Muscle cramp susceptibility is associated with muscle-specific modulation of human motoneuron discharge patterns.

Muscle cramps are painful, involuntary contractions thought to arise from altered spinal motoneuron excitability, potentially involving persistent inward currents (PICs). We examined whether cramp susceptibility is associated with altered PIC-related motoneuron behaviour and whether electrically induced cramp modifies these estimates. Twenty-eight healthy, active adults performed triangular plantar-flexor contractions before and after a cramp threshold frequency test designed to evoke cramp in gastrocnemius medialis. Participants were classified as crampers or controls according to cramp test outcome. Contributions of PICs to motoneuron discharge were estimated in soleus and gastrocnemius medialis from high-density surface EMG recordings during triangular contractions decomposed into motor unit spike trains. Discharge rate hysteresis (ΔF), brace height, associated slopes and self-sustained discharge duration characterized PIC-related modulation of motor unit firing. Spinal reflex excitability and centrally mediated neuromuscular responses were assessed using H reflexes and wide-pulse, high-frequency neuromuscular electrical stimulation. At baseline, ΔF in gastrocnemius medialis was lower in crampers than in controls, whereas other PIC-related estimates, wide-pulse, high-frequency-evoked torque, sustained EMG activity, warm-up effect and H-reflex amplitudes did not differ between groups. After the cramp-inducing protocol, ΔF increased in gastrocnemius medialis and self-sustained discharge duration increased in both muscles, while brace height remained unchanged in gastrocnemius medialis but decreased in soleus. Maximal discharge rate in gastrocnemius medialis decreased in crampers but not in controls. These findings indicate that cramp susceptibility is not associated with generalized motoneuronal hyperexcitability, but with subtle, muscle-specific alterations in motoneuron discharge modulation.