Motor unit discharge and torque steadiness is reduced in people with Multiple Sclerosis when performing voluntary muscle contractions.

Abstract

Although people with MS often report symptoms of muscle weakness and motor fatigue, it is largely unknown how motor unit discharge and force generation is affected by the disease. The current study recruited 13 individuals with relapsing remitting MS (age range: 33-56 yr) and age-, sex-, and height-matched controls to perform submaximal isometric knee extensions at 30%, 50%, and 70% of maximal voluntary contraction (MVC). Motor unit discharge properties were extracted from high-density electromyography (HD-EMG) of the vastus lateralis, and coefficient of variation of torque was used to assess torque steadiness. All data were collected before and after a motor fatigue task, which consisted of sustained MVCs that reduced knee extension torque to 60% of baseline MVC. Compared to healthy controls, the MS group had reduced maximal knee extension torque (-23%, p = 0.042), reduced discharge rate (-0.88 pps, p = 0.003) and increased torque coefficient of variation (CV) (+0.014, p = 0.006) during submaximal baseline contractions. The MS group had a shorter time-to-task failure for sustained MVCs (-19.3s, p < 0.001), reduced discharge rate (- 0.95 pps, p = 0.002), and increased torque CV (+0.039, p > 0.001) compared to controls following the sustained MVCs. Rating of perceived exertion was higher in the MS group compared to controls after the performance of the sustained MVCs (+18%, p < 0.001). These findings indicate that reduced motor unit discharge and torque steadiness are a feature of MS regardless of whether submaximal contractions are performed in the presence of motor fatigue.