I’ve read that using gravity to “fall forward” is the most efficient way to run. Will using gravity help me to run faster and with fewer injuries? – Brad
Gravity doesn’t cause things to “fall forward.” Things fall straight down. During runs and races, you maintain your pace by directing force down with each footstrike. In response, the ground force reaction directs force up (see Newton’s third law), causing you to bounce. Inertia (see Newton’s first law) then becomes responsible for most of your forward motion.
Gravity is the force of attraction that makes things fall straight down. Or more accurately, it’s what makes things on Earth fall toward the center of the planet. Or more accurately still, it’s what causes both a thing’s and the Earth’s centers of mass to accelerate toward each other.
Okay, so defining gravity is a process with hiccups. But one thing we can be certain of is this: Unless you’re running downhill, gravity doesn’t cause your center of mass (the point representing the average location of all the mass that makes up your body) to fall forward. As Sweat Science author and physics Ph.D. Alex Hutchinson puts it, “One thing I can remember from first-year physics is that there’s some pretty basic conservation of momentum and energy laws that say you can’t fall forward for all the run unless you’re running down Mt. Everest.”
The Spring-Mass Model
The spring-mass model proposes that when you run, your legs work like a pogo stick.
Let’s start by looking at the two distinct phases of running:
Acceleration: This is the phase where you increase speed from either a motionless start or a slower pace. It lasts roughly 5-30 meters, depending on your starting speed and your targeted velocity.
Targeted velocity: This is the pace you settle into after acceleration, whether you’re running slow distance, a sprint, or something in-between.
While sprinters usually have a single acceleration phase, transitioning to a maximum velocity (top-end speed) phase, distance runners might accelerate multiple times during a run — turning a corner, stopping for a red light, surging during a race, accelerating into a hill, etc. Acceleration requires your legs to push in a horizontal direction. Think of pushing a stalled car. You lean into the car, directing force backward to move the car forward — you apply “horizontal force.” When you accelerate from a motionless start, about 90 percent of the force your legs generate is applied in a horizontal direction. The remainder is directed vertically — you apply force downward to propel yourself up (that’s how you get off the ground).
Once you reach your targeted velocity, horizontal and vertical force flip. Now you exert up to 90 percent of the force you generate vertically, with only a fraction applied horizontally. You no longer focus on pushing to move forward. Instead, you bounce.
Which brings us back to the pogo stick.
A pogo stick has an internal spring that compresses each time the pogo stick lands on the ground, then subsequently decompresses, releasing all the energy stored during compression and launching pogo stick and rider back into the air.
Your body employs the same principle. Your legs compress as your foot strikes the ground, storing “elastic energy” in your tendons and muscles, then releases that energy as your foot passes beneath your hips. Elastic energy accounts for up to 50 percent of the energy you use to power each stride. The remainder comes from muscle contractions. You use all this energy to apply force into the ground, then utilize Newton’s third law — “For every action there is an equal and opposite reaction” — to trigger an equal (upward) vertical ground force. The result is that you’re propelled into the air. You bounce like a pogo stick.
But even though you bounce “up,” you move forward. This is due to two things:
Inertia – Newton’s first law dictates, “A body continues in a state of rest or uniform velocity unless acted upon by an external force.” Think of a ball rolling across a smooth floor after an initial push. This inertia — your body’s resistance to a change in motion — keeps your body moving forward during each bounce, slowed only by braking (the slight deceleration that occurs each time your foot strikes the ground) and wind resistance.
Horizontal force – Even while bouncing, you devote at least 10 percent of the force you generate to “pushing” — you dedicate however much is needed to offset the slight deceleration caused by braking and wind resistance.
So running at your sustained, targeted velocity is basically bouncing over and over like a pogo stick, while allowing inertia (aided by a slight push) to move you forward.
Braking, Contact Time, and Your Odds of Getting Injured
They argue two things. First, they suggest that braking slows you down. Second, they postulate that braking and the extended contact time (the length of time your foot is on the ground) increase the load on your legs, which leads to injury.
Braking is required for sufficient elastic energy creation. And the extended ground contract time (approximately 125 milliseconds in a non-elite sprint, 300 milliseconds during a distance run) associated with a normal footstrike is the minimum needed to generate efficient muscular force. Therefore, landing beneath your hips slows you down by decreasing both elastic energy storage and muscle force production.
Gravity does play a role in your stride. It helps create the impact force that you’ll store as elastic energy and then release to get airborne. But it isn’t falling forward that creates a fast, efficient stride. It’s inertia, maintained by means of a forceful bounce.
Pete Magill is a running coach, world-class runner, and author. As a coach, Magill has led his masters clubs to 19 USATF National Masters Championships in cross country and road racing and has worked with athletes of all ages and abilities. He holds multiple American and world age-group records and is a 5-time USA Masters Cross Country Runner of the Year. Magill is author of Fast 5K,SpeedRunner, Build Your Running Body, and The Born Again Runner.