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

Area of Science:

• Biomedical Engineering
• Neuroscience
• Exercise Physiology

Background:

• Functional electrical stimulation (FES) commonly uses low-frequency pulsed currents.
• A significant limitation of low-frequency FES is rapid muscle fatigue.
• Alternative stimulation parameters are needed to improve FES efficacy.

Purpose of the Study:

• To investigate the impact of kilohertz (kHz) frequency alternating currents on muscle fatigue during electrically induced contractions.
• To analyze the effects of varying kHz frequencies on skeletal muscle fatigue components.
• To explore the potential of kHz alternating currents as an improved FES stimulus.

Main Methods:

• Transcutaneous application of alternating currents at frequencies from 1 to 15 kHz.
• Stimulation delivered in 3-s surges at 50 Hz with a 1:1 duty cycle for 10 minutes.
• Measurement of electrically induced wrist extensor torque to assess fatigue.
• Analysis of torque decline into "fast" and "slow" fatigue components.

Main Results:

• A frequency-dependent effect on muscle fatigue was observed.
• Higher kHz frequencies (1-10 kHz) showed an increased contribution from fatigue-resistant muscle fibers.
• "Fast" torque decline, attributed to fast-fatigue fibers, decreased with increasing frequency.
• "Slow" torque decline, attributed to fatigue-resistant fibers, increased with increasing frequency.

Conclusions:

• Kilohertz frequency alternating currents demonstrate potential for reducing muscle fatigue in FES.
• The observed effects suggest selective activation or reduced fatigue in fatigue-resistant muscle fibers.
• kHz alternating currents may offer a more effective stimulation strategy for FES applications.