Modeling and Animating Human Figures - Computer Animation: Algorithms and Techniques - page 299

Run cycle
The run cycle can also be described as a sequence of phases. It differs from the walk cycle in that both
feet are off the ground at one time and at no time are both feet on the ground. As in the walk cycle, the
stance is the duration that a foot is on the ground. Thus, the left stance , defined by the left heel strike and
left toe lift, has the right foot off the ground. This is followed by a period of flight , during which both
feet are off the ground, with the right foot swinging forward. The flight is terminated by the right heel
strike, which starts the right stance (see Figure 9.15 ) . Notice that the left and right stances do not over-
lap and are separated by periods of flight.

Pelvic transport
For this discussion, let the pelvis represent the mass of the upper body being transported by the legs.
Using a simplified representation for the legs, Figure 9.16 shows how the pelvis is supported by the
stance leg at various points during the stance phase of the walk cycle. Figure 9.17 shows these positions
superposed during a full stride and illustrates the abutting of two-dimensional circular arcs describing
the basic path of the pelvis as it is transported by the legs.

Pelvic rotation
The pelvis represents the connection between the legs and the structure that separates the legs in the
third dimension. Figure 9.18 shows the position of the pelvis during various points in the walking cycle,
as viewed from above. The pelvis rotates about a vertical axis centered at the stance leg, helping to

left heel strike right heel strike left heel strike

left toe off ground right toe off ground

single
support

single
support flight flight

left stance left swing

right
stance

right swing right swing

one cycle (stride)

FIGURE 9.15
Run cycle [ 12 ] .