Single-leg power, straight up.
Push through the top foot, don't bounce off the bottom leg to get up.
The step-up is single-leg strength at its most honest, one quad hauls your bodyweight plus two dumbbells straight up onto the box while the glute locks the hip out at the top. The trailing leg is only allowed to come along for the ride, no push-off, no bounce, so each side earns every rep alone. It earns its place for building legs that produce force from a dead position, the kind of strength that shows up in sprinting, climbing and everyday life.
Around knee height is the sweet spot for most people, thigh roughly parallel when your foot is planted. Lower boxes make it easier and more quad-dominant through a shorter range, higher boxes increase glute demand but invite cheating.
Slow the start of each rep and think about the bottom foot going quiet. Some lifters lift the back toes just before driving up. If you cannot step up without a bounce, the box is too high or the dumbbells too heavy.
They train similar muscles with different demands. Step-ups start from a dead position with no stretch reflex, so they build raw drive from the front leg, while lunges load a deeper stretch. Strong programmes tend to rotate both.
