Sunday, January 17, 2010

The Fall

Update to post Sept 5th 2011

I wrote this post a while ago, and I've been meaning to update it. I basically wrote that the body is vertical from the hips up, but that description was incorrect and poorly worded. The upper body does lean forward, but it does not stay in a completely straight with the hips and the ankle though out the fall. 

Original Post

Falling is how one moves forward when running. Most people are under the impression that moving forward is accomplished by pushing with the back leg. This is probably the most common misconception concerning running technique.  Any pushing by the back leg will result in vertical movement, which is undesirable because vertical motion waists energy and time that should be applied to moving horizontally (forward).

How is falling accomplished? It is accomplished by the runner leaning forward with his or her hips and letting gravity pull him or her forward using gravitational torque. The further the runner leans forward, the more off-balance he or she is, and the more off-balance he or she is, the more gravity can pull him or her forward, and the faster he or she moves forward.

There are some very important points about leaning that need to be made.
  1. The lean is from the ankle through the hip only!  The torso remains vertical from the hip to the head. Do not lean with the upper body or the head!   
  2. Also, for reasons that will become clearer in future posts, do not drop the lead foot out of the figure 4 position until the back foot breaks contact with the ground.
  3. Speed and stride length are determined by of the angle of the lean. The greater the angle, the faster the runner falls forward and the longer the stride becomes. So naturally sprinters lean more than middle distance runners, and middle distance runners lean more than Marathon runners.

The two pictures above show the runner starting in the pose (right) and leaning with the hips to fall (left).  Notice that the front foot is still held in the figure 4 position while leaning. To illustrate the lean a little better, look at the illustrations below with angle lines drawn.

Here is an illustration (below) to help visualize what is meant by not leaning with the upper body or the head.

For those who like math:
For those of you who like equations, moving forward is accomplished through gravitational torque which is described by the equation F=mg . sin theta. Force (F) equals mass (m) times gravitational acceleration (g) times the sin of the angle theta (which is the angle of the lean of the runner). Through a series of algebraic manipulations that I won't go through here, this equation can be rewritten as (a = g . sin theta). Acceleration (a), which is the movement forward, equals gravitational acceleration (g) times the sin of the angle of theta, which is the angle of the lean.

  • I'll go more into the forces that affect running in future posts. 
  • Theta should be represented as the Greek letter for theta, but I didn't bother to get the HTML code for the theta for this post.

Here is quick quiz to see how well you understand the relationship between gravity and running speed.

Question - If the gravity of the Earth were reduced, how would it affect running speed? Would one be able to run faster?

Answer - If gravity were reduced, it would slow down running speed, because the acceleration forward do to gravitational torque would be reduced. In other words, you can not fall forward as fast in lower gravity. When the astronauts were on the Moon, they were completely unable run or walk because the gravity did not allow them to fall forward fast enough. They ended up getting around by hopping.

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