Quiet standing: The Single Inverted Pendulum model is not so bad after all

Area of Science:

• Biomechanics
• Human Motor Control
• Robotics

Background:

• The Single Inverted Pendulum (SIP) model, focusing solely on ankle rotations, has been a long-standing paradigm for balance and postural control.
• Recent kinematic analyses reveal significant hip motion during quiet standing, necessitating a more complex model like the Double Inverted Pendulum (DIP).
• Existing DIP models suggest bi-axial active control to minimize Center of Mass (CoM) acceleration.

Purpose of the Study:

• To propose and simulate a novel hybrid control mechanism for the Double Inverted Pendulum (DIP) model of human postural control.
• To investigate an alternative control strategy involving intermittent active feedback to the ankle and passive stiffness at the hip.
• To validate the model's ability to reproduce experimentally observed ankle-hip coordination patterns and CoM acceleration minimization.

Main Methods:

• Development of a DIP model incorporating a Virtual Inverted Pendulum (VIP) to represent the dynamics between the ankle and CoM.
• Implementation of a hybrid control strategy with intermittent active feedback to the ankle joint based on the VIP's state.
• Simulation of the DIP/VIP model under the proposed hybrid control to analyze joint interactions and CoM dynamics.

Main Results:

• The DIP/VIP model successfully reproduced the in-phase and anti-phase oscillatory patterns observed between the ankle and hip joints in experimental studies.
• Simulations demonstrated that the hybrid control mechanism indirectly achieves minimization of CoM acceleration through the interplay of active ankle and passive hip control.
• The model's findings suggest a functional equivalence between the proposed hybrid control and simpler models for specific experimental contexts.

Conclusions:

• The proposed hybrid control mechanism for the DIP model provides a more comprehensive explanation of human postural control than the traditional SIP model.
• Intermittent active ankle control combined with passive hip stabilization effectively explains observed balance dynamics and CoM acceleration minimization.
• While the SIP model is an oversimplification, it remains practically acceptable for studies focusing on CoM oscillations due to its functional correctness in certain scenarios.