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Fig. 1. (A,B) Peak downward center of mass displacement during stance (COM), peak
`leg' compression and peak `surface' compression vs surface stiffness
for hopping at (A) 2.2 Hz and (B) 3.0 Hz. Positive leg compression values
indicate shorter legs at midstance than at landing, and positive values for
COM displacement and surface compression indicate downward movement. (C)
Combined vertical stiffness of the leg and surface (Kvert)
vs surface stiffness during hopping at 2.2 and 3.0 Hz. Values are
means and S.E.M.s for all subjects, lines are least squares
regressions and many error bars are hidden by symbols. The arrows indicate
when the surface stiffness equals the preferred vertical stiffness at 2.2 Hz
(filled arrows) and 3.0 Hz (open arrows). At this surface stiffness, center of
mass displacement equals surface displacement. The dashed line in (C)
indicates equal vertical stiffness and surface stiffness over the entire
range. Vertical stiffness exceeded surface stiffness on the softest surfaces
because the legs extended while the surface simultaneously compressed in the
first half of stance.
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