The Physics of Running

Posted by Danny Dreyer on Mon Apr 30th, 2001, 2 comments

I got a "C" in high school physics. It wasn't because I wasn't smart - it was because I didn't study. I was fascinated by all the cool experiments that my teacher would demonstrate, but I just couldn't bring myself to do the math and to tell you the truth, I was lazy. That's water under the bridge at this point, but what I'm discovering now is that I'm applying some of what I was actually able to absorb from that class to my running. A huge benefit of following these laws of physics is that when I'm working with them, my running becomes more efficient because the laws of motion are on my side. When I'm working against them, I pay for it big time ... with tired or sore body parts. I'm still lazy, but I love to run.

Basic High School Physics 101 - The Law of the Pendulum:
"The speed at which a pendulum swings depends on the length of the pendulum, not on the amount of weight at the bottom."

When you want to get a pendulum to swing faster, you need to shorten it. If your grandfather clock is running a little slow, you don't change the weight of that big round disk at the bottom, you just slide the disk up a little. In essence, you're shortening the pendulum. So, what does this have to do with running, you ask?

Plenty. Your arms and legs are both pendulums. Think about it: when you swing your arm, your shoulder is the top of a pendulum and your hand is at the bottom. Likewise, when you swing your leg, your hip is at the top of a pendulum and your foot is at the bottom. Therefore, in order to get your pendulum (either your arms or your legs) to swing faster, you need to shorten them. There are a number of ways to accomplish this, but I will only tell you the quickest and easiest way: bend.

That's right. When you bend either your arms or legs, you are actually shortening your pendulum because you're moving the center of gravity towards the top of the pendulum. This is how to get your arms and legs to swing faster without using more muscles. This accomplishes a couple of things. First, it prevents you from having a slow tempo, which translates to more time spent on each foot which requires additional muscle usage. And secondly, when you swing faster, you run faster. It's that simple.

Try this. Stand on one leg and swing the other leg forward and back. Keep your leg straight while you're doing this. Take a few swings and then stop. Now do it again and try to swing your leg faster (still keep it straight). Did you notice an increase in effort? You should. That's because it takes muscle to make your "pendulum" swing faster than it wants to. Your quads pull your leg forward and your glutes pull it back, and so on.

To get your "pendulums" to swing faster and easier just bend your leg at the knee and bend your arm at the elbow. Try this. Stand up and let your arms hang straight down at your sides and then begin to swing them without bending at the elbow. Now bend your arms until your forearm is parallel to the ground, which will mean that your arms are "L" shaped. Swing your arms and notice how much less effort it takes to swing your arms. This same rule applies to your legs. If you don't bend your knees when you run, your pendulum will be too long and your tempo will be slow. When you run, focus on always keeping your arms and your legs bent at 90º at mid-swing. If it's any easier for you to remember, your shins will be parallel to the ground when your leg swings forward.

Another use of my high school physics was when I realized the importance of leaning and letting gravity do the work for me. That's right, just let yourself fall forward. Here's how it works according to Vern Vanderbuilt, a rocket scientist at NASA in Mountain View, CA:

The Law: Energy expenditure is directly proportional to how much you go up and down. If you can minimize up and down movement, you can lower your energy expenditure. The more you lean forward when you run, the more it adds a horizontal component to your energy usage.

The key to letting your lean do the work is to have impeccable posture. When you lean, you should lean from your ankles, not from your waist. Let your imagination take you away and pretend you're a Nordic ski jumper gracefully extending yourself out over the tips of your skis, body fully extended. Leaning is so efficient, in fact, that if you want to run faster all you have to do is lean more and gravity will pull you forward at whatever speed your legs will allow. Your lean is your gas pedal. The more you lean, the faster you go. The less you lean, the slower you go. There, we just cut the workload on your legs in half! Instead of pushing, you are being pulled!

Here's an exercise to help you work on your lean: Stand up straight about 6" away from a wall, facing it. Now, without changing any part of your posture, allow yourself to fall forward by merely relaxing your ankles. Don't tilt your head forward or bend at the waist. Just hold your hands in front of your body with your elbows bent and stop yourself with your hands so that you don't break your nose. Do this repeatedly until you can comfortably fall forward with your posture remaining intact. Remember to lean from your ankles. While doing this exercise, try to familiarize yourself with the feeling of gravity pulling you forward because that's what you want to feel when you're out there on the road. If it helps you, I prefer the word "falling" to describe my lean while I'm running. Another image to use is to imagine you're running downhill all the time. Be sure that as you are leaning, you are not reaching farther out front with your stride. My rocket scientist friend pointed out that, if you take a plumb bob and drop it down from your center of gravity while you're running, your foot should never land in front of that imaginary point. If you do, you're adding a force in the opposite direction which means that you're putting on the brakes. So, when you lean, just be sure that your foot comes down underneath you, not in front of you.

Here's something that you can combine with your leaning practice. It may seem radical, but once again, our NASA rocket scientist confirms it's true. If you're like 99% of the runners out there, here's how you run you propel yourself forward by pushing with your legs. You're driving with your quads and then finishing off your stride by pushing off with your toes. What's wrong with this picture? Not much, really, other than the fact that you're working about twice as hard as you need to. I weigh 140 lbs., and that means that if I'm pushing off, I'm using my leg muscles to do an upside down 140 lb. leg press every time I take a stride. That takes a lot of effort, burns a lot of fuel, generates a lot of lactic acid, and requires that I breathe a lot of O2 to oxygenate those big quads (actually I have sort of small quads).

Try this instead: lift your legs instead of pushing with them. The amount of energy it takes to lift the weight of your leg (in my case, I figure about 30 lbs. apiece) is considerably less than pushing your entire body weight up into the air. It doesn't take a rocket scientist to understand this basic premise. As I said earlier in this article, if you're leaning, then gravity is pulling you forward at whatever speed you'll let it. So, what is optimal here is to lift your legs forward to keep up with your forward fall. This mode of using your legs also has the wonderful side effect of reducing the impact to your knees because the upward lift of your knees counterbalances the downward motion of your other leg. In layman's terms, it lightens your footstep. According to physics, you're creating a force in the opposite direction, which counter-balances the force in the first direction.

Also, when you lift your legs, you can eliminate pushing with your calf muscle. Here's why you don't want to have your calf muscles energized.
Try this out on yourself. Stand flat-footed for 10 seconds, then stand up on the balls of your feet for the same amount of time. When you're up on your toes, you're propelling yourself forward by using your calf muscles. From physics, if youíre just standing there youíre not expending energy because your skeletal system is supporting your weight. When youíre up on the balls of your feet you are expending energy in your muscles just to support your weight. This is not only a fatal mistake made by even the best runners but can lead to shin splints. If you keep your heels down it forces those quads of yours to do the job for which they were designed.

Next is your stride length. Have you ever thought of yourself as having a set of gears, like on a bicycle? Well, that's exactly what your stride length is for. Look at it this way, you don't pull away from a stoplight in 4th gear for the same reason that you don't take off on a run with a long stride. Imagine taking off from a standing start on your bike and being in your highest gear. You can probably feel your quads burn. That's what your low gear is for! Well, your body is no different and a short stride length is the equivalent of a low gear. Let your runs begin with a shorter stride and let it gradually lengthen as your muscles get warmed up and looser. When you hit your cruising speed your stride will be the right length, not too long and not too short.

While we're on the subject of gears, how about a little hill lesson? Pop quiz "What do you do when you're riding your bike and you come to a hill?" I hope you answered "Downshift." Well, guess what? Your legs are no exception to the rule. You still need to downshift on hills, even when you're running. This involves shortening your stride, which in turn reduces the level of muscle usage on your legs. It's easy to do but you have to remember to do it or you'll get to the top of a hill and wonder why you're quads are so wasted.

Are you ready for another no-brainer? If you have a 150 lb. body that you need to carry across town, which would be the easiest way to get the job done?

A.) Let one person do it.
B.) Let 6 people do it.

If you answered with B, read on.

Just imagine all of your individual body parts working together as a team. Then imagine that if that were the case, then no single part of your body would be carrying more than it's fair share of your total weight. Your legs wouldn't be working any harder than they need to. Your arms wouldn't be working any harder than they need to. Everyone would share in the load equally, and that's exactly what happens when all of your various body parts are involved in your running. Each part doing a little work is much more efficient than one or two parts doing most of the work. And, when all of your body parts are doing what they each do best, you've got the "Dream Team" in your court.

There are lots of runners out there that look as though some area of their body is not really participating. It's limp or stiff and generally not being a good helper for the work done by the rest of the body. I've also seen the opposite and everything in between. The next time you watch the New York City Marathon on TV, keep your eyes on the Kenyan that's in the lead and challenge yourself to find some part of his body that he's not using. Good luck.

Exercise: Do this on your next run. Once you're warmed up and running, ask yourself if there is any part of your body that is just along for the ride? Or, is there any part that is working too hard and doing more than it's fair share? To help you out, here's a little check- list:


Here's how the physics works out. Each of these body sections has an interdependent relationship with the rest. They each have a "job" to do, and if everyone does their job, the machine runs smoothly and efficiently. If one of these parts isn't moving as much as it should, it could inhibit the movement of the other parts. For example, if you have a stiff neck, it will inhibit the movement of your shoulders, which will reduce your arm-swing, which will increase the effort of your legs, which will make your feet work harder than they need to get it?

Just spend a little time checking in with your body parts to see if there's anybody not doing their job. If you want to become a more efficient runner, learn to keep your body parts working as a team.

There is no question, high school physics has been valuable to me even though I didnít give it much focus. Optimal running form can be confirmed with the basic laws of physics, so I use those laws to help me tune in to the most energy efficient way to run. What Iíve discovered is that my body prefers it when Iím really efficient (even though my competitors donít!!) and I still get to be lazy while I'm running to my hearts

Sound Interesting? There's more in my 200+ page book, ChiRunning: A Revolutionary Approach to Efforless, Injury-free Running. Even better, watch how you can do it while I explain in my step-by-step program available on DVD.

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2 CommentsLeave a comment below

I was just at another site with the same metaphor of the pendalum and the grandfather clock.

Thomas Putnam Oct 1st, 2014 09:54pm

very nice article. Really helped me with my running.I’m seeing results and am very thankful to Danny who wrote this.

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