Motor unit discharge and torque steadiness are reduced in people with multiple sclerosis when performing voluntary muscle contractions

Although people with multiple sclerosis (MS) often report symptoms of muscle weakness and motor fatigue, it is largely unknown how motor unit discharge and force generation are 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 the 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 with healthy controls, the MS group had reduced maximal knee extension torque (-23%, <i>P</i> = 0.042), reduced discharge rate (-0.88 pps, <i>P</i> = 0.003), and increased torque coefficient of variation (CV) (+0.014, <i>P</i> = 0.006) during submaximal baseline contractions. The MS group had a shorter time-to-task failure for sustained MVCs (-19.3 s, <i>P</i> < 0.001), reduced discharge rate (-0.95 pps, <i>P</i> = 0.002), and increased torque CV (+0.039, <i>P</i> < 0.001) compared with controls following the sustained MVCs. Rating of perceived exertion was higher in the MS group compared with controls after the performance of the sustained MVCs (+18%, <i>P</i> < 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.<b>NEW & NOTEWORTHY</b> Multiple sclerosis (MS) is characterized by inflammation and scarring in the nervous system that can cause severe motor dysfunction. This study reveals that discharge rate and torque steadiness are reduced in MS compared with age- and sex-matched healthy controls, which is consistent for regular submaximal contractions and submaximal contractions that are performed in the presence of motor fatigue. The findings highlight consistent impairments in muscle activation that contribute to MS motor dysfunction.