Author(s): POPESCO, T., DOS SANTOS, R., CASSOLI, I., TRAJANO, G.S., PEARCEY, G.E.P., MAFFIULETTI, N.A., DONNELLY, C., KAYSER, B., PLACE, N., Institution: UNIVERSITY OF LAUSANNE, Country: SWITZERLAND, Abstract-ID: 1914

INTRODUCTION:
Muscle cramps are painful, involuntary contractions affecting 30–50% of healthy individuals with unclear pathophysiology (Schwellnus, 2007). While dehydration theories have historically dominated, emerging evidence implicates a dysregulation of spinal motoneuron excitability (Minetto et al., 2011), potentially involving persistent inward currents (PICs). This study examines whether cramp susceptibility is associated with enhanced PIC-related motoneuron activity and whether an electrically-induced cramp acutely modifies these estimates.
METHODS:
Twenty-eight healthy adults performed voluntary triangular contractions of the plantar flexors before and after a cramp threshold frequency test designed to evoke a cramp in the gastrocnemius medialis (GM). Participants were classified as crampers (cramp elicited) or controls (no cramp elicited). Contributions of PICs to motoneuron discharge were estimated in soleus (SOL) and GM muscles from high-density surface electromyographic (EMG) signals that were decomposed into motor unit spike trains. Maximal discharge rate was used as an index of overall motor unit firing capacity and net excitatory drive. The paired motor unit analysis technique assessed discharge rate hysteresis (ΔF); brace height and related slopes assessed nonlinearity of the ascending discharge rate profile; and self-sustained duration (SSD) assessed motor unit discharge prolongation. Spinal reflex excitability (H reflex) as well as the torque and EMG responses to wide-pulse high-frequency neuromuscular electrical stimulation (WPHF NMES) were compared between both groups.
RESULTS:
At baseline, ΔF in GM was lower in crampers than in controls, whereas the other PIC-related estimates were not different between groups. No differences were found in WPHF-evoked torque, sustained EMG activity, or H-reflex amplitudes between crampers and controls. In both groups, following the cramp-inducing protocol, ΔF increased in GM and SSD increased in both muscles, whereas brace height remained unchanged in GM but decreased in SOL. In addition, maximal discharge rate in GM increased in controls but not in crampers.
CONCLUSION:
These findings suggest that cramp susceptibility is not associated with generalized motoneuronal hyperexcitability but with an altered, subtle muscle-dependent modulation of motoneuron discharge dynamics. Cramp induction appears to further modulate PIC contributions to discharge patterns in the targeted muscle, with distinct responses observed in synergistic motoneuron pools.

Minetto, M. A., Holobar, A., Botter, A., Ravenni, R., & Farina, D. (2011). Mechanisms of cramp contractions: Peripheral or central generation? The Journal of Physiology, 589(Pt 23), 5759–5773.
Schwellnus, M. P. (2007). Muscle cramping in the marathon: Aetiology and risk factors. Sports Medicine (Auckland, N.Z.), 37(4–5).