The heel-elevated side step-up keeps the torso more upright and shifts emphasis toward the quads and VMO. It is a smart variation when standard step-ups turn into too much forward lean.
Seated leg extensions can light up the quads when constant tension is the goal. Higher reps, smooth tempo, soft lockout, and a brutal final hold turn this isolation move into serious work.
The original hack squat was not a machine exercise. This post brings back the barbell version with a heels-elevated twist for quad emphasis.