STIMULATION FREQUENCY ALTERS RATE OF FORCE DEVELOPMENT

Abstract

At sub-tetanic stimulation frequencies, muscles partially relax in the time between consecutive stimuation pulse. It is generally assumed that the tetanic stimulation frequency does not provide enough time for muscle relaxation between pulses. However, it is well known that a short series of relatively high frequency stimulation pulses (e.g. “doublet”) at the beginning of a stimuation pulse train increases the rate of force development (df/dt) relative to a pulse train at the tetanic frequency. We hypothesized that the brief high frequency pulse train acts to prevent relaxation wthin the interpulse interval. Purpose The purpose of this investigation was to determine the effects of stimulation frequency on maximum force and maximum df/dt and to determine if partial relaxation occurred at or above tetanic stimulation frequency. Methods The gastrocnumious of Syrian Golden Hamsters (N=5) was stimulated in situ for 250ms at 30 to 200hz via the sciatic nerve. Maximum force and maximum df/dt were determined for each frequency. Fourier analyses were performed to determine if the stimulation frequency influenced the frequency content of the force or df/dt signal. Results Maximum force increased to a peak at approximately 100hz and declined with further increases in stimulation frequency. Maximum df/dt increased across all frequencies. The stimulation frequency was evident within the df/dt signal at all frequencies. Conclusions Based on these data, we conclude that inter-pulse relaxation of muscle occurs in as little as 5ms in this mixed fiber type muscle (approximately 70% fast twitch). These data support our hypothesis and suggest that high frequency stimulation trains allows higher df/dt by minimizing inter-pulse relaxation during force development. Supported by ALA RG-013N