47 An acute session of high-frequency, low-intensity, wide-pulse electrical stimulation evokes fatigue adaptations in an intrinsic foot muscle

Background Compromised intrinsic foot muscle strength and tonic control underlie a variety of common over-use injuries associated with the foot and ankle complex. The abductor hallucis (AH) muscle plays an important role in the support of the longitudinal arch. Due to its superficial location, neuromuscular electrical stimulation represents a viable option as a strengthening modality. Objective To investigate the effects of an acute session of high-frequency, low-intensity wide-pulse electrical stimulation (WPS) applied to AH on peripheral muscle excitability and contractility. Design Within-day-repeated-measures design comprising a WPS intervention session. Setting Laboratory. Participants Nine healthy male volunteers free from any orthopaedic, neurological or vascular pathology identified as exclusion criteria. Intervention Participants received 24 × 15-sec of 2-sec alternating WPS (20Hz-100Hz-20Hz), interspersed with 45-ses rest, to the motor point area of their right AH. Square wave (1 msec) pulses (400 V) were delivered (constant current) at a stimulation intensity of 150% motor threshold. Main outcome measurements Medial plantar nerve stimulation at 130% maximal Mwave (Mmax130) was performed at PRE, POST and 30 min after (RET) WPS. Dependent variables were: latency (ms), peak amplitude (mV), area (mV•s-1) and duration (a.u; Mmax130 area/P-P Mmax130) of Mmax130; and electromechanical delay (EMD; ms), peak (pTw; N) and half relaxation time (HRT; ms) of the evoked twitch force. Results At POST, an increased latency (p < 0.01), area (p < 0.05) and duration (p < 0.0001) were observed in Mmax130. Latency and duration were still significantly elevated at RET (p < 0.01 and p < 0.0001, respectively). Also, HRT was significantly increased at POST (p < 0.01) and RET (p < 0.05). Conclusions In healthy persons, an acute intervention of WPS induces lasting peripheral fatigue in AH muscle, which is required for gains in muscle strength in response to training. The clinical implication of our findings is that WPS appears to be a worthwhile rehabilitation modality for persons with suboptimal neuromuscular foot function.