If you're heading off to town on your bike or going for a relaxing cycle in the countryside, be sure not to pedal too quickly. That’s the advice of a group of physiologists who have found that people burn more energy than they need to when pedaling furiously, elite cyclists excepted. The researchers reached that conclusion by developing a new equation to describe cycling performance, which they say should help people get into better shape.
Many bicycles already contain a device designed to prevent overpedaling: gears. Muscles work best when contracting neither too quickly nor too slowly. To travel fast, you will use less energy in a higher gear even though you have to push on the pedals harder, because your leg muscles are far more efficient at the lower contraction rate. Similarly, lower gears are better for going uphill because they keep you from pedaling far more slowly than you should.
To find out exactly how pedaling rate affects energy consumption, physiologist Federico Formenti of the University of Oxford in the United Kingdom and colleagues studied how 10 men of various ages and fitness levels performed on an exercise bike in a laboratory at the University of Auckland in New Zealand (where Formenti was previously based). Each cyclist pedaled faster and faster against different loads while a face mask monitored how much oxygen he was consuming—a measure of metabolic rate.
Formenti and co-workers made 160 such measurements (16 for each participant), and compared them with rates of oxygen uptake predicted by an equation recommended by the American College of Sports Medicine (ACSM). This equation involves just two variables: a cyclist's mass and "work rate," equal to the resistive force of the exercise bike multiplied by the distance that the bike would travel were it not attached to the floor.
The researchers found that they could get a better match to observed values of oxygen uptake by modifying the equation to add a third variable: the pedaling rate, they report today in Physiological Reports. The ACSM equation estimates oxygen uptake fairly well most of the time, Formenti says, because the formula for work rate already incorporates pedaling rate. But, he points out, the equation fails to capture the relative toll imposed by resistive force and rate of pedaling. In fact, he and his colleagues found that at high pedaling rates and low work rates, the test participants used up most of their energy simply spinning their legs. "Muscles' physiological response to exercise is very different if they are contracting slowly against a high resistance, compared to their contracting rapidly against a low resistance," he says.
Oxygen uptake during exercise is often used to measure fitness. Formenti says the new equation should improve fitness monitoring in gyms that aren’t equipped to analyze breathing directly. He cautions that his group's results need to be confirmed by wider studies of exercisers, including women, but he notes that data from earlier studies totaling 50 cyclists gave a good match to the new equations’ values of oxygen uptake.
Ernst Hansen, a sports scientist at Aalborg University in Denmark who was not involved in the research, agrees that equations describing cycling performance should include pedaling rate. But he argues that the new equation is relatively simplistic because it does not account for individual differences in muscle-fiber efficiency and choice of pedaling rate. Formenti acknowledges that such variations are beyond the scope of his group's study but says they could explain why elite cyclists cycle efficiently even though they pedal very quickly.
As to exactly why so-called recreational cyclists are better off pedaling more slowly, the jury appears to be out. Formenti and colleagues think it’s due to what they call "internal work," the energy cyclists consume as they move their legs up and down—as distinct from the "external work" (work rate) that propels the bike forward. The researchers measured internal work during their tests by tracking pedaling motion with infrared beams bounced off the cyclists’ body. They found that internal work jumped 10-fold when a cyclist increased his pedaling rate from 50 to 110 revolutions per minute, regardless of the external work done. As a result, they say, an average-sized man cycling at the higher rate would use about 60% of his energy just to spin his legs.
Others, however, disagree with this interpretation. Steven Kautz, a biomedical engineer at the Medical University of South Carolina in Charleston, agrees that estimates of oxygen uptake need to factor in pedaling rate but says the reason has nothing to do with internal work. Internal and external work are "not independent additive quantities," he says, because the former to some extent generates the latter. Instead, he maintains, the effect of pedaling rate is probably due to the subtleties of muscle dynamics.