This study aimed to quantify, characterise and localise motor fatigue in people with motor neuron disease (MND) using a combination of clinical assessments, neurophysiology and muscle phosphorus-31 magnetic resonance spectroscopy (31P-MRS).

We quantified fatigability using the fatigue index (FI) in twenty patients and 10 healthy controls. F-wave amplitudes, motor unit number index (MUNIX) and 31P-MRS were acquired pre- and post-maximal voluntary contraction to investigate fatigability at different sites of the motor system (spinal cord, peripheral nerve and muscle, respectively). Between-group differences and associations were assessed using regression models.

There were no between-group differences in FI (p=0.115). MUNIX (p=0.008) and f-wave amplitude (p=0.024) declined significantly post-contraction in controls, but not in patients (MUNIX p=0.284, f-wave p=0.264). FI was associated with resting intracellular magnesium (R=0.869, p=0.001, FDR-corrected) in controls, but not patients. Resting magnesium and post-contraction MUNIX decrease were associated with greater fatigue in controls. A decrease in post-contraction f-wave amplitude was associated with greater fatigue in patients, after accounting for denervation (MUNIX) and magnesium.

There is a differential response to fatigue in MND compared to healthy controls. Fatigability appears related to spinal excitability (f-waves) in patients, whereas peripheral (MUNIX) and muscular (magnesium) components predominate in healthy controls.