As illustrated by Figure 1 at right, the swing phase begins when the foot lifts off the ground behind the body, and ends when it makes contact with the ground in front of the body. It is essential to the process of walking because it allows the walker to string together a series of steps and, thereby, maintain the momentum gained prior to, and during, the swing phase. Without the swing phase, the walker would fall flat on his face at the end of a single step.
 
While swinging the leg forward might seem like a very simple act, the use of proper technique not only makes the motion more efficient, it also plays a very important role both in generating propulsion through torque, and in properly preparing the swing leg to make contact with the ground in front of the body--with a strightened knee.
 

TORQUE: While the swing leg is off the ground and can not move the hip forward (as the stance leg does), it can apply torque at the forward-moving hip to move the swing leg forward faster than the hip is moving. In this way, the swing leg moves body mass (the mass of the swing leg) forward as a counterbalance to the mass of the stance leg which is moving aft. This process helps maintain the body's momentum--and does, as I read the literature, constitute a propulsive force.
 
FALLING: As the swing leg move forward, it will magnify the increase in mass in front of the stance foot (which is, of course, moving aft) and, thereby, magnify the falling force created by gravity.
 
PUSHING: Having no contact with the ground, the swing leg and foot can not apply a pushing force.
 
 

Figure 2. Runner/Walker Study

FOLLOW THROUGH As illustrated by Figure 2 at right, the movement of the walker's trailing leg after toeoff is dramatically different from that of a runner. While the runner will lift the trailing foot very high and bend the knee to a sharp angle before moving the foot forward, the walker wastes no time in stopping the aft and upward motion of the foot, and will begin to move the relatively straight-kneed swing leg forward. Much of the difference can be attributed to the walker's lack of a flight phase. While the runner has time to reposition both legs without constraint prior to landing, the walker's trailing leg must, at toeoff, immediately begin to counterbalance the stance leg which is beginning its journey aft. (Note the very different locations and angles of the runner's and walker's swing legs when intial contact is made by the stance leg.)
 
HIP: The swing-side hip rotates foreward, arcing downward and then upward, as it compensates for the opposite actions taken by the stance-side hip (as the latter compensates for the vaulting action required to pass over the straightened stance leg)..
 
KNEE: In his effort to move the swing leg forward rapidly, the walker puts only a minor bend in the swing-side knee. The angle of the bend is only that required to compensate for the hip drop on that side, and the need to keep the toes of the swing foot from scraping the ground. Too much of a bend in the knee will attract unwanted attention from the judges as they check for lifting ("running"), and too little bend will lead to unwanted scuffing of the shoes. Sometimes, the need to have a straightened knee during the stance phase is interpreted by novices as a reason to keep the knee too straight during the swing phase. In any case, it is important to find the proper angle for the swing-side knee--through experience, instruction, or feedback from judges.
 
FOOT: Having extended the foot during the propulsion subphase, it is important to quickly flex (raise) the foot as it starts to pass under the body in its travel forward. Raising the foot quickly helps avoid scuffing, and also helps to have the toes in a raised position prior to making initial contact with the ground at the end of the swing phase.
 
The process of rapidly flexing the foot is the main cause of shin pain in novice walkers, and is due in large measure by the significant difference in size and power between the large calf muscles (which extend the foot) and the much smaller tibialis anterior muscles that flex the foot (see Figure 3). Because the speed of flexing is involved, novice walkers need to slow their walking speed until the smaller, flexing muscles can be properly developed.