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The Physics of ChiRunning, Momentum

A previous post (The Physics of ChiRunning, Lean … Analysis) with a physics analysis suggested you can use the external force of gravity to your advantage. Doing this adds the concept of a “controlled fall” to your running gait cycle. You can get the feeling for this concept in just a few moments as described in this post (The Physics of ChiRunning, Lean). But the key to this is allowing the fall to happen as described in this post (It’s Not (All) About the Lean).

“Allow is a huge study word.” – Danny Dreyer, ChiRunning/ChiWalking Author

You can think in terms of allowing yourself to fall subtly with each step, but you can also think in terms of allowing yourself to maintain momentum. I have found that once you get a good body sense for a balanced forward position, the next level of efficiency and performance comes from thinking in terms of maintaining momentum.

The physics of momentum or velocity is very simple:

Newton’s First Law: “An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.” 

But the running motion is complex with numerous internal and external forces that can act upon the runner and running motion at various times during the gait cycle. Some of these forces can result in acceleration and some can result in deceleration.

Velocity (now) = Velocity (before) + Sum (accelerations) – Sum (decelerations)

But how can this be if a runner is moving at a constant pace? Question … when a runner is moving at a constant pace, do they have a constant velocity? My answer is no. Although a runner may appear to have a constant pace, within each gait cycle velocity will vary slightly. A runner’s “pace” is the average velocity over a gait cycle or longer.

So then the question becomes: “What can I do to efficiently maintain velocity?” My answer is to efficiently utilize the forces that create acceleration and decrease the effect of the forces that create deceleration. In simpler terms this might be restated as “reducing forward resistance while maintaining forward momentum”. And the more you reduce resistance in each step, the less you have to re-create momentum.

The point here is once you get moving with momentum … a focus on “allowing” that forward motion to continue can be a powerful way of looking at it. A focus on a subtle lean becomes secondary to stepping out of the way of your own momentum.

So what are some things that allow forward motion to continue with less resistance? Here is a partial list and diagram (click to enlarge):

1. Posture

  • Alignment allows greater relaxation, period.

2. Landing position as close to under the COM as possible (green position)

  • Why? Reduce braking resistance and geting to the falling phase sooner
  • How? Forward aligned body position, reducing reach forward with drills, higher cadence

3. Relaxed lower legs to move through the gait cycle with limited “hinge” resistance

  • Why? Falling acceleration increases and potentially longer to re-create lost velocity/momentum
  • How? Ankle rolls body loosener, increase overall ankle range of motion, develop balance through one-legged practice drills
  • Ref: Blog post from above It’s Not (All) About the Lean

4. Hip mobility and extension to move through the gait cycle with limited leg muscle resistance

  • Why? Falling acceleration increases and potentially longer to re-create lost velocity/momentum
  • How? Dynamic core strength through the gait cycle with increasing (loosening) range of motion, numerous core strength exercises and a focus all day long – sitting, standing, walking, running

5. Arm swing for balance

  • Why? Balance supports relaxation and efficiency across the whole machine
  • How? Arm (elbow) swing to the rear, shoulders still

And not to forget … an efficient cadence:

  • Why? Reduce time duration of all of the above, including time on your feet. Also reduces reach which decreases time to the falling phase. And supports an efficient gait cycle with the falling phase occurring more often.
  • How? Think light and quick, but fullfoot. Use a metronome or think waltz count attached to a form focus.

And the all time favorite … run downwind!

  • Reduces the effect of air resistance
  • It can even add momentum …

So as you can see there are plenty of things to focus on with this alternate way of looking at it. My key focuses:

  • Developing and engaging dynamic core strength all day long
  • Imagining “letting my engaged center move forward” with balance

My experience is that technical “running/falling” feels very different from a softer “allowing my forward motion to continue” mindset. Once the primary technique elements are in place, see if you can use this mindset to create a greater level of relaxation, efficiency and perhaps performance.

[See the Physics Resource Page for more information.]


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David Stretanski is a holistic health, fitness and wellness coach – and Certified ChiRunning®/ChiWalking® Instructor.  For more information on David, please see his About MeContact page or his website at

ChiRunning® and ChiWalking® are registered trademarks of ChiLiving, Inc.


Posted on Monday, Jul. 30th 2012 4:41 PM | by echifitness | in All, ChiRunning, Gravity | No Comments »

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