Frequency Matters: The Influence of Stimulation Frequency on Force Loss for Human Females and Males

Most studies applying repeated neuromuscular electrical stimulation (NMES) to assess intrinsic contractile properties employ frequencies considerably greater than the mean motor unit discharge rate (MUDR) for a given force level. It is hypothesized that force loss increases with stimulation frequency, but this has not been evaluated in the same pool of participants when other parameters are unchanged. Furthermore, there is a paucity of research investigating possible sex-based differences for force loss during an NMES protocol, with the presence or absence of a group difference seemingly dependent on stimulation frequency. To address these limitations, we compared force loss of electrically evoked contractions at (10 Hz), slightly above (15 Hz), and well above (30 Hz) the expected mean MUDR of the quadriceps at 25% maximal voluntary force.

On three separate occasions, 24 participants (12 females) received 3 min of intermittent NMES (10, 15, or 30 Hz) over the quadriceps of the dominant leg.

Force impairment increased with NMES frequency (19.8 ± 14.5, 42.6 ± 8.1, and 52.9 ± 4.7 for 10, 15, and 30 Hz, respectively), with no significant differences between sexes. Relative to the start of each task, the rates of force development (RFD) and relaxation (RFR) slowed markedly during the 10-, 15-, and 30-Hz fatiguing protocols (RFD: 42.1 ± 13.5, 61.6 ± 13.2, and 75.9 ± 9.8; RFR: 38.0 ± 13.9, 64.2 ± 9.1, and 80.4 ± 5.0, respectively). RFD impairment was less at 10 compared with 15 and 30 Hz, whereas the slowing of RFR increased with NMES frequency. Post-hoc analysis revealed no sex-based differences at any time point for RFD or RFR.

These findings underscore the impact of stimulus frequency on muscle fatigability and highlight a lack of sex-based differences for electrically evoked force loss, emphasizing the need for appropriate frequency selection in NMES protocols.