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Fig. 2. Vertical displacement during contact. (A) Old subjects, (B) young subjects,
with data from A (gray) superposed for comparison. The upward and downward
vertical displacements of the centre of mass taking place when the foot is in
contact with the ground (Sc) are divided by the total
upward and downward vertical displacements (Sv) to obtain
the fraction of the vertical displacement during contact
(Sc/Sv). The fractions so obtained,
are plotted separately during the lift
(Sc,up/Sv,up; open circles) and during
the descent (Sc,down/Sv,down; filled
circles). The vertical bars indicate the standard deviation (s.d.) of the
mean; numbers near the symbols indicate the number of items in the mean. Lines
represent the weighted mean of all the data (Kaleidagraph 4.03). Their only
purpose is to be a guide for the eye: they do not describe the underlying
physical mechanism. Note that at very low speeds
Sc/Sv
1 because the aerial phase is
often nil (e.g. Fig. 1A,B).
With increasing speed an aerial phase of progressively greater extent takes
place during the step and the fraction of vertical displacement during contact
decreases. The decrement is less during the lift than during the descent (i.e.
during the aerial phase the lift is smaller than the fall). This is
particularly true in the old subjects who maintain contact with the ground for
almost the whole of the lift (see also light-blue dotted lines in
Fig. 3). It must be pointed out
here that heel-strike and toe-off, i.e. start and end of foot contact, do not
properly describe landing and take-off of the bouncing system. As described in
the text, landing and take-off, in a physical sense, coincide with the
instants where vertical force becomes greater than body weight (after heel
strike) and falls below body weight (before toe-off). The correct
landing–take-off asymmetry of the bouncing system is described in
Fig. 3.
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