Mechanisms underlying the training effects associated with neuromuscular electrical stimulation

Area of Science:

• Neuromuscular Physiology
• Exercise Science

Background:

• Neuromuscular electrical stimulation (NMES) enhances muscle force production.
• The mechanisms of force enhancement via NMES may differ from voluntary exercise.

Purpose of the Study:

• To investigate if NMES recruitment order of motor units differs from voluntary activation.
• To compare motor unit recruitment patterns between NMES and voluntary muscle activation.

Main Methods:

• Comparing Hoffmann reflex (H-reflex) and direct motor (M-response) evoked muscle twitches.
• Assessing the impact of sub-threshold NMES on H-reflex-induced twitch force.
• Analyzing time to peak twitch force as an indicator of motor unit population.

Main Results:

• Sub-threshold NMES preferentially activated faster-contracting motor units during H-reflex tests.
• NMES application led to a quicker activation of specific motor unit populations.
• This suggests a distinct recruitment pattern compared to voluntary muscle activation.

Conclusions:

• NMES preferentially recruits fast-twitch motor units.
• These fast-contracting units are typically engaged during high-intensity voluntary exercise.
• NMES may provide a unique pathway for activating specific motor unit types.