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Fig. 2. Vertical displacement during contact and amplitude of oscillation of the
elastic system. The fractions of the vertical displacement of the centre of
mass of the body taking place during contact,
Sc/Sv (filled circles: downward
displacement, open circles: upward displacement), and when the vertical force
is greater than body weight, Sce/Sv
(filled squares: downward displacement, open squares: upward displacement) are
given as a function of the running speed. It can be seen that
Sc/Sv decreases markedly with speed.
Except at very low speeds when the aerial phase may be nil
(Sc/Sv
1: e.g. upper left panel in
Figs 1 and
7), the downward displacement
while the foot is in contact with the ground is smaller than the upward
displacement during contact: the ballistic fall of the centre of mass is
greater than the ballistic lift (see Fig.
7). In contrast, the amplitude of the vertical oscillation below
the equilibrium point, Sce/Sv, changes
less with speed and approaches one half of the vertical displacement
Sv both during compression (down) and recoil (up) of the
spring. The vertical bars indicate the standard deviation of the mean
calculated in each velocity class; the figures near the symbols indicate the
number of items of the mean. Lines represent the weighted mean of all the data
(Kaleidagraph 3.6.4); their only purpose is to be a guide for the eye: they do
not describe the underlying physical mechanism.
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