Stand on upper platform with stirrups grasped to sides.
Extend one leg back on forefoot. Lower body on other leg by flexing knee and hip of front leg until knee of rear leg is almost in contact with lower platform. Return to original standing position by extending hip and knee of forward leg. Repeat by alternating rear lunge with opposite leg.
Keep torso upright during lunge; flexible hip flexors are important. Forward knee should point same direction as foot throughout movement. A long lunge emphasizes Gluteus Maximus; short lunge emphasizes Quadriceps. See Cable Rear Lunge for similar exercise.