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...

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.

Among men who regularly lift weights, those who kept their usual number of sets improved their maximum squat strength more than those who added 30% or 60% more sets, suggesting that increasing training volume beyond a certain point may reduce gains in maximal strength.