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Fig. 3. Example leg force vs leg length change for the same subject as in
Fig. 2 hopping at (A) 2.2 Hz
and (B) 3.0 Hz on a range of surfaces. Surface stiffness values are above each
curve. Thick lines represent early stance and thin lines represent late
stance. On the stiffest surface, the spring-like legs compressed as force rose
to its peak at midstance (thick line) and then extended as force fell until
takeoff (thin line). In contrast, on the softest surface, legs extended as
force rose to its peak and then compressed as force fell to zero at takeoff.
The slight increase in area of the leg force–displacement work loops on
softer surfaces offset the slightly greater energy dissipated by the surface
when it compressed further. The increase in stiffness at peak force on the
softest surfaces in A was observed in four of the eight subjects.
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