Recent research is beginning to show a negative relationship between the amount of subcutaneous fat you have covering your muscle and the amount of force that muscle can produce per unit of area. That just means that as subcutaneous fat builds up on top of the muscle, its ability to generate force begins to decline.
In light of this recent and growing body of research, a group from the University of Michigan wanted to see if this same effect was evident in non-obese and otherwise healthy young people. In addition, they wanted to see if 12 weeks of training would influence the effects of fat on the muscle. The study included 634 non-obese subjects (253 males and 381 females, with an average age of 23). Subcutaneous adipose tissue (SAT), muscle mass (magnetic resonance imaging–derived SAT and biceps muscle volume), isometric and dynamic biceps strength, and muscle quality (strength/muscle volume) were measured at the beginning and after 12 weeks of resistance exercise of the nondominant biceps muscle.(1)
As expected, muscle quality, or the strength per unit of muscle volume, was negatively associated with the amount of SAT covering the muscle at baseline. Interestingly, though somewhat disheartening, was the finding that not only did the fat covering the muscle hurt strength at the beginning of the trial, but it also negatively affected adaptation to the training regimen! Ouch! Keep in mind both the relationship between adipose tissue and muscle strength, as well as the relationship between the adipose tissue and the muscles’ adaptability to training, were both statistically significant. In females, however, there was no negative effect on strength gains after training; the effect was observed only in males.
Easy Does It
So what’s behind all this? Recent research has shown that “crosstalk” is going on between muscle stem cells and fat stem cells. The stem cells from fat are sending signals that trigger simultaneous muscle degeneration and formation of new fat cells, scar tissue, and/or collagen deposition within the muscle.(2) This ultimately leads to a decrease in strength and hampers your muscles’ ability to recover properly from training.
In practical terms, this new knowledge tells us that gaining a lot of fat from perpetual “mass cycles” will, at some point, begin to directly hurt your gains. Keep in mind that the subjects involved in this study were not obese by any stretch. Your best bet is to try to stay under 12%–13% body fat in the off-season or when you’re just trying to put on some significant scale weight. Once you’re done putting on size, take some time and lean out before you hit it again. Your muscles will thank you for it!
References: 1) M.D. Peterson et al., Int. J. Obes. (Lond.)., 35(8):1095–2103, 2011. 2) M.S. Rodeheffer, Nat. Cell Biol., 12:102–04, 2010.