Technique > SOURCES OF PROPULSION
 
The literature discusses three main sources of propulsion accessed by race walkers. One source is gravity, and the other two sources are muscle power using an assist from gravity and friction. The three are:
  1. torque applied by muscles of the upper legs and lower torso to make the thighs rotate fore and aft around the hip joints,
  2. falling forward due to the pull of gravity, and
  3. pushing forward by using the calf muscles to extend the forefoot.
The animation at right shows the legs of a race walker. The name of each of the three main sources of propulsion is visible when that source is active--as described below. The three sources of forward motions are almost exclusively generated by the supporting leg--shown in blue in the animation.
 
Torque
Torque is the application of force to make something rotate about a pivot. On a vehicle, it is the application of power to make the wheels rotate about an axel. On a body, it is the application of muscle power to make a body part rotate about the joint of two or more bones. In walking or running, it includes the application of muscle power to make parts of the legs (the thigh, shin, or foot) rotate about a hip socket, knee, or ankle.
 
One of the most important activities in any form of walking is the repetitive movement of the legs fore and aft as each leg alternatively takes the responsibility of supporting the moving torso. The thighs form the basis of this pendulum action, and the lower part of the legs assist in the process with help from the knees, ankles, and feet. The quadriceps at the front of the thighs provide most of the power to move the thighs forward by contracting (i.e., flexing), and the hamstrings and glutius medius muscles at the back of the upper thigh provide most of the power to move the thigh aft--again by contracting (flexing). This pendulum action (sometimes referred to as a scissor action) serves two primary purposes.
  • First, it propels the person forward. It does so, however, only because the one foot is on the ground and is held steady by gravity-induced friction between the foot (i.e., the shoe) and the ground surface. As the foot on the ground tries to move aft, friction prevents it from doing so. The net force resulting from this conflict causes the upper part of the leg to propel the hip (and therefore the torso) forward. (The same result occurs with your car. The power train tries to rotate the bottom of the drive wheels aft but, because of friction, the wheel can not comply. In response, the axel of the wheel moves forward instead--along with the car and driver.) For the walker, the forward propulsion resulting from torque applied to the legs (fore or aft) occurs throughout the walking cycles except for that brief moment when the thighs come to a stop prior to reversing direction. (In the animation at the top of the page, Perez has no torque propulsion because he has no contact with the ground.)
     
  • Second, the pendulum action, repositions the non-supporting leg from the back of the body to the front so that it is in position to take its next turn in supporting the body and propelling the body forward. This action is crucial if the body is going to maintain its momentum from one step to the next. Newton's First Law of Motion--on inertia--states that an object in motion tends to remain in motion unless a force is applied to slow it down--and even stop it. Without having the non-supporting leg ready to take over when the supporting leg has moved as far aft as possible, walking (or race walking, or running) would not be possible. The repositioning of the non-supporting leg forward does not, in itself, provide any propulsive force (although we will see a bit later that, by moving forward, it helps increase the "falling" force).
Falling
As the support leg passes under the body's center of mass moving aft, the race walker will begin to fall forward due to gravity--especially if there is already a slight body lean forward from the ankles. As the supporting foot moves further aft, the falling force increases rapidly until it is stopped by the planting of the non-supporting foot as it begins its support phase. The strength of the falling force can easily be demonstrated either by leaning forward just beyond the ability of your toes to keep you upright, or by seeing how fast a broom handle hits the floor after being released from a "near vertical" position. The falling force is amplified by the non-supporting leg moving in front of the center of mass. Conversely, the falling force slows a walker down if he begins the next step by planting the foot too far in front of the body. The falling force will then be negative (i.e., directed aft)--and cause a loss of momentum. (In the animation at the top of the page, Perez has no falling force because I suspended gravity.)
 
Pushing
As the support leg passes under the body's center of gravity moving aft, and especially after the heel starts to rise off the ground, the race walker is able to begin putting pressure on the forefoot and toes of that foot using the calf and foot muscles.  While avoiding pushing the body upward, he can gain forward motion by pushing aft with increaing pressure until that foot loses contact with the ground. The race walker is able to apply this pressure because of the downward force of gravity induces friction between his shoe and the ground surface. (In the animation at the top of the page, Perez is airborne, has no contact with the ground and, therefore, has no way to push himself forward.)
 
You will note that I have not tried to indicate which propulsive force is greater. The amount of force applied by each of the three sources depends on a race walker's technique and style--and, for example, whether he even tries to push off with the forefoot of his supporting leg.
 
Until the forces slowing him down exceed the forces moving him forward, the race walker will go faster. When those forces are in balance, he will simply maintain his current speed. Four things can slow him down:
  • resistance (e.g., a head wind or a tree),
  • friction - an inevitable byproduct of contact with the ground,
  • overstriding - which can cause a braking action, and can counteract the falling mechanism as the body goes through a much higher arc to get over the straight support leg, and
  • intentionally slowing down by taking shorter steps, less frequent steps, or simply stopping.
Good race walking technique is designed to help you move forward efficiently by maximizing the torque, falling, and pushing actions. It is also designed to minimize those forces that can slow you down.
 
For many years, race walkers overstrided in front of the body because they believed the most important propulsive force was a "pulling" action, and overstriding gave them more time to pull. That concept is no longer accepted. What they were sensing as a "pulling" action was the resistance supplied by friction to the torque applied to the support leg. In reality, overstriding was slowing them down because of the negative (backward) falling force created by having the supporting foot so far in front of the body's center of mass.
 
SOME SIMPLE TESTS
 
I learned a lot about the physics and biomechanics of walking by performing a few simple tests.
  1. Standing still with your arms at your side, try to push yourself forward without first leaning forward to begin falling. Unless you flex your body in some way, very little happens. There is simply no torque, falling force, or pushing force to make you move forward.
     
  2. Repeat Test #1 but now use your ankles to lean forward. It is amazing how fast you begin to fall forward, and how quickly you must put one foot forward to stop yourself from falling.
     
  3. Repeat Test #2 but as you start to fall, start pushing forward with the toes on one foot. While you will quickly have to put the other foot forward to stop yourself from falling, you will probably notice that the push from your toes helped accelerate your forward motion.
     
  4. Now repeat Test #3 but repeat the test immediately after you have moved the one foot forward. Don't try to stop yourself after the first step..
You can quickly see how the three sources of forward motion (plus momentum) work together to let you walk very fast.
 
Now try a few other tests.
  • While standing, face a counter or a wall and push your one knee forward against the vertical surface. Use your hand to find the muscles and ligaments that firm up as they apply the torque force. You should feel most of the tension at the top end of the quads and in the groin area. (You may need to press down hard with your finger tips on those areas to feel their tightness (flexion).
     
  • While standing with your back to a firm object about knee high, push your knee back against the object. Again, use your hand to find the muscles and ligaments that firm up as they apply the torque force. You should feel most of the tension at the top end of the hamstrings and fairly deep at the very bottom of your bottom.
     
  • You have now located the muscles that play a major role in performing the leg swinging action as you move forward. By being creative, you can also check for the muscles that tighten when you try to move a leg forward using hip rotation rather than leg muscles, and for those muscles that help you perform most of the technical elements of race walking technique. (The muscles of the abdomen and lower back which allow you to rotate and drop your hips are the same ones that have the primary responsibility of stabilizing the hips. They are quite busy during normal walking, and doubly busy during race walking.)
As you gain familiarity with the muscles used in race walking, begin to sense what they are doing when you are on training walks. It is interesting to me how a better familiarity with the physics and biomechanics of race walking can translate into an easier time learning to use good race walking technique.
 
 
OK. Class is over. There will be a quiz on this material the next time you race walk ... and the next time ... and the next time ...
 
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