In the animation below, the blue leg is in the stance phase and the green leg is in swing phase.

Figure 1. This segment of the gait shows the blue leg in the propulsion subphase just prior to lifting off the ground.

Figure 2. Marinelli Study
As illustrated by Figure 1 at right, the propulsion subphase beings when the heel of the stance leg first lifts off the ground, and ends when the stance leg loses ALL contact with the ground (usually at toeoff). It is during this subphase, and only during this subphase, that the walker has an opportunity to take advantage of all three methods of propulsion at the same time.
Learning to use proper technique during this period is critical to becoming a good race walker. Unfortunately, many (if not most) beginning-to-intermediate walkers tend to miss this opportunity. They either lift their stance foot off the ground prematurely, or they are so focused on the impending landing of the other (swing) foot, that they fail to even focus on the technique applied during this period.

TORQUE: Friction between the shoe and the ground continues to translate rotational forces in the area of the hip into a propulsive force. The resulting rearward pressure on the stance leg will sometimes hyperextend (or further hyperextend) the stance knee.
FALLING: As the stance foot moves further behind the body's center of mass, and the stance leg leans forward at a greater angle, gravity applies increasing pressure for the torso to fall forward. The greatest falling force is realized during this period.
PUSHING: As the stance foot moves further behind the body's center of mass, the walker can more effectively use his calf and foot muscles to push aft (versus upward) and, thereby, propel the body forward. Almost all of the benefits of pushing are realized during this subphase.

A top walker will try to lengthen his step by keeping the stance foot in contact with the ground as long as possible. This allows him to increase his speed both through a longer step length (behind the body), and by using the longer period of contact in this subphase to take advantage of the opportunity to push himself forward. His biomechanics reflect these goals.
HIP: The stance-side hip continues to rotate aft as the walker tries to keep the stance-foot on the ground as long as possible.
KNEE: Most top walkers will keep the stance-side knee straight well into the propulsion subphase. How long they keep it straight, however, will depend both on their flexability and on their walking style. The walker in Figure 2 is keeping her stance-side knee straight until the very last moment before toeoff. While some top walkers (even faster than the woman in Figure 1) will begin to bend their knees sooner, almost all walkers of beginning-to-intermediate skills tend to begin bending the knee very shortly after the stance leg passes under their body's center of mass.
FOOT: The propulsion subphase is an important part of the process of rolling over the foot from heel plant to toeoff. While most walkers, during this period, are focused on raising up the toes of the swing-side foot prior to heel contact, also focusing on the extension of the foot while pushing aft on the stance side can often help improve the entire rolling process. It is, of course, difficult to focus on two things at once but, with practice, a walker can develop an awareness of the entire rolling process that can help not only the heel plant and toeoff processes, but also the effort to establish and maintain a straightened knee.
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