Duration of electrically induced muscle cramp increased by increasing stimulation frequency

Context: Electrically induced muscle cramps (EIMC) do not last long enough to study many cramp treatments. Increasing stimulation frequency lengthens cramp duration; it is unknown which frequency elicits the longest EIMC. Objective: To determine which stimulation frequency elicits the longest EIMC and whether cramp duration and stimulation frequency are correlated. Design: Randomized, crossover. Setting: Laboratory. Participants: 20 participants (12 male, 8 female; age 20.7 ± 0.6 y; height 174.9 ± 1.9 cm; mass 76.6 ± 2.2 kg) with a self-reported history of muscle cramps in their lower extremities within the 6 mo before the study. Interventions: The dominant leg's tibial nerve was percutaneously stimulated with 2-s-duration electrical stimuli trains starting at a frequency of 4 Hz. After 1 min of rest, stimulation frequency increased in 2-Hz increments until a cramp occurred in the flexor hallucis brevis. The stimulation frequency at which a cramp occurred was termed cramp threshold frequency (TF). Cramp duration was determined using strict clinical criteria (loss of hallux rigidity and return of hallux neutral). On the next 4 consecutive days, participants were stimulated at 5, 10, 15, or 20 Hz above TF, and cramp duration was reassessed. Main Outcome Measures: Cramp TF and duration. Results: Cramp TF was 16.9 ± 5.1 Hz. Cramp duration was longer at 15 and 20 Hz above TF (77.9 ± 37.6 s and 69.5 ± 36.9 s, respectively) than at TF (40.8 ± 34.0 s; P <.05). Cramp duration and TF were highly correlated (r =.90). Conclusions: Stimulating at 15 and 20 Hz above cramp TF produces the longest-lasting EIMC.