Muscle Cramp Susceptibility Increases Following a Prior Muscle Cramp

Abstract

Many athletes develop multiple muscle cramps in the same muscle during exercise. This phenomenon may be due to cramp-induced increases in nervous system excitability. The effect of prior cramps on future cramp risk or how long prior cramps affect cramp risk is unknown. PURPOSE: We tested 2 hypotheses: (1) volitionally-induced muscle cramps will decrease cramp threshold frequency (TF) thereby indicating greater cramp risk and (2) cramp TF would be lower 5 minutes post-cramp but return to baseline after 30 minutes. METHODS: A cross-over, counterbalanced, repeated measure experimental design guided the study. Fifteen subjects capable of volitionally inducing a flexor hallucis brevis (FHB) cramp completed the study (11 males, 4 females: age=25±5 y, height=180.8±12.1 cm, mass=85.0±11.8 kg). On 4 days, we measured cramp TF (the lowest electrical stimulation frequency necessary to induce cramp). On day 1, baseline cramp TF was determined by electrically stimulating the tibial nerve (two square-wave bursts, burst duration=1 s, pulse width=1 ms, pulse delay=1 ms, stimulus intensity=80V). Initial burst frequency was 4 Hz. If cramp was not induced, subjects rested 1 min and frequency was increased by 2 Hz. This process continued until a cramp occurred. Subjects rested 10 minutes, volitionally induced a FHB cramp, and rated the volitionally-induced cramp’s intensity (0=weakest cramp, 100=strongest cramp). On testing days 2-4, subjects first voluntarily induced a FHB cramp and then waited either 5 minutes, 30 minutes, or 60 minutes before cramp TF was reassessed. Volitionally-induced cramp amplitude, cramp duration, and perceived intensity were measured each day to ensure consistency of volitionally-induced cramps. RESULTS: Volitionally-induced cramp amplitude (F3,42=1.1, P=0.37), cramp duration (F3,42=0.7, P=0.54), and perceived cramp intensity (F3,42=1.2, P=0.33) were similar each day. Volitionally-induced muscle cramps significantly decreased cramp TF (F2,25=4.5, P=0.02). Baseline cramp TF (18±6 Hz) was higher than the 5 minute (14±6 Hz), 30 minute (14±5 Hz), or 60 minute TF (14±5 Hz, P<0.05). CONCLUSIONS: Prior muscle cramps increase cramp risk for at least 60 minutes. Clinicians should continue treatment interventions to prevent cramps for >60 minutes to decrease the probability of recurrence.