Older men who start out weaker or with smaller fast-twitch muscle fibers tend to get bigger gains from heavy weight training, but even knowing how weak they were at the start doesn’t fully explain why some people improve more than others.

When older men do heavy weight training, some get much stronger and build more muscle than others—even when they do the same workout—and those differences are bigger than any mistakes we might make when measuring them.

When older men and women over 60 do strength training, the men tend to get stronger in their arms and legs than the women do, even when they’re doing the same workouts.

When older men and women over 60 start lifting weights, women tend to get stronger faster relative to how strong they were at the start—especially in their arms and legs—compared to men doing the same workout.

Some people get much bigger and stronger from weight training than others—even when they do the exact same workouts—because everyone’s body responds differently to exercise.

When older men get stronger quickly at the start of weight training, it’s often just because they’re learning how to lift better—not because their muscles are growing or getting stronger for real. So, early gains don’t mean they’ll keep getting stronger over time.