1. Your maximum number of muscle fibers
2. Your percentages of fast- and slow-twitch fibers
3. The shapes of your muscles when fully developed
On the downside, unless you were born to anchor the 4×100 relay at next summer’s Olympics, you can forget about ever reaching that goal. The athletes at the extremes—the fastest and strongest, the ones with the best-looking muscles, and the ones capable of the greatest endurance—were already at the extremes from the moment sperm swam headlong into egg.
The upside is that there’s a lot of wiggle room in between. Few of us ever approach our full genetic potential. You probably will never be a freak, but with the right kind and amount of work, you can always be a little freakier than you are now.
The best way to do that is to learn to use your muscles’ very own juice machine.