Gross metabolic cost of transport and preferred walking velocity. (a) PWA using the bionic prosthesis (blue diamonds) had nearly the same average metabolic cost of transport (COT) as non-amputees (green squares) (p > 0.50 for 0.75–1.50 m s−1; p = 0.17 for 1.75 m s−1). Cost of transport is the metabolic energy needed to transport unit weight a unit distance, equal to (metabolic demand)/(body weight × distance travelled). PWA using a passive-elastic prosthesis (red circles) had 11–25% greater cost of transport (p < 0.05) at 1.00–1.75 m s−1 compared with non-amputees. (b) Average preferred walking velocities of PWA using the bionic prosthesis (blue bar) and non-amputees (green bar) were equivalent (p = 0.97). PWA using a passive-elastic prosthesis (red bar) preferred to walk significantly slower (p = 0.008). Values within bars indicate averages (s.e.m.). Asterisks (*) indicate significant differences between PWA using the bionic prosthesis compared with using a passive-elastic prosthesis. Error bars indicate s.e.m. Second-order polynomial curve equations in (a) are, non-amputee: COT = 0.303v2 – 0.846v + 0.934, r2 = 0.49; bionic prothesis: COT = 0.308v2 – 0.808v + 0.895, r2 = 0.39; and passive-elastic prosthesis: COT = 0.295v2 – 0.750v + 0.295, r2 = 0.52. COT is calculated in Joules per Newton body weight per metre. v is velocity in metres per second.