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

ABSTRACT People with Multiple Sclerosis typically exhibit greater levels of exercise-induced fatigue compared to healthy individuals. However, it is unknown if voluntary muscle activation is affected over a range of contraction forces in people with MS who have exercise-induced fatigue. The purpose of this study was to use transcranial magnetic stimulation (TMS) and electrical muscle stimulation to examine muscle activation during exercise-induced fatigue. Ten people with relapsing-remitting MS (39 ± 7 years) and 10 healthy controls (40 ± 5 years) performed elbow flexions at 25%, 50%, 75%, 90%, and 100% MVC while electromyography (EMG) of the biceps brachii was recorded. Sustained elbow flexion MVCs were then performed until force declined to 60% of baseline MVC, and the target contraction intensities of 25%, 50%, 75%, 90%, and 100% MVC were examined again. The Fatigue Severity Scale was higher for the MS group ( P < 0.01). Exercise-induced fatigue caused a reduction in biceps EMG amplitude for the MS group across all contraction intensities ( P < 0.01), which was not aligned with changes in MEP amplitude ( P = 0.25). Exercise-induced fatigue reduced motor cortical voluntary activation in the MS group across all contraction intensities ( P < 0.01), as well as increased MS time-to-peak force ( P < 0.01) and half relaxation time for TMS evoked twitches ( P = 0.03). These findings provide evidence that MS-related fatigability during maximal contractions is due to the inability for the motor cortex to drive the muscle, with possible contributions from altered contractile properties in the MS muscle. NEW & NOTEWORTHY We use transcranial magnetic stimulation to demonstrate that people with relapsing-remitting Multiple Sclerosis (MS) have a reduced ability to activate muscle following maximal effort fatiguing contractions. Although our MS participants reported greater symptoms of fatigue via the Fatigue Severity Scale, their reduced ability to activate muscle was more associated with the duration of disease.