points, but the fatiguing contractions were omitted. Soleus motor neuron PIC and resting H reflexes in people with concussion were not different from the controls at any time point. When the groups were collapsed, maximum voluntary torque declined to 92.63±8.67% (prd (pth (p=0.026) set of fatiguing contractions, returning to baseline by the end of the fatigue protocol (p=0.562). The slopes of the H reflex recruitment curves did not change. It is likely that increased monoaminergic drive seen during exercise activates soleus motor neuron PIC to enhance motor output. The increased gain provided by PICs may serve to enhance motor output during fatiguing muscle activity.
Compton, Chris, "The fatigue-associated changes in estimates of persistent inward current in human motor neurons" (2018). Theses and Dissertations (Comprehensive). 2082.