Fig. 11. The instantaneous work traces of the front and back limb calculated using the individual limbs method (ILM) and the method proposed by Bastien et al. (2003) are plotted as a function of gait cycle duration. The shaded area indicates the double support period. The ILM separately sums the vertical and fore–aft components of each limb to obtain the work performed by the front limb (W_front) and back limb (W_back). The dark grey area between both curves indicates the amount of opposite work. Using Bastien's method, instantaneous work in the vertical direction performed by the front and back limb is summed (W_z). Then a transfer of energy is allowed from W_z (if W_z is decreasing) to fore–aft work performed by the back limb (W_yback). Another transfer of energy is allowed from the negative fore–aft work of the front limb (W_yfront) to increase W_z during the second half of double support. Allowing for these energy transfers causes a flattening of the instantaneous work curves of the front and back limb. Consequently the amount of calculated opposite work is smaller.