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.