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

The most commonly used stimulus for functional electrical stimulation is lo w frequency, short duration pulsed current. A disadvantage is the associate d rapid fatigue. The present study investigated the effect of kHz frequency alternating currents on the rate of fatigue with electrically induced skel etal muscle contractions in normal subjects. Alternating current with frequ encies 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-mi n period, during which time electrically induced wrist extensor torque was measured. The decline in torque with time was analysed in terms of 'fast' a nd 'slow' fatigue components. A systematic frequency dependence was found i n each. 'Fast' torque decline is interpreted as reflecting fast-fatigue mus cle fibre activity and 'slow' torque decline, that of fatigue-resistant fib res. With this interpretation, over the frequency range 1-10 kHz, the propo rtion 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 cur rent for functional electrical stimulation. (C) 2001 IPEM. Published by Els evier Science Ltd. All rights reserved.