Postural maintenance during movement: simulations of a two joint model

Area of Science:

• Biomechanics
• Motor Control
• Human Physiology

Background:

• Voluntary upper body movements in standing individuals are typically preceded by anticipatory postural adjustments (APAs) in the lower body.
• The prevailing hypothesis suggests APAs counteract center of gravity perturbations caused by arm movements.

Purpose of the Study:

• To investigate the functional roles of anticipatory postural activity during rapid arm movements using computational modeling.
• To explore different neurological control strategies for maintaining postural stability.

Main Methods:

• Developed a computational model of a double-joint system (shoulder and ankle) with idealized antagonistic muscle pairs.
• Modeled muscles as nonlinear viscoelastic elements with proprioceptive feedback.
• Simulated various control strategies including cocontractions, stretch reflexes, and anticipatory/synchronous postural activities.

Main Results:

• No simulated postural maintenance strategy fully eliminated the initial backward body thrust.
• A significant destabilizing perturbation was identified as a subsequent forward sway, posing a risk of falling.
• Anticipatory extensor silencing followed by brief activation (descending control) and synchronous reflex activity (feedback control) proved most effective.

Conclusions:

• The combination of descending and feedback control mechanisms is crucial for inhibiting forward sway and maintaining stability during arm movements.
• This strategy aligns with existing experimental evidence on human postural control.