Does a two-element muscle model offer advantages when estimating ankle plantar flexor forces during human cycling?

Area of Science:

• Biomechanics
• Motor Control
• Computational Neuroscience

Background:

• Traditional Hill-type muscle models often underestimate joint torques in rapid, high-force tasks.
• This discrepancy may stem from models failing to differentiate motor unit types.

Purpose of the Study:

• To investigate if accounting for different motor unit types in muscle models improves force reproduction.
• To determine if multi-element muscle models reduce the required muscle excitation for simulations.

Main Methods:

• Forward simulations of ankle mechanics during cycling at varying cadences (60-140 RPM).
• Comparison of a 1-element Hill-type model with a 2-element model incorporating slow and fast motor units.
• Systematic variation of total excitation and distribution between elements in the 2-element model.

Main Results:

• The 2-element model required significantly less total excitation to match forces generated by the 1-element model.
• At 140 RPM, excitation demand was reduced by 23% using the 2-element model.
• Model performance improved with higher cadences.

Conclusions:

• A 2-element muscle model, differentiating motor unit properties, enhances apparent muscle strength in simulations.
• This approach can improve the fidelity of muscle-driven simulations for tasks with dynamic mechanical demands.
• Future models should incorporate motor unit heterogeneity for greater accuracy.