Fig. 1. Simulation of the transduction between kinetic and potential energy of the centre of mass when the maximum in kinetic energy is set to lag behind the minimum in potential energy by a value of =10° (A) and =20° (B), which covers the range of mean experimental values measured in this study during walking (see values of in Table 1). is the phase shift between the maximum of the kinetic energy E_k and the minimum of the potential energy E_p. Upper panels: the total energy of the centre of mass of the body (E_cg, thin continuous line) is simulated as the sum of two sine waves representing its potential energy (E_p=-sinx; dotted lines) and kinetic energy [E_k=sin(x-10°) in A, and E_k=sin(x-20°) in B: broken lines) during a step cycle, expressed in degrees. The fraction of the mechanical energy recovered at each instant by the pendular transduction within the cycle, r(x) (thick lines and right-hand ordinates), is calculated according to Equation 5 from the relative changes in the E_k, E_p and E_cg curves. r(x) is zero when the changes in the E_k and E_p curves have the same sign, and attains unity when the E_cg curve is at a maximum or at a minimum. Lower panels: the area under the r(x) curve divided by 360°, defined as , attains the value R_int(360°)=R_int at the end of each cycle. Time-averaged R_int is less than R_step, calculated according to Equation 1 from the total amplitude reached by the E_p, E_k and E_cg curves during the cycle. The relationship between R_int and R_step for different values of is shown in Fig. 2.

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