Higher frequencies do not enhance muscle force during fatigue

During contractions elicited by electrical stimulation before and after a voluntary fatigue task, significantly more force is lost at high versus low frequencies. This phenomenon has been termed 'low frequency fatigue'. The purpose of this study was to determine if progressively increasing the stimulation frequency during contraction would increase overall force output and reduce the rate of fatigue. This technique artificially generated neural activity in order to overcome lost functions of paralyzed, incontinent or sensory impaired persons. The study compared force and EMG parameters during a constant frequency train of 20Hz for 3 minutes to a train that increased linearly from 20Hz to 40Hz over the same time period. Ten subjects participated in this study and performed the two fatigue tests on different days. Six of the subjects returned for a third experiment to determine changes in the force frequency relation after a 3 minute voluntary sustained isometric fatiguing contraction of 45% maximum voluntary contraction (MVC). The force-frequency relationship shifted to the right with fatigue and significantly more force was lost at the low frequency than at the high frequencies. During the fatigue task experiments, average maximal voluntary contraction force (20 Hz: 47.8 ± 5.0N, 20-40 Hz: 53.2 ± 5.2 N) and the starting force at 20 Hz (20 Hz: 22.9 ± 3.1 N, 20-40 Hz: 23.3 ± 2.4 N) did not differ between experiment days. Surprisingly, there was no significant difference in the overall force time integral (20Hz: 3.25 ± 0.04 KNs, 20-40Hz: 3.40 ± 0.05 KNs) for the two stimulation protocols or the peak force at the end of the 3 min evoked contraction (20 Hz: 8.2 ± 1.8 N, 20-40 Hz: 7.6 ± 1.1 N). These data indicate that the force output is highly dependent on the starting frequency and minimally affected by the number of stimulation pulses delivered (20 Hz: 3600 pulses, 20-40 Hz: 5400 pulses) when stimulating within the physiological range.