Fig. 6. Extensor muscle mean EMG during the stance phase (mean ± S.E.M.) vs surface stiffness for hopping at (A) 2.2 Hz and (B) 3.0 Hz. Extensor muscles are SOL, MG, LG, VM, VL, RF and ST. Mean EMG values are expressed as a percent of the mean EMG during stance phase on the 81 kN m^-1 surface during 2.2 Hz hopping. Solid lines are least squared regressions. Arrows indicate when the surface stiffness equals the preferred vertical stiffness at 2.2 Hz (filled arrows) and 3.0 Hz (open arrows). Extensor muscle activation was minimized on the 27 kN m^-1 surface when hopping at 2.2 Hz, whereas absolute leg work was minimized on the 17 kN m^-1 surface (see Fig. 4).