Metabolic cost underlies task-dependent variations in motor unit recruitment

Area of Science:

• Biomechanics
• Muscle Physiology
• Computational Modeling

Background:

• Mammalian skeletal muscles comprise motor units with slow and fast-twitch fibers.
• The 'size principle' typically recruits slower units for low-intensity contractions, which is metabolically efficient but poses a paradox for fast, low-intensity movements.

Purpose of the Study:

• To evaluate a novel, mixed cost function in musculoskeletal simulation to predict muscle fiber type recruitment.
• To determine if this cost function accounts for metabolic cost and matches fiber recruitment to contraction speed and load.

Main Methods:

• Utilized a musculoskeletal simulation incorporating a mixed cost function that includes metabolic contraction costs.
• Evaluated the model's ability to predict muscle fiber recruitment across various loads and contraction speeds.

Main Results:

• The metabolically informed cost function successfully predicted the favored recruitment of slower muscle fibers for slower/isometric tasks.
• It also predicted the favored recruitment of faster muscle fibers for higher velocity contractions.
• The model's predictions align with experimental observations and suggest reduced metabolic cost.

Conclusions:

• Varying motor unit recruitment strategies based on movement demands (speed and load) is mechanically and metabolically advantageous.
• This approach optimizes the deployment of different motor unit types for task-specific efficiency.
• The findings support a more nuanced understanding of muscle recruitment beyond the traditional size principle.