Transition of COM-COP relative phase in a dynamic balance task

Area of Science:

• Biomechanics
• Dynamical Systems Theory
• Human Postural Control

Background:

• Postural control involves complex coordination of multiple body segments.
• Identifying collective variables is crucial for understanding the dynamics of whole-body systems.
• The relationship between center of mass (COM) and center of pressure (COP) is key to maintaining balance.

Purpose of the Study:

• To determine if COM-COP coordination serves as a collective variable in whole-body postural control.
• To investigate the dynamic transitions in COM-COP coordination during a balance task.
• To analyze the characteristics of these transitions, including hysteresis and variability.

Main Methods:

• Utilized a moving platform to perturb balance in the anterior-posterior direction across a range of frequencies (0-3.0 Hz).
• Measured center of pressure (COP) using a force platform and derived center of mass (COM) from 3D motion capture of joint angles (ankle, knee, hip, neck).
• Analyzed COM-COP coordination patterns and joint coordination during sinusoidal support surface translations.

Main Results:

• COM-COP coordination exhibited a non-equilibrium phase transition with hysteresis and increased variability near the transition frequency.
• Ankle-hip coordination transitioned consistently at 0.3 Hz, preceding the COM-COP transition and showing low variability.
• The observed COM-COP transition dynamics are characteristic of critical fluctuations and hysteresis.

Conclusions:

• COM-COP coordination functions as a candidate collective variable for the multi-segmental postural control system.
• The system demonstrates non-equilibrium phase transition dynamics, with COM-COP coupling playing a significant role.
• This research provides insights into the fundamental organizational principles of human balance control.