Modeling of postural stability borders during heel-toe rocking

Area of Science:

• Biomechanics
• Neuroscience
• Human Movement Science

Background:

• Maintaining balance requires the central nervous system (CNS) to generate muscle forces counteracting destabilizing torques from gravity and inertia.
• A potential trade-off exists between movement frequency and amplitude, influencing balance control strategies.

Purpose of the Study:

• To investigate the relationship between movement frequency and amplitude during heel-toe rocking.
• To examine how the CNS adjusts muscle forces and joint stiffness to maintain stability at different frequencies.

Main Methods:

• Experiments involving 15 participants performing heel-toe rocking at 0.33 Hz and 0.66 Hz with maximum amplitude.
• Mathematical modeling using an inverted pendulum model with a rotational spring to predict ankle stiffness and center of gravity (COG) position.
• Measurements of whole-body center of gravity (COG) and center of pressure (COP) positions, and ankle torque.

Main Results:

• Doubling the rocking frequency reduced the maximum anterior COG position by 11% and increased the minimum anterior COG position by 8%.
• Ankle stiffness significantly increased with frequency (from 787 to 1625 Nm/rad).
• Peak ankle torque and COP positions remained consistent across frequencies, suggesting torque allocation adjustments.

Conclusions:

• COG amplitude decreases as movement frequency increases during heel-toe rocking.
• This reduction is attributed to the fixed peak ankle torque and the need to allocate torque between gravitational and inertial demands, which increase with the square of frequency.