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Abstract
When humans hop in place or run forward, they adjust leg stiffness to accommodate
changes in stride frequency or surface stiffness. The goal of the present study was
to determine the mechanisms by which humans adjust leg stiffness during hopping in
place. Five subjects hopped in place at 2.2 Hz while we collected force platform and
kinematic data. Each subject completed trials in which they hopped to whatever height
they chose ("preferred height hopping") and trials in which they hopped as high as
possible ("maximum height hopping"). Leg stiffness was approximately twice as great
for maximum height hopping as for preferred height hopping. Ankle torsional stiffness
was 1.9-times greater while knee torsional stiffness was 1.7-times greater in maximum
height hopping than in preferred height hopping. We used a computer simulation to
examine the sensitivity of leg stiffness to the observed changes in ankle and knee
stiffness. Our model consisted of four segments (foot, shank, thigh, head-arms-trunk)
interconnected by three torsional springs (ankle, knee, hip). In the model, increasing
ankle stiffness by 1.9-fold, as observed in the subjects, caused leg stiffness to
increase by 2.0-fold. Increasing knee stiffness by 1.7-fold had virtually no effect
on leg stiffness. Thus, we conclude that the primary mechanism for leg stiffness adjustment
is the adjustment of ankle stiffness.