How to pedal efficiently: “dead centre size”
[UPDATE 9/17: Check out reader Phil Koop's analysis of the paper in the Comments section of this post. Definitely worth a click.]
An interesting Norwegian study on pedalling efficiency has been posted online at Medicine & Science in Sports & Exercise, proposing a new measurement to determine whether cyclists are getting the most out of their pedal strokes.
The basic goal of cycling, obviously, is to convert your effort into forward motion of the bike. To do that, experts have long believed that a quantity called “force effectiveness ratio” (FE) should be optimized. FE basically tells you how much of the force you’re applying with your foot at any given moment is directed perpendicular to the crank. For example, when the crank is parallel to the ground and you’re pushing straight down, FE is very high. But when the pedal is at the very bottom of the cycle, if you’re still pushing down instead of back, your FE will be lower. Averaged over the whole pedal stroke, a typical FE might be about 50%.
This makes sense in theory, but no one has been able to show that better cyclists have higher FE (at least if you’re comparing elite and sub-elite, as opposed to complete novices):
From an energetic-mechanical point of view, this should be the best “technique” parameter because the energy used for producing the ineffective, static component of force will not contribute to external power. However, it is not necessarily so that man is able to produce the highest FE at the lowest metabolic cost: the coordinative challenge of generating power while creating a rotation of the crank by extending the lower extremity may require additional, apparently ineffective, energy expenditure.
The Norwegian researchers propose instead a parameter called “dead centre size” (DC). They look at the two weakest parts of the pedal stroke — the top and bottom — and compare how much useful force you’re generating to the average useful force over the whole pedal stroke. A typical value is about 25%. The idea is that the better these “low points” in the stroke are, the smoother your overall stroke must be, so you’ll avoid wasting energy accelerating and decelerating and so on.
So they did a study with 21 competitive cyclists, using force-sensitive pedals and high-speed video and so on, and sure enough found that DC was much better at predicting the overall efficiency of the cyclists than FE. So what does this mean? Well, it’s consistent with the idea that you shouldn’t worry too much about trying to generate power on the upstroke, since that’s a hopeless task. Instead, focus on keeping the whole cycle smooth, not letting power dip too far at the top and bottom. One thing I couldn’t tell was whether it’s possible for anyone without a high-tech laboratory to get a measurement of DC, in order to see whether they’re improving their form over time. It would be pretty cool if local bike shops were able to offer the service.