Multiple features of motor-unit activity influence force fluctuations during isometric contractions

Area of Science:

• Neuroscience
• Biomechanics
• Human Physiology

Background:

• Voluntary muscle contractions exhibit force fluctuations.
• Understanding these fluctuations is key to deciphering motor control mechanisms.

Purpose of the Study:

• To identify the mechanisms underlying force fluctuations during voluntary contractions.
• To compare experimental measurements with simulated forces across a range of contraction intensities.

Main Methods:

• Measured index finger abduction force during isometric contractions of the first dorsal interosseus muscle in 10 adults.
• Simulated force using computer models of motor-unit recruitment and rate coding (120 motor units).
• Analyzed force fluctuations in time and frequency domains, comparing experimental and model data.

Main Results:

• Experimental force variation (coefficient of variation) showed a minimum at ~30% maximum voluntary contraction (MVC) and a plateau thereafter, which models failed to replicate.
• A model incorporating short-term synchrony best approximated the experimental force variation, showing independence from discharge-rate variability.
• Only a model with low-frequency excitation oscillation matched experimental power spectra peaks below 2 Hz, while all models produced a secondary higher-frequency peak absent in experimental data.

Conclusions:

• No single mechanism fully explains force fluctuations across the entire muscle operating range.
• Motor unit synchrony and excitation oscillations appear to be significant contributors to force variability.
• Discharge-rate variability and synchrony interact to influence force fluctuations, but their precise roles require further investigation.