We all know as we get bigger and stronger that the Body Mass Index is not a good metric for athletes. More useful, for us, would be separated data with Lean Mass Index (sometimes called “Fat Free Mass Index”) and Fat Mass Index, so you could target each metric independently. For my part, my own long term goal is to hit a lean mass index of 27 with a fat mass index of 3.0 - that way I can have a body mass index of 30 - technically obese - but at 10% body fat.
Lean mass index can be defined as (kg of fat free mass)/(height in meters)^2
Fat mass index is (kg of fat)/(height in meters)^2. If you prefer pounds and inches, go ahead, just multiply by 703 afterwards to make the units come out right.
In 2002, a paper was published in Switzerland with some statistics that show what the population statistics were at the time. I find this useful in my own training as I decide whether to target losing fat or gaining muscle in any particular workout cycle.
The graph below shows lean and fat mass index graphed for the 5th through the 95th percentile for different age categories.
For my part, I’m age 55 so I get to use the orange and green bands. At 5’11", 192 pounds and 18% body fat (DEXA), that gives me a fat mass of about 35 pounds and a lean mass of 157. My LMI is thus 22 and my FMI is 4.8. How do I measure up?
On the upper set of lines, I see my lean mass index is in the 94th percentile. A long way to go, sure, but I’m basically doing better than 18 guys out of 20, close to 19. Not terrible, but more work ahead, to be sure.
On the lower set of lines, I’m showing more like the 70th percentile. I plotted the percentiles there in reverse because I wanted to show the “direction of goodness” to be the same in both sets of data.
What I draw from this is that while I want to continue my progress in adding muscle, I still have some work to do in cutting fat. To be where I am relative to the population in fat mass index, I would have to drop around 13 lbs with no muscle loss. So that’s my goal for the rest of the summer.
Paper reference is in the title. For Americans, it’s probably a little conservative - this was almost 20 years ago, and in Switzerland.