Fig. 1. Diagram of the model. The spring contacts the ground at the origin of the coordinate system (0,0) at some angle ₀ to the vertical. At foot contact, represented as the spring on the far left, the point mass (filled circle) has initial horizontal and vertical velocities V_xi and V_yi, respectively. During stance phase the horizontal velocity of the mass decreases to a minimum at midstance and then increases, whilst the spring compresses and then extends as the mass sweeps over the foot, moving from left to right. For an explanation of symbols used, see list.