The Walking Cycle
Human walking is a remarkably efficient form of locomotion, evolved over millions of years of bipedal adaptation. Each gait cycle — from heel strike to the next heel strike of the same foot — divides into stance phase (foot on ground, ~60% of cycle) and swing phase (foot in air, ~40%). During walking, there is always at least one foot on the ground, with brief double-support periods at transitions. This distinguishes walking from running, which includes an aerial phase.
The Inverted Pendulum
Biomechanists model walking as an inverted pendulum: the body vaults over the stance leg, converting kinetic energy to potential energy and back. This passive exchange recovers up to 65% of the mechanical energy, making walking metabolically cheap at about 3 J/kg/m. The optimal speed for this mechanism is predicted by the Froude number Fr = v²/(gL), with the walk-run transition occurring near Fr ≈ 0.5 — a universal scaling law that applies from toddlers to elephants.
Ground Reaction Forces
Every step generates a ground reaction force (GRF) equal and opposite to the forces the body exerts on the ground. In walking, the vertical GRF follows a characteristic M-shaped double peak: the first peak (~1.1 BW) at heel strike from impact deceleration, a valley at midstance as the COM rises, and a second peak (~1.1 BW) at push-off. This simulation visualizes the GRF curve in real time as you adjust walking parameters.
Clinical Applications
Instrumented gait analysis — combining force platforms, motion capture, and electromyography — is the gold standard for diagnosing movement disorders. Children with cerebral palsy, stroke survivors relearning to walk, and amputees with prosthetic limbs all benefit from quantitative gait assessment. Modern wearable IMU sensors and machine learning algorithms are bringing gait analysis out of specialized labs and into everyday clinical practice.