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Fig. 4. Mechanical work by the legs during the landing (circles) and takeoff (squares) parts of the stance phase vs surface stiffness for hopping at (A) 2.2 Hz and (B) 3.0 Hz. Values are means and S.E.M.s for all subjects, lines are least squares regressions and error bars are hidden by symbols. Arrows indicate when the surface stiffness equals the preferred vertical stiffness at 2.2 Hz (filled arrow) and 3.0 Hz (open arrow). On all surfaces, the magnitudes of the positive and negative work were equal because the surfaces dissipated negligible energy. On stiffer surfaces, the legs absorbed mechanical energy (i.e. performed negative work) during landing and then performed positive mechanical work during takeoff. In contrast, on softer surfaces, this sequence was reversed. The legs performed positive work during landing and then absorbed mechanical energy during takeoff. Leg work was not minimized when surface stiffness and vertical stiffness were equal due to small transient changes in leg length in early and late stance (see Fig. 2B) that were not reflected in the values for leg compression between touchdown and midstance in Fig. 1.

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