It is unclear whether motor fatigability and perceived fatigue share a common pathophysiology in people with multiple sclerosis (PwMS). This cross-sectional investigation explored the relationship between the mechanisms of motor fatigability from cycling and fatigue severity in PwMS. Thirteen highly fatigued (HF) and thirteen non-fatigued (LF) PwMS, and thirteen healthy controls (CON) completed a step-test until volitional exhaustion on an innovative cycle ergometer. Neuromuscular evaluations involving femoral nerve electrical stimulation and transcranial magnetic stimulation were performed every three minutes throughout cycling. One-way ANOVA at baseline and exhaustion uncovered evidence of consistently smaller motor evoked potential (MEP) amplitudes (P = .011) and prolonged MEP latencies (P = .041) in HF, as well as a greater decline in maximal voluntary contraction force (HF: 63 ± 13%; LF: 75 ± 13%; CON: 73 ± 11% of pre; P = .037), and potentiated twitch force (HF: 35 ± 13%; LF: 50 ± 16%; CON: 47 ± 17% of pre; P = .049) in HF at volitional exhaustion. Hierarchical regression determined that fatigue severity on the Fatigue Severity Scale was predicted by prolonged MEP latencies (change in R2 = .389), elevated peripheral muscle fatigability (change in R2 = .183), and depressive symptoms (change in R2 = .213). These findings indicate that MS-related fatigue is distinguished by disrupted corticospinal responsiveness which could suggest progressive pathology, but fatigability from whole-body exercise and depressive symptoms also influence perceptions of fatigue in PwMS.