Fig. 4

a External, internal and total mechanical energy during level and decline walking for rigA-F and hyA-F prostheses. Regression lines for the relationships between mechanical work and speed are shown in grey (rigA-F) and black (hyA-F). b The interchange of potential and kinetic energies of the BCoM, quantified as the recovery index (%), are shown for level and decline walking for rigA-F and hyA-F prostheses. Data are mean ± SEM. The external mechanical work and recovery index (dashed lines) in non-amputees, previously reported in the literature are plotted for comparison (Minetti et al., 2011). Note that this figure highlights absolute speed rather than percentage of customary; the value plotted is the mean absolute speed for each category of relative customary speed category (0.8, 1, 1.2, 1.4 and 1.6 of customary speed: NB group mean customary speed was 0.98 m s^− 1)