Set-up: Find an immovable, sturdy bench or other surface which is roughly knee height. Place one foot flat on the bench with the upper leg angled out a few degrees.
Movement: Keeping the back leg straight, drive upwards through the forward leg and gently place your off-leg onto the bench. Step down under complete control, flexing through the lead knee and bracing through the core.
Emphasis: The top leg should perform all of the work in the step-up. While pushing with the back leg will allow you to lift more weight, it will degrade the purpose of the exercise.
Repetitions: Perform all prescribed reps on one side, then on the other. This counts as one set.
Benefits: Step-Ups are a great exercise for training the VMO (Vastus Medialis Oblique) - one of the component muscles of the quadriceps that crosses the knee joint and is important in maintaining knee stability. If your knees cave inwards while squatting or when landing while jumping, the VMO may be weak and the step-up an excellent choice.