BMI is a measure of the proportion of weight to height. It is calculated by dividing the bodyweight expressed in kilograms by height expressed in meters, squared. It is widely used by health experts to assess population changes in obesity. It doesn’t work in many athletes because they have more muscle mass than normal.

For example, bodybuilding champion Ronnie Coleman is 5′11″ and weighs 315 pounds during the off-season, which gives him a BMI of 43.9. This places him in the morbidly obese category, even though he is approximately 5 percent fat, which is far below the 20 to 25 percent fat typical in men his age.

Researchers from Michigan State University – led by Joshua Ode – concluded that BMI was a poor measure of fatness in college athletes. It overestimated fatness in male and female athletes and male nonathletes, but underestimated fatness in female nonathletes. BMI is simple and convenient, but a poor measure of excess fat and obesity in athletes. [sources: Medicine & Science in Sports & Exercise, Fitness RX]