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Fig. 8. Hula-hoop body motions. Angular momentum, horizontal ground reaction force
and ground references points are plotted in the medio-lateral (x) and
anterior–posterior (y) directions. In this experiment a
participant rotated his hips while standing in double support, similar to how
one twirls a hula hoop, at an increasing and then decreasing rotational speed
for approximately 10 s (see A for one representative cycle). (B) The
horizontal components of normalized angular momentum are plotted
versus time. For ease of comparison with walking values shown in
Fig. 3A, the angular momentum
is normalized by the product of the participant's mass, CM height and
self-selected gait speed (MVH; see participant no. 1 in
Table 1). (C,D) The horizontal
ground reaction forces measured experimentally (thin blue line) are plotted
along with the calculated zero-moment forces (thick red line) versus
time for the same participant and trial as in B. Both experimental and
calculated zero-moment forces are normalized by the stiffness term,
Fz/zCM, and the radius of the ground
support base in the appropriate direction (see Eqns
9 and
10). In the medio-lateral
(x) direction, the radius was measured while standing in double
support, and was equal to one-half the distance from the lateral edge of the
right foot to the lateral edge of the left foot. In the
anterior–posterior (y) direction, the radius was equal to
one-half the participant's foot length. (E) Plotted are the CP (blue dashed
line), CMP (red solid line) and CM ground projection (green dash-dotted line)
trajectories and corresponding footprints. As in A, only one hula-hoop cycle
is shown from 7.2 to 8 s. The ground CM projection remains within the support
envelope while the CMP often falls outside the region.
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