Originally Posted by hamster

Using the same principle as BMI, you can calculate a number called "fat-free mass index". That's simply your fat free mass divided by height squared. If you have BMI of 25 and 6% body fat, your FFMI is 25*0.94=23.5.

An average untrained and sedentary adult might have FFMI on the order of 19-20 if it's a male or 16-17 if it's a female (slowly decreasing with age). Endurance sports do not increase FFMI beyond these levels and, in fact, may even lower it slightly. The only way to raise it is through weight training. A male college baseball player might be at 20-22, a football player might be at 24, and Mr. Olympia bodybuilders from the pre-steroid era averaged 25.4. It is thought that you can't raise FFMI much above 25 without steroids.

Having naturally wide bones might move you up a point or two at the most. Your reference to being at 6% body fat and overweight suggests to me that you were seriously hitting the weights at the time.

Conversely, a FFMI below ~18 for a male is a sign of malnutrition (starvation or a protein-deficient diet) and it is generally neither healthy nor sustainable. You rarely see people below 17-18 in the developed world except for cases of extreme dieting.

This all gives you a natural way to look at the BMI and the effect of training. At 10% body fat, BMI 19 = FFMI 17.1 (starvation), BMI 22 = FFMI 19.8 (normal for a skinny untrained person), BMI 25 = FFMI 22.5, BMI 30 = FFMI 27 (not happening without steroids). At 20% body fat, BMI 19 = FFMI 15.2 (not happening unless it's a formerly morbidly obese person in the middle of a poor-quality extreme diet), BMI 22 = FFMI 17.6 (ditto), BMI 25 = FFMI 20 (average for a sedentary and borderline overweight person), BMI 30 = FFMI 24 (college football player in a defensive position), BMI 34 = FFMI 27.2 (steroid-bulked national-level football player).



That would put your FFMI at the time at 21.8. Somewhat high for an average person but healthy.