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Percent Muscle and Fat Content
I address this discussion especially to athletes looking to improve performance over time.
Loss of muscle + gain of fat = reduced performance
Shown below is a comparison for my own DEXA scans, taken in 2006, 2007 and multiple scans in 2011. My biking fell off heavily in 2008-2012, and only in 2011 did I resume training seriously.
The DEXA scan shows that I gained 5117g = 11.2 pounds of body fat through March 31, 2011. Intensive training through September 2011 reversed all of that fat gain, and more.
More important to cycling performance, it shows that I lost 3193g / 7.0 pounds of muscle. Iintensive training in 2011 regained nearly all of that muscle.
Percent body fat
My March 2011 body composition was 20.6% body fat, twice where I wanted to be. Try strapping on a 25 pound backpack climb, and you’ll quickly find it a huge burden, even on flat ground. I like being light and nimble, and 20% body fat is no fun.
With intensive training, weight dropped by 12 pounds to 180 at 15.5% body fat by mid-may 2011. When cycling uphill, it’s a massive difference in comfort and fatigue.
The key point is to MAINTAIN OR INCREASE MUSCLE WHILE LOSING BODY FAT.
The graph below shows the relative proportions at various time intervals, fat and non-fat.
Comparing DEXA over time for a muscle group
I checked my records for lean mass in my legs. The 4203 gram lean tissue loss seen above from 8/2/06 to 3/31/11 is found to be (see below) as 3186 grams from the legs, or 75% of the difference in lean tissue (muscle). That leg muscle mass was gained back by later in 2011, and reached a new high by September 2011.
August of 2006 I achieved an all-time personal best for my key climbing benchmark (Old La Honda Road at 19:09), showing that peak muscle mass was the key factor involved. At that time I was also speed skating intensively as cross-training, which no doubt increased leg strength considerably. In 2011, I nearly matched my best effort without even training specifically for that time trial— the increased muscle mass and lower body weight account for that.