Hack Squats

With your back on the sliding pad, place your feet about 6-8 inches apart on the center of the platform and your toes pointing forward at their natural angle; place your hands on the handles located above your shoulders.
 
Straighten your legs, unhook the safety catches, and then bend your legs slightly; keep a good arch in your low back, with your back and abdominal muscles tensed throughout the movement.
 
Squat down until your thighs are slightly below parallel to the floor, being sure to keep your knees moving out in the same direction as your feet.
 
Reverse the movement and return to the starting position.
 
The entire motion should be done in a smooth, controlled manner that lasts about 4 seconds, with the downward movement no faster than the upward.
 
Do not bounce at the bottom of the movement and never lock your knees at the top.