Figure 1. The dynamic bias model.

The body is represented by an inverted pendulum with its centre of mass (CoM) indicated. The gastrocnemius and soleus muscles together are represented by the contractile element (CE). These muscles act through a spring-like element which connects them to the ground through the foot. The total stiffness of this elastic link is represented by K. The system operates by dynamically altering the length of the CE thus altering the position of one end of K. We refer to the length of the CE as the bias of the spring. In angular terms, the length of the spring is given by the angle of the CoM relative to the vertical (θ) minus the length of the bias (θ₀). Ankle torque is then given by T = K(θ−θ₀). All quantities are expressed in angular terms.