Background: Multiple sclerosis (MS) is an autoimmune demyelinating disease that disrupts corticospinal functioning. Transcranial magnetic stimulation (TMS) of the motor cortex allows assessment of both corticospinal excitability, inhibition, and conduction time via recordings of motor evoked potentials (MEPs) and silent periods (SPs), respectively. A change indicates a central origin of neuromuscular fatigue. Research has shown that corticospinal integrity is impaired at rest in MS. This study investigated how sustained isometric exercises of the wrist extensors affects corticospinal responses in people with relapsing-remitting MS compared to healthy controls.
Methods: 22 people with relapse-remitting MS (3 males; Expanded Disability Status Scale < 3.5) and 22 healthy matched controls (CG) performed an all-out 3-minute effort, and 8 minutes later, a 5-minute submaximal effort (17.2 ± 6.6 % Maximal Voluntary Contraction (MVC)). Active motor threshold (AMT) was quantified as the minimum stimulator output to evoke 3/6 MEP > 0.5mV at 20% MVC. Ten single-pulse TMS (120% AMT) were delivered to the primary motor cortex during a 10% MVC, before the 3-minute all-out effort, immediately after, and 9 minutes following the 5-minute submaximal exercise bout. Surface electromyography was recorded from the extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB) muscles. MEP amplitude, latency, and silent period (SP were measured for each MEP. Student t-tests and linear mixed models were used to assess group, time, and interaction effects. Critical p value was set at 0.05. Ethical approval was obtained by the Health Research Authority (IRAS: 260176).
Results: AMT was significantly higher in the MS group (MS: 55 ± 10 % of maximum stimulator output (MSO); CG: 47 ± 8 % of MSO (t(21) = -3.66, p = .001). Averages and standard deviations for both groups at each time point for all variables can be viewed in Table 1. A significant effect of time was found for ECRB SP only (F(2, 70.7) = 3.77, p = .02). MEP latency and SP were overall longer and absolute ECRB MEP amplitude smaller for the MS group (ECRL latency: F(1, 29.3) = 8.51, p = .007; ECRL SP: F(1, 20.3) = 4.67, p = .04; ECRB latency: F(1, 37.6) = 6.79, p = .01; ECRB SP: F(1, 24.2) = 7.08, p = .01; ECRB MEP amplitude: F(1, 36.7) = 7.82, p = .01). No group x time interactions were observed (p > .05).
Conclusion: These findings support the notion of impaired corticospinal function in MS, as evidenced by the higher AMT, longer SP, delayed MEP latencies and smaller MEP amplitudes in the MS group both at rest and following exercise of the wrist extensors. Exercise did not induce changes in corticospinal excitability or inhibition exclusive to the MS group.
