People with multiple sclerosis have reduced TMS-evoked motor cortical output compared with healthy individuals during fatiguing submaximal contractions

Abstract

By combining TMS and motor nerve stimulation during a low-intensity exercise task, we were able to uncover the contribution that different levels of the CNS have during fatiguing exercise in PwMS. Our findings are novel and revealed that PwMS experienced decreased voluntary drive from the motor cortex during a low-intensity sustained fatiguing task that was associated with heightened levels of performance fatigability.

Voluntary muscle activation in people with Multiple Sclerosis is reduced across a wide range of forces following maximal effort fatiguing contractions

“…Interestingly, the mechanism underlying reduction in MS muscle activity may not be associated with excitability of neurons in the motor pathway, as there was no change in MEPs during the contractions performing with, or without, the exercise intervention. This aligns with previous investigations that report people with MS have similar corticospinal excitability to healthy individuals during fatiguing exercise (5,12,21,22). Notably, previous studies of the upper limb showed that a fatiguing contraction (sustained MVC (5,21) and sustained low-intensity ( 12)) has similar effect on the MEP area in both fatigued MS (FSS > 4) and healthy controls.…”

“…The findings in the current study support previous work where individuals with MS exhibited a reduction in biceps brachii EMG during a fatiguing contractions during a brief maximal contraction of the elbow flexors (12) and knee extensors (9). Interestingly, the mechanism underlying reduction in MS muscle activity may not be associated with excitability of neurons in the motor pathway, as there was no change in MEPs during the contractions performing with, or without, the exercise intervention.…”

“…We recently used transcranial magnetic stimulation (TMS) and motor nerve stimulation to examine performance fatiguability during prolonged low-intensity contractions of the elbow flexors (15% of maximal voluntary contraction [MVC] for 26 min). In doing so, we revealed that superimposed twitch responses, and hence voluntary activation of muscle, was compromised in relapsing-remitting MS compared to healthy controls (12). Not surprisingly, a strong association existed between higher scores in the clinically validated fatigue severity scale (FSS) and reductions in voluntary activation of muscle following prolonged low-intensity contractions in people with MS.…”

“…All participants were asked to refrain from any form of CNS stimulant or depressant such as caffeine, alcohol, or moderate-to-high intensity exercise for twelve hours before testing. A priori statistical analysis using effect sizes based off group differences seen in neuromuscular activation (12) and was conducted in G*Power (v3.1.9.7) to determine the adequate sample size. To achieve an alpha of 0.05 and a power of 0.8 for 2 groups with a repeated measures ANOVA, 20 subjects were required to participate in the study.…”

“…For isometric elbow flexions, the performance of a maximal contraction that reduces force to 60% of an unfatigued MVC can reduce the ability of activate the muscle by up to 14% (14). Given that people with MS can have a reduced ability to activate muscle even before undertaking exercise (12), it is likely that fatiguing maximal contractions will inflict a significant challenge on the MS motor system (15). Functional magnetic resonance imaging has revealed that intracortical activity is lower in MS than healthy individuals during and following maximal fatiguing contractions (7).…”