The Inverted Pendulum
Standing upright is a continuous act of balance — the human body is an unstable inverted pendulum with its center of mass (COM) high above a relatively small base of support (BOS). Unlike a table with four widely-spaced legs, bipedal stance places the COM at about 55% of body height, supported by feet that are only 25-30 cm apart. The central nervous system must continuously adjust muscle activity to keep the vertical COM projection within the BOS boundaries.
Stability Margin
The stability margin — the distance between the COM projection and the nearest BOS edge — quantifies how close a person is to falling. Wider stances increase the margin; standing on one foot or on tiptoe shrinks it dramatically. This simulation computes the stability margin and shows how external perturbation forces shift the COM toward the BOS boundary. When the margin reaches zero, a corrective step is required to prevent falling.
Balance Strategies
The nervous system employs three hierarchical strategies for perturbation recovery. Small, slow perturbations are handled by the ankle strategy — co-contraction of ankle dorsiflexors and plantarflexors generates torque to rotate the body back. Larger perturbations trigger the hip strategy — rapid hip flexion shifts the COM faster than ankle torque alone. For perturbations exceeding in-place capacity, a protective step widens the BOS. Aging, neurological disease, and sensory loss impair these strategies, increasing fall risk.
Clinical Significance
Falls are the leading cause of injury death in adults over 65, with 30% of community-dwelling elderly falling each year. Quantitative balance assessment — measuring sway area, velocity, and stability margins with force platforms — predicts fall risk better than subjective clinical judgment. Balance training programs that challenge all three strategies reduce fall rates by 30-40% in high-risk populations.