Your quads work hard during squats no matter how heavy the weight, but your glutes really kick in only when the weights are heavy or you're almost too tired to keep going.

When you squat with heavier weights, your butt muscles (glutes) have to work way harder than your thigh muscles (quads), more than you'd expect just from the added weight.

When strong guys do fast squats, their thigh muscles turn on in a set order—one muscle first, then another, then a third—and that order doesn’t change no matter how heavy the weight is.

When women who lift weights do squats with 80% of their max weight, their butt muscles work way harder compared to their hamstrings — more than with lighter or heavier weights.

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.