When strong guys do squats as fast as they can, the heavier the weight, the slower they move — even if they're trying to go fast. The heaviest weights always result in the slowest lifts, and this seems to be true for everyone.

When strong guys do fast squats with heavy weights, their leg muscles don't work harder the heavier the weight gets—until they go all the way up to their max. At max weight, two of the three main quad muscles actually fire way more.

When strong guys do fast back squats with heavy weights, their butt and hamstring muscles work harder the heavier the weight gets — especially the butt muscle, which really kicks in when the weight is 80% or more of their max lift.

When strong guys do fast squats with heavy weights, their butt muscles kick in later during the upward push compared to when they use lighter weights — it’s like their body adjusts how it uses those muscles depending on how heavy the weight is.

When women who lift weights do squats with a really heavy weight (close to their max), their leg and butt muscles have to work a lot harder than when they use lighter weights.

Lifting heavier weights for fewer reps makes your legs much stronger than lifting lighter weights for more reps, but both types of training help your upper body a little bit too, and this might work differently for different muscles.