Several muscles contribute as agonists, or prime movers, in step-ups. Your quadriceps and hamstrings -- the muscles on the front and back of your thighs -- as well as your gluteus maximus, provide most of the power during this exercise. Other muscles contribute, some provide power, others stability, but none work as hard as your legs and backside. Consult a health-care practitioner before beginning any strength-training program.
Quadriceps and Knees
Your quadriceps muscle is the most active muscle in the step-up, according to a 1984 study published in "Physical Therapy." The higher you have to step, the more your quadriceps has to work to straighten your leg out. Your quadriceps works hardest when your knee is bent, but continues to work until both your knee and hip straighten. Your leverage improves as your leg straightens, reducing the strain, but not the recruitment, on your quadriceps, making this muscle active through the entire range of motion of the exercise.
Gluteus Maximus and Hips
Your gluteus maximus is the largest muscle of your posterior. This muscle serves two functions during the step-up -- pulling your thigh back into line with your torso and helping keep your torso upright. All of this work occurs at the hip joint. The degree of work your gluteus maximus performs depends on the height of your box or platform and the amount you lean forward. Leaning forward may force your gluteus maximus to work harder, but this puts more strain on your lower back. If you wish more work for your backside, use a higher box.
Hamstrings, Knees and Hips
Your hamstrings work to stabilize your knee joint and your torso at the hip. The degree of activation during a step-up depends on box height, torso angle and how far you step forward in addition to stepping-up. The more you have to pull yourself forward as you rise, the more your hamstring will work. This limits the height you can step-up to, so there will be a trade-off between quadriceps and hamstring recruitment during a step-up.
Numerous muscles provide support during a step-up, including your calf which stabilizes your knee and ankle joints. Your abdominals and lower back contract to keep you from leaning forward or backward. The muscles at the sides of your waist -- your obliques -- contract to keep you from wobbling from side to side. This stability occurs around the pelvic joint. While all of the muscles work to a degree, none of them work hard enough to consider the step-up an effective training tool for these muscles.