Improved muscle activation using proximal nerve stimulation with subthreshold current pulses at kilohertz-frequency

Area of Science:

• Neuroscience
• Biomedical Engineering
• Rehabilitation Technology

Background:

• Transcutaneous electrical nerve stimulation (TENS) aids motor function recovery in neurological disorders by external muscle activation.
• Conventional TENS often causes simultaneous muscle fiber recruitment, resulting in twitch forces synchronized with stimulation.

Purpose of the Study:

• To investigate a high-frequency (HF) electrical nerve stimulation mode for less synchronized finger flexor muscle activation.
• To compare HF stimulation with conventional low-frequency (LF) stimulation regarding muscle activation patterns and pain.

Main Methods:

• Delivered clustered, narrower pulses to median and ulnar nerves at 10 kHz or 7.14 kHz (HF mode) and 30/40 Hz (LF mode).
• Matched elicited muscle forces between HF and LF modes for comparison.
• Analyzed force variation, high-density electromyogram (EMG) signals, and stimulation-induced pain.

Main Results:

• The 10 kHz HF mode showed asynchronous action potential cancellations, indicated by lower amplitude and wider duration compound action potentials compared to LF.
• Reduced force fluctuations in the 10 kHz HF mode demonstrated less synchronized motor unit activation.
• Pain levels were not reduced in HF mode, possibly due to higher charge density.

Conclusions:

• HF electrical nerve stimulation recruits nerve fibers asynchronously, mimicking physiological activation patterns more closely than LF stimulation.
• This asynchronous activation is achieved through summations of subthreshold depolarizations.
• The 10 kHz HF mode shows greater potential for achieving physiological muscle activation compared to the 7.14 kHz mode.