Adjusting the Hack Squat Machine
Position yourself with your feet on the platform and your back against the hack squat machine's back pad; both platform and pad are usually angled back at about a 45 degree angle.
Check that you can take the full weight of the machine's shoulder pads with your knees still slightly bent. If the shoulder pads are too high for this, now's the time to adjust the sled a little bit lower.
Adjust the sled position, as necessary, by first supporting the weight of the shoulder pads and sled, then flipping the safety stopper handles out to release the safety locks. They're usually located at about waist level and stick straight out from the machine. Move the sled to a lower position on the safety stoppers and check your body position again.
Using the Hack Squat Machine
Load the bars on either side of the hack squat machine's sled with weight plates. The sled is the part that moves up and down with you as you exercise; because the machine moves in only one place, you don't need to put weight collars on the bars.
Position yourself in the machine again: feet flat on the platform, back against the machine's back pad, shoulders tucked firmly beneath the shoulder pads. Adjust your feet so that when you squat down, your knees won't bend forward past an imaginary line leading straight up from your toes.
Take the weight of the sled on your shoulders, then flip the safety stopper handles out to release the safety stoppers. Let go of the stopper handles.
Keep your hips and shoulders against the back pad and your heels flat on the foot platform as you bend your knees, squatting down to about a 90-degree angle. Some less-conservative experts recommend stopping just short of full flexion; as a general rule, only go as far as is comfortable.
Straighten your legs, pressing the sled back up to the starting position. Aim for a set of eight to 12 slow, controlled reps; once you can do more than 12 repetitions, the American Council on Exercise recommends increasing the amount of weight you're lifting by 5 to 10 percent.
Reengage the safety stoppers by moving the sled as high on the machine as possible, then flipping the stopper handles back in toward your body. Carefully lower the sled until you feel it engage with the stoppers on both sides.