Manipulation of stimulation frequency to optimize electrically-evoked muscle output

To assess intrinsic neuromuscular properties (e.g., muscle contractility and neuromuscular propagation), neuromuscular electrical stimulation (NMES) is commonly applied over a muscle belly or nerve to evoke muscle responses with limited influence from the central nervous system. Repetitive, intermittent electrically-evoked muscle contractions are used to evaluate fatigability (force loss) of a single muscle or muscle group. Moreover, NMES is used in clinical rehabilitation, primarily to limit muscle atrophy for people with a comprised ability to perform voluntary contractions. Although NMES has been clinically validated, stimulation parameters, such as pulse-width, duty-cycle, and frequency, can be optimized to increase the amount of ‘work’ completed during the stimulation protocol. To limit discomfort (i.e., generate the desired muscle force with a relatively low current), repetitive NMES is typically applied at a frequency ≥30Hz. However, this approach could be detrimental to muscle output as higher frequencies are purported to exacerbate force loss to a greater degree than low frequencies. In a clinical setting, greater time-under-tension should enhance the benefits of NMES, which means that high-frequency stimulation could undermine its efficacy. Additionally, as high-frequency (i.e., ≥30Hz) NMES involves non-physiological activation rates that are two- to three-fold higher than those of the central nervous system (CNS), its utility in assessing fatigability within a research setting may be limited. Therefore, to better inform future application of the technique, further investigation is needed to determine the optimal stimulation frequency for minimizing force loss in settings using NMES. Accordingly, the studies of this dissertation systematically manipulated stimulation frequency to characterize muscle fatigability, with the aim of limiting force loss from intermittent NMES. Even when frequency was ~1 Hz higher, a lower stimulation frequency better attenuated force loss compared to higher frequencies. Furthermore, the results indicate that a shorter interpulse interval, rather than the total number of pulses delivered, is the primary determinant of the greater evoked force decline observed at high vs. low stimulation frequencies. Future work should aim to implement the current findings in a clinical setting in an effort to reduce force loss and improve time-under-tension for individuals relying on NMES for health-related benefits.