The human walking step is composed of two different phases.
The first phase is the swing phase or single support phase
when one foot is on the ground while the other swings.
This phase begins with the moment of liftoff and ends
with the collision of the swing foot with the ground.
This phase makes up the majority ( 
) of the duration of the
walking
step in human walking.
The second phase is called the double support phase as both feet
are on the ground while the body is moving forward.
This phase usually makes up only a small part
( 
) of the human walking step.
Also of interest are the transitions between phases. Immediately at the beginning of the swing phase is the moment of liftoff. Here, the foot is just propelling the body forward so that the leg loses contact with the ground. The other transition between the swing and double support phases is characterized by a collision of the swing foot with the ground. Figure 1 gives a graphical depiction of our biped model first in the swing phase, then in the double support phase.
Some additional detailed definitions can be useful. The cadence is defined as the number of steps in a standard time frame (e.g. steps/min). The step length is the distance between the same point on each foot during the double support phase. The stride length, on the other hand, is the distance traveled between two successive foot strikes of the same foot and is equal to two step lengths. Each stride is, thus, composed of one right and one left step length. All measurements given will be in meters.