Muscle cramp susceptibility increases following a volitionally induced muscle cramp

Introduction: Muscle cramping may increase peripheral nervous system excitability. It is unknown if, and how long, cramp susceptibility is affected by previous cramping. We tested whether volitionally induced muscle cramps (VIMCs) lowered cramp threshold frequency (TF_c) and how long TF_c was affected post-VIMC. Methods: Fifteen cramp-prone participants volitionally induced a flexor hallucis brevis (FHB) cramp on 4 separate days. FHB TF_c was measured before VIMC (i.e., baseline) and 5, 30, and 60 min post-VIMC. VIMC electromyography (EMG) amplitude, VIMC duration, and perceived VIMC intensity were measured to ensure consistency of VIMC between days. Results: VIMC EMG amplitude, duration, and perceived intensity were similar between days (P > 0.05). VIMC lowered TF_c; baseline TF_c (18 ± 6 Hz) was higher than 5-min (14 ± 6 Hz), 30-min (14 ± 5 Hz), and 60-min TF_c (14 ± 5 Hz; P < 0.05). Discussion: Acute VIMCs increase cramp susceptibility. Clinicians should apply treatments for at least 60 min postcramp to decrease the probability of cramp recurrence. Muscle Nerve 56: E95–E99, 2017.