Whenever you're running, your body comes under the influence of two forces: 1) the constant downward pull of gravity and 2) the force of the road coming at you as you run. In ChiRunning, you'll cooperate with these two forces and make them your allies with every step.
The Pull of Gravity
You can either cooperate with the pull of gravity and let it help you down the road, or you can fight it, causing friction and extra work for your leg muscles. Here's the science behind this statement: Newton's law states that "a body at rest tends to stay at rest unless acted upon by an external force." Anytime you run with your body upright (which is how most adults run) your center of mass is located directly over your feet and your body is vertically aligned with the downward pull of gravity.
In this position, your body is at rest because your center of mass (pelvic area) is directly over your feet. The only way to get your body moving is to use your legs, which is why we call it "power running." When you run upright, as soon as you stop pushing yourself forward, you stop moving. All of your motion is dependent on your legs, and because you are pushing yourself off the ground to move forward, you're working against gravity.
With the ChiRunning technique, you cooperate with the pull of gravity by leading with your upper body and falling forward into the pull of gravity. Your body then becomes a forward-falling object, like a tree that's just been chopped down. Because your center of mass is just ahead of your feet, your upper body falls forward. When you learn to balance in this "perpetual fall" you'll be cooperating with the same force that pulls a unicycle rider forward. (See p. 108 in the new ChiRunning book for more information about how to maintain the perfect balance of lean.)
So if gravity is pulling you, just go with it and you'll run more efficiently than you ever thought possible. Can you imagine how happy your legs would be if they were used only for momentary support between strides, and they didn't have to be used for propulsion? This is especially great for anyone doing distance running.
The Force of the Road
Here's how to cooperate with the other force you'll be dealing with: the force of the road coming at you. Whenever your body moves forward, the road is always moving in the opposite direction at the same speed, relatively speaking. Most power runners reach forward with their legs, swinging them into the force of the oncoming road, and land with a heel strike in front of their body.
The impact of a heel strike, coupled with the forward momentum of your entire body, can send a shock wave up your leg, potentially damaging your heels, ankles, shins, knees, hips or lower back, depending on where your weakest link is.
Having your foot strike in front of your body is like putting on the brakes because your feet are stopping your forward momentum each time they hit the ground. Pretty inefficient, I'd say. If you don't believe me, just look at the bottoms of your running shoes and check to see if the heels are worn down. If they are, it means you're running with your brakes on. Would you ever drive your car with one foot on the gas pedal and one foot on the brake pedal at the same time? No.