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Fig. 1. Fluctuations of the external and internal mechanical energy during one
stride of walking. The three upper curves present the mechanical energy
changes of the centre of mass of the body (COM):
Ek is the kinetic energy due to its velocity relative to
the surroundings, Ep is the potential energy and
Eext is the external energy, which is the sum of the
Ek and Ep curves. Due to a
pendular-like energy transfer between Ek and
Ep, the variations of the Eext curve
are smaller than those of the Ek and
Ep curves. The increment a represents the work
done on the COM during the first double contact phase of the stride.
The internal work (Wint,dc) made by one leg against the
other is presented as a function of time in the fourth and fifth curves: the
positive work done by the back leg during the first double contact phase
(increment b) is equal to the negative work (decrement d)
absorbed in the front leg. The
and
curves are the kinetic
energy changes of the lower and upper limbs, respectively, due to their
velocity relative to the COM. The increment c represents the
positive work to accelerate the front lower limb during the double contact
phase. The internal energy–time curve of the lower limb
(
) is the sum of the
and
Wint,dc curves. This procedure assumes that the internal
positive work done by the back leg during the double contact phase (increment
b in Wint,dc) increases passively the backward
velocity of the front leg relative to the COM (see Materials and
methods). Consequently, the internal work done by the front leg is reduced
(increment e in
). The `stick-man' at the
bottom of the figure shows the position of the limb segments each 10% of the
stride: thick lines refer to the segments on the right side of the body that
were recorded by infrared cameras; thin lines refer to the segments of the
left side of the body that were reconstructed on the assumption that the
movements of the left segments during one half-stride were equal to the
movements of the right segments during the other half-stride. The vertical
broken lines delimit the two double contact phases of the stride and were
determined from the force traces. The curves are from a 20-year-old woman
(mass, 70.1 kg) walking at 1.5 m s–1.
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