Bicycle Biomechanics [Mechanical engineering]

Bicycle Biomechanics

The Biomechanics Of The Pedal Stroke

Photo: Michael Dodge/Getty Images/Challenge Family

For most of the century or more that humans have ridden bikes, we didn’t know very much about the pedalling 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 pedalling, 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 pedalling 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.

Pedalling Is a Restrictive Athletic Motion
The pedalling 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.



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