Book excerpt: The biomechanics of pedaling, from Andy Pruitt’s Complete Medical Guide for Cyclists
Editor’s note: This article was excerpted with permission of the publisher from Andy Pruitt’s Complete Medical Guide for Cyclists by Andy Pruitt, Director of the Boulder Center for Sports Medicine, with Fred Matheny. The book is available in bookstores, bike shops, and online. The table of contents and another excerpt are available at velopress.com.
For most of the century or more that humans have ridden bikes, we didn’t know very much about the pedaling motion or the forces that are exerted at different points in the pedal stroke. Without the technology of high-speed cameras to help them, riders and coaches tried to eyeball the spinning foot. They used this plus feedback from their bodies — in the form of pain in their quads— to guess what was going on.
From this, researchers deduced that skilled cyclists pushed down hard on the downstroke while at the same time they pulled up on the backstroke. It seemed like a logical conclusion based on watching cyclists legendary for their smooth pedaling, like five-time Tour de France winner Jacques Anquetil. Of course he pulled up at the back of the pedal stroke. How else could he time trial so fast with so little obvious show of exertion?
Pulling up was assumed to be crucial to fast riding — it unloaded the pedal, assuring that not only was the “rear” foot not going along for the ride while the other foot was trying to push down on the power stroke, it was also adding to the total power transferred to the pedal.
Because of advancing technology and the development of new ways to observe and measure biomechanics in action, we know a great deal about the pedal stroke. And one of the things we know is that even the best pedaling stylists don’t produce power when they pull up on the backstroke. The most they can hope for is to unweight the rear foot so it adds less drag to the power output of the foot that is pushing downward. But it’s not possible even to get the back foot out of the way entirely.
Sophisticated force-measuring pedals can tell us exactly what forces are being exerted during the pedal stroke and at what angles. Biomechanists like Jeff Broker, Ph.D., developed early models at UCLA in the 1980s. At the Boulder Center for Sports Medicine, we devised our own force-measuring pedals to help the 1996 U.S. Olympic pursuit team hone their skills.
So what is going on when we pedal? Cycling biomechanists often use a so-called clock diagram to illustrate the forces involved in pedaling.
As you can see in the clock diagram, these biomechanical findings are complicated and technical (see Figure 18.3). For the purposes of this book, I’ll offer a simplified version. Call it the layperson’s guide to pedaling mechanics. Here’s what we’ve learned.
Pedaling Is a Restrictive Athletic Motion
The pedaling motion takes place through a relatively small range of motion. If you’re using 170 mm crank arms, the legs move in a circle with a diameter of only 340 mm — less than 14 inches. Contrast that to the huge mobility required by basketball players, gymnasts, or triple jumpers.
So while cycling is less likely to cause muscle pulls due to excessive motion, a stretching program is crucial since the muscles aren’t stretched in their daily routine of pedaling.
As we saw in the section of this book on bike fit, the bicycle is a fixed machine that can be adjusted by such means as raising or lowering the saddle and changing the reach to the handlebars. Humans are also machines, and while adjustment isn’t possible (short of an operation to lengthen your femurs), the human body is adaptable.
The Foot Rarely Pushes Straight Down on the Pedal
The only point at which the foot is pushing straight down is at about the 3 o’clock position, as you can see from the clock diagram. The rest of the time, force is applied tangentially to the pedal, increasing shearing force and reducing the percentage of power from the quads that’s actually applied to the bike’s forward motion.
Fast Pedaling Lowers Force, Slow Pedaling Increases It
Lance Armstrong has made it popular once again to climb at a fast cadence. He and his coach, Chris Carmichael, know that low-cadence pedaling (60 to 80 rpm) requires large muscular forces, while fast cadences (around 100 rpm) lessen the load on the quads and transfer it to the cardiovascular system. Because the quads fatigue faster, and recover more slowly, than the heart, it makes sense to train your cardiovascular and neuromuscular systems to pedal rapidly.
The Best Cyclists Don’t Produce Power When They Pull Up on the Backstroke
As mentioned earlier, force-measuring pedals show us that no cyclists, not even track pursuiters who are capable of silky-smooth pedal strokes at 130 rpm, really exert upward force when the pedal is coming up from dead bottom center.
Mountain Bikers Most Closely Approach the “Ideal” Pedal Stroke
How could it be that mountain bikers get closest to the ideal pedal stroke? We tend to think of mountain bikers using a forceful, hammering pedal stroke as they ride up technical climbs.
But in fact, riding loose surfaces and steep climbs requires an extremely smooth pedal stroke. If the rider emphasizes the downstroke, the surge of power applied to the rear wheel causes it to lose traction on sand and gravel trails.
This phenomenon is painfully evident on Moab’s fabled Slickrock Trail. The surface isn’t loose; rather, it’s smooth sandstone that provides incredible grip to the tires. So it’s possible to climb insanely steep pitches—but only if you avoid any power surges to the rear wheel. The slightest jerkiness in the pedal stroke breaks loose the rear wheel and causes a painful slide down the “slick rock.”
Getting your skin rubbed off by Utah sandstone provides instant feedback, teaching skilled off-roaders to apply power all the way around the pedal stroke. They still can’t pull up, unless they’re pedaling at a very low rpm, but they come close.
Even Though There’s No Such Thing as a Perfect Pedal Stroke, It’s Still a Goal to Work Toward
You can improve your pedal stroke by doing the following drills:
Concentrate on the top and bottom of the pedal stroke. At around 90 to 120 rpm the pedaling motion is so rapid it’s nearly impossible to focus on and modify the different parts of the stroke. The feet simply go around too fast. The trick is to anticipate the motion you want and initiate it early. That means starting the upward pull of the pedal when the pedal is at dead bottom center and initiating the downward push as the pedal comes over the top and begins its descent.
Greg LeMond first described pulling through at the bottom of the stroke saying it’s “like scraping the mud off your shoe.” The image still works. But pulling through at the bottom is only half the story. You should also concentrate on pushing the knee toward the handlebars as it comes over the top and begins the power phase of the stroke.
By starting both motions well before you want their actions to take effect, you’re assured that by the time your command is sent from your brain to your legs, they’ll do the right thing at the appropriate time.
Do one-leg pedaling drills
Set your bike on a trainer and warm up. Then unclip one foot and rest it on the rear trainer support or on a chair or stool. Pedal with the other foot, emphasizing good form. The switch feet and repeat. Start by doing several sets of one minute for each leg in a low gear, and increase to sets of five minutes and larger gears.
One-leg pedaling forces you to pedal all the way around the stroke. It will be awkward at first, but with practice you’ll improve rapidly. And the pedaling efficiency you acquire will transfer to normal two-leg pedaling on the road.
Most cyclists now use indoor trainers, but old-school rollers can help you improve pedal form. The reason is that it takes a smooth stroke to even ride the things!
If you pedal awkwardly on rollers, you’ll weave all over the rollers or be unable to stay upright. Rollers are the ultimate biofeedback device for smooth pedaling.
As I mentioned earlier, riding loose-surfaced, steep climbs on a mountain bike is a great way to work on your pedal stroke. You don’t have to live in the mountains to get the benefits of this pedaling workout. Even short climbs are helpful.
Andrew Pruitt, EdD, PA-C, is the founder of the Boulder Center for Sports Medicine. Pruitt has worked with elite athletes for years and has become the world’s foremost expert in 3D bike fit analysis. His patients include some of the most formidable cyclists in the world including Fabian Cancellara, Andy and Frank Schleck, Jens Voigt, Alberto Contador, and other ProTour riders who either see Pruitt as part of their team, or fly to Boulder from around the world on their own accord. BCSM and VeloNews.com have partnered to produce cycling training and health columns on this site.