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, F_z/z_CM, 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.