Watch an idealised walker as a skeleton with muscle schema: joint angles, ground reaction force, pressure footprints and energy cost. Educational and non-diagnostic.
The science
Gait cycle. One stride = 2 steps. Heel strike (phase 0) starts stance, toe-off ends it at stance fraction beta = 0.65 - 0.04 v (about 60 % at 1.3 m/s); the rest is swing, and both feet are on the ground for two double-support windows of beta - 50 % each. Speed v = s f: cadence f = 1.82 (v/1.31)^0.55 steps/s, step length s = v/f (about 0.415 H at normal speed).
Joint angles. Hip, knee and ankle curves are smooth normative-style shapes (hip cosine, knee loading and swing peaks, ankle rocker and push-off). They are illustrative, not patient data. The hip amplitude is solved so the foot-flat ankle does not slide on the floor, which makes v = s f consistent with the skeleton.
Ground reaction force. The centre of mass is lowest at double support. Its vertical path is the inverted-pendulum arc excursion dh = L - sqrt(L^2 - (s/2)^2) reduced by a knee-flexion factor 0.45 (determinants of gait). Then Fz = m (g + y''): two humps of about 1.1 to 1.3 body weight with a mid-stance dip, whose mean is exactly body weight. Each foot carries a smooth share of it; shares of the two feet sum to 1. The horizontal force follows the angle of the leg line, mu_required = tan(angle at heel strike), compared with the friction coefficient of the chosen surface.
Joint moments. External moment about each joint = (centre of pressure - joint) x GRF in the sagittal plane; segment inertia is ignored (quasi-static), so peaks during fast swing are not meaningful. Energy. Ralston's fit E = 2.23 + 1.26 v^2 W/kg; the cost per metre E/v is minimal at v = sqrt(2.23/1.26) = 1.33 m/s. Froude number Fr = v^2/(g L); walking becomes a run near Fr = 0.5.
Honest simplifications. Sagittal plane only; one idealised adult; steady speed; rigid segments; the rendered pelvis bobs about 10 cm from rigid-leg geometry (measured COM motion is 3 to 5 cm, which the force model uses); muscle brightness is an illustrative activation envelope, not EMG. No pathology, no advice. Models built with Blender (skeleton, muscle schema, silhouette) and rendered with Three.js.