Fig. 3.

Limb loading and work following the perturbation are consistent with the mass–spring model. (A) Shown is the relationship between leg spring contact angle (ϕ_o), velocity at contact (V_o), and total ground impulse (J) in a mass–spring system (Eq. 3). If the virtual leg at contact is perpendicular V_o, J is zero. (B) Measured ground impulse magnitude (|J|) (see Methods) decreases with increasing limb contact angle θ_o. (C) An estimate of mass-specific leg spring energy based on the model (see Methods) predicts most of the variation in absolute limb work during stance (R² = 0.85; P < 0.0001).