Part 2: Zygote Stew
With all the tools we have to manipulate diet and training programs, and all the ways that lifestyle choices affect your physique, it's hard to believe that genes play such an outsized role in the results. We can accept that genetics determine our height and hairlines. But our muscles?
Start with satellite cells. These are stem cells within your muscles that provide extra nuclei, giving them a more powerful growth stimulus. The only way to know how many satellite cells you have would be to take biopsies of the muscles and run sophisticated and presumably expensive tests.
That's what researchers at the University of Alabama-Birmingham did for a 2008 study in the Journal of Applied Physiology. They found that the relative number of satellite cells predicted who would gain the most muscle over a 16-week training program. A quarter of them didn't gain any muscle at all in their quadriceps, while a quarter increased their quad mass by more than 50 percent.
More from Men's Health: The 17 Best Glutes Exercises
So if nothing else, we can conclude that Tom Platz, the bodybuilder with turbines for legs, started with a ridiculous load of satellite cells.
Muscle-building potential isn't entirely, invisibly mysterious. Sometimes you can look at someone before he starts training and see the potential. A guy who looks like an athlete, with wide shoulders and the kind of frame that doesn't disappear when he turns sideways, will probably look even more athletic when he's spent some time in the weight room.
The converse is someone like me. In my teenage years even fat guys thought it was fair to make fun of my skinny arms and legs. The size and shape of those muscles is limited by the length of tendons, relative to the length of bones.
Think of your biceps. A bigger muscle belly will have a shorter tendon connecting it to the forearm. The most genetically gifted lifter will have biceps that appear to begin right at the elbow joint; when his arms are flexed, there'll be little if any space between his upper- and lower-arm muscles.
More from Men's Health: The 23 Best Biceps Exercises
Individual muscles aside, when we talk about building muscle, more often than not we're also talking about gaining weight. That requires eating more than your body needs to maintain its current size, and it involves a different but equally complex set of variables.
Part 3: Weight, What?
It's easy to gain weight if you don't care what kind of weight you gain. But most guys who think it through will choose to do a "clean bulk"—muscle gain with minimal fat gain.
Alan Aragon, my coauthor on The Lean Muscle Diet, estimates that an entry-level lifter can gain 2 to 3 pounds of muscle mass in a month without adding much fat. An intermediate can gain 1 to 2 pounds a month, and an experienced lifter will be lucky to add a half-pound.
There's also the "dirty bulk," in which you lift hard and eat anything that doesn't move fast enough to get out of your way. How much you gain during a mass overfeeding, and how much of it is muscle, depends on two key variables. (Not sure how to get the best results from your workouts? Check out What and When You Should Eat to Build Muscle.)
The first is how lean you are to begin with.
Dr. Gilbert Forbes, a pioneer in the study of body composition, showed that fat and lean tissue increase or decrease in relationship to each other. When a lean person overeats, 60 to 70 percent of the additional weight will be lean tissue. It'll be the opposite for someone with high body fat, who'll gain 60 to 70 percent fat and just 30 to 40 percent lean mass.
But there's a lot of individual variation, and yet again your genes seem to be running the show.
Perhaps the greatest weight-gain experiment ever conducted began in the late 1980s at Laval University in Quebec. The research team took 12 pairs of identical twins, all relatively lean but sedentary young males, and overfed them for 100 days. The average increase was 18 pounds—about two-thirds lean tissue and one-third fat—but the range was from 9 to 29 pounds. The number-one predictor of how much an individual gained was how much his twin gained. (Genes also determined where they added the fat.)
Why didn't everyone gain the same amount of weight? In a 2014 study in the International Journal of Obesity, the researchers showed that those with the highest VO2 max (a measure of aerobic fitness) and the highest percentage of Type I muscle fibers (the ones responsible for long-duration, low-intensity work) gained the least weight, with the highest proportion of lean mass.
On the flip side, the men with the highest percentage of Type IIA muscle fibers—the ones that produce speed, strength, and power—gained the most weight, and the largest proportion of fat. (Find out Why Dwayne "The Rock" Johnson is Technically Obese.)
The same applied to men whose muscles had the most glycolytic potential, a measure of their ability to produce energy when they're moving too fast to use their aerobic energy system. For most of us, the glycolytic system is what we use for all-out efforts lasting 30 to 60 seconds (although well-trained athletes can use it for up to two minutes).
Thus, the men who were wired for long-distance sports gained the least weight when overfed, and gained the lowest percentage of fat. That's the good news for hardgainers. And the men who were predisposed to be better at lifting or sprinting not only gained weight easily, they gained a higher proportion of fat.
More: Why You Might Be Losing Muscle Tone and How to Stop It
