Fig. 4

Changes in net metabolic rate. A Net metabolic reduction in stiffness experiment. The metabolic reduction of NE condition (control group) was set as zero level. The blue color indicates walking conditions, and red indicates running conditions. The blue and red dashed lines are the third-order best fit to mean metabolic reduction from stiffness conditions in walking and running (*p = 0.007; *p = 0.037). B Net metabolic reduction in speed experiment. The metabolic reduction of NE condition (control group) was set to zero level. The figure shows the metabolic reduction during walking and running at different speeds. The blue dashed line is the third-order best fit to mean metabolic reduction from speed conditions in walking and running (R² = 0.99, *p < 0.001). C Net metabolic changes in common optimal stiffness experiment. The net metabolic rates of walking and running with the 83 Nm Rad⁻¹ were reduced by 7.2% ± 1.2% (mean ± s.e.m, paired t-test, p < 0.01) and 6.8% ± 1.0% (p < 0.01) respectively compared to walking/running NE condition. The asterisk symbol indicates a statistically significant reduction compared to NE condition