The variation in fatigue rate with frequency using kHz frequency alternating current.

The most commonly used stimulus for functional electrical stimulation is low frequency, short duration pulsed current. A disadvantage is the associated rapid fatigue. The present study investigated the effect of kHz frequency alternating currents on the rate of fatigue with electrically induced skeletal muscle contractions in normal subjects. Alternating current with frequencies between 1 and 15 kHz, interrupted at 50 Hz and applied in 3-s surges with an effective 1:1 duty cycle, was applied transcutaneously for a 10-min period, during which time electrically induced wrist extensor torque was measured. The decline in torque with time was analysed in terms of "fast" and "slow" fatigue components. A systematic frequency dependence was found in each. "Fast" torque decline is interpreted as reflecting fast-fatigue muscle fibre activity and "slow" torque decline, that of fatigue-resistant fibres. With this interpretation, over the frequency range 1-10 kHz, the proportion of fatigue-resistant fibres contributing to the torque increases. It is argued that this is due to selective dropout of fast-fatigue fibres. The findings suggest the potential usefulness of kHz frequency alternating current for functional electrical stimulation.