Fig. 2
Net metabolic rate across stiffness and speed. Steady state metabolic cost for five exoskeleton stiffness conditions (kexo = 0, 50, 100, 150, 250 Nm rad− 1) and three walking speeds (1.25, 1,50, 1.75 m s− 1). A significant relationship was found between exoskeleton rotational stiffness and net metabolic rate at slow and fast walking speeds (n = 11; mixed model ANOVA with second order term k2exo; 1.25 m s− 1 p2stifffness = 0.022;1.75 m s− 1 p2stifffness = 0.009). No significant relationship was observed for the intermediate 1.50 m s− 1 speed. The lowest stiffness (50 Nm rad− 1) resulted in metabolic reductions of 4.2 ± 1.7% (mean ± s.e.m) at 1.25 m s− 1 and 4.7 ± 1.3% at 1.75 m s− 1 (* = paired t-test α = 0.025). The solid line is a quadratic best-fit curve where the stiffness at the minima is indicated by the vertical dashed line