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Body weight has a measurable, negative effect on cycling and running performance. All else being equal, the lighter you are, the better you will perform as a cyclist or runner. But, of course, all else is not equal. As you lose weight, various aspects of your internal physiology change, especially if your weight loss is primarily training induced. It is difficult to separate the weight loss effect of training from other performance-boosting effects such as increased mitochondrial density, increased neuromuscular efficiency, etc. It’s not so much that all of these changes are physiologically interconnected—although many are, to a degree—as it is that they all result from the same stimuli simultaneously. As you get fitter, you typically get lighter and leaner.
Only to a point, however. It is, of course, possible to become too light and lean. Actually, it is not possible to be too light per se. If you could weigh one pound and maintain the same muscle contractile force and VO2max you have now, you would perform a lot better than you can now. But that’s just it: After a certain point, you can only become lighter and leaner by effectively starving yourself and thereby depriving your body of crucial fuel and dismantling parts of your body that it needs to perform at a certain level. Getting too skinny is not unheard of in endurance sports. It is especially widespread among female collegiate runners. But it is far more common for endurance athletes to carry too much fat on their bodies.
Naturally, optimal body weight and body fat levels are individual. But how does the individual athlete know what his optimal racing weight is? There must be some kind of formula you can use to determine exactly how much you should weigh for maximum performance. Right? Actually, there is not, and there probably never will be. The relationships between body weight and the training and dietary factors that influence it and between body weight and the dozens of other performance-relevant physiological factors affected by training and diet are so complex that it is impossible to reduce it all to a simple formula. Optimal racing weight can really only be determined functionally. Get in the best racing shape of your life, step on the scale, and there’s your optimal racing weight.
This answer is unsatisfying to the many endurance athletes who feel that they have never attained their optimal racing weight at any level of fitness. It leaves them without any kind of concrete goal to pursue. They know they could race faster if they got leaner, but without a specific number to chase, it’s all a muddle. That’s why I created an optimal racing weight prediction tool for my recently published book, Racing Weight. The crux of this tool is a body fat percentage table created from data compiled from thousands of body fat tests on men and women of all ages and fitness levels. The data are distributed into percentile rankings by age and gender. Do remember when you took the SAT, and your 550 verbal score (or whatever) put you in the 85th percentile among the whole population of students who took that test? This body fat table works the same way. For example, if you are a 40-year-old woman and your body fat percentage is 26.5, you are in the 50th percentile among women your age, meaning about half of 40-year-old women are leaner than you and half are fatter.
The tool I created operates on the premise that all endurance athletes are able to reach at least the 80th percentile for their age and gender group, while most athletes who are already between the 80th and 90th percentiles when not in peak shape are able to move a little higher still. To use my Racing Weight tool, you have to get an initial body fat measurement, which you can then use, with the aid of guidelines I present, to set an appropriate optimal body fat percentage estimate. Finally, you can use this estimate to calculate how much weight you would lose in reaching that body composition, hence a projected optimal racing weight.
It’s not an exact science, but it serves the purpose of enabling the athlete to set a realistic goal. Even if that goal turns out to be a little off the mark, it will increase the chances that you will actually attain your perfect racing weight by motivating you to train and nourish yourself into the best shape of your life.