Motor unit recruitment by size does not provide functional advantages for motor performance

Area of Science:

• Neuroscience
• Motor Control
• Biomechanics

Background:

• Motor unit recruitment by size is a widely accepted principle in motor control.
• The functional benefits of this orderly recruitment for movement performance are often assumed.
• Motor neuron excitability is linked to size and innervation number, potentially predisposing them to size-ordered recruitment.

Purpose of the Study:

• To investigate whether orderly motor unit recruitment by size is inherently beneficial for motor performance.
• To determine if size-ordered recruitment optimizes motor output compared to other recruitment strategies.
• To explore the underlying reasons for the prevalence of size-ordered motor unit recruitment.

Main Methods:

• Utilized evolutionary algorithms to optimize neuromuscular model parameters without prior assumptions.
• Set various criteria for motor performance to guide the optimization process.
• Assessed motor output and performance metrics under different recruitment order scenarios.

Main Results:

• Optimized model parameters aligned with known neuromuscular properties.
• No optimization criterion consistently favored size-ordered recruitment unless imposed by size-excitability links.
• Imposing size-ordered recruitment did not enhance motor performance compared to random recruitment.
• Optimal motor performance consistently required a broad distribution of motor neuron innervation numbers, favoring smaller ones.

Conclusions:

• Orderly motor unit recruitment by size does not inherently offer significant functional advantages for motor control.
• The prevalence of size-ordered recruitment is likely a consequence of optimizing motor functions through a diverse range of motor neuron innervation numbers.
• Motor neuron properties, rather than functional demands, may primarily drive size-ordered recruitment.