Start: Stand in front of a sturdy step or bench with a barbell resting on your traps. Put your nondominant foot flat on the step-that's the leg that's going to do the work. Make sure your lower back is in its naturally arched position, your shoulders are pulled back, and your eyes face forward.
Finish: Push down through your working heel to lift your other leg and brush the step with that foot. Then return to the starting position. It's important to keep all your weight on your working foot-your other foot is just along for the ride. Finish the set, then switch and do the same number of repetitions with your dominant foot on the step.
Do the standard stepup, but step all the way on and all the way off the bench each time without pausing between reps. You'll use momentum and sacrifice strength gains, but you'll get that hear rate up.