How to Perform the Hack Squat Machine
Powerlifters require exceptionally strong quads to lockout 400, 500, and 600+lb squats in a powerlifting meet. The bodybuilding community praises large sweeping quads that contribute to an aesthetically pleasing and balanced physique.
Regardless of your training style, most leg workouts begin with a heavy compound barbell movement like the back squat, deadlift, or clean. As the workout progresses volume increases and the complexity of movements decreases.
The hack squat machine is an exceptional compound push exercise to target the quadriceps and glutes. it's an excellent auxiliary lift for those looking to improve the back squat, those with injuries, and those looking for a brutal final exercise on leg day. This plate-loaded machine can be found in even the most hardcore gyms, using a lever or sled apparatus to hold the weight.
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This exercise allows the lower body to move in a motion similar to the squat with the addition of a pad to support the upper body and glutes. Some lifters find they're able to move their legs through an increased range of motion and use more weight compared to barbell back squats.
A barbell variation of the hack squat exists but form typically deteriorates quickly; many raise their hips too quickly, don't start at a low enough depth, allow their shoulders roll-in and slump, and fail to maintain straight back and arms throughout the motion.
The hack squat machine targets the quadriceps, a muscle group comprised of four heads (Rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis (Internus)).  This exercise engages the gluteus maximus, adductor magnus (inner thigh), and soleus (calf) to assist in completing the movement.
The hamstrings and gastrocnemius (calf) act as stabilizers during the exercise.  If you're looking to trigger serious quadriceps growth and willing to endure seriously delayed onset muscle soreness (DOMS) then you must include the hack squat machine in your routine.
How to Perform the Hack SquatApproach the hack squat machine and select the appropriate working weight. Distribute the weight evenly on both sides of the lever or sled apparatus.
don't place 55lbs on one side and 35lbs on the other side of the machine; doing so won't improve your gains and will likely lead to an injury. If this is your first time performing the exercise then pick a conservative weight that you can safely lift for 8 to 12 repetitions.
After selecting the working weight place your back on the padded support and ensure your shoulders are touching the shoulder pads. Raise your legs and place your feet flat on the lever apparatus or sled platform.
Take a shoulder width or slightly wider stance with the toes pointed forward or slightly outwards. Your stance should be roughly in-line with the center of the platform. At this point your knees may still be slightly bent; this is okay as the weight is resting on the safety pins.
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After setting your stance, take a deep breath, brace your abdominals, and push through your heels to fully extend the hips and knees. Release the safety pin apparatus and then hold the handles on either side of the machine's shoulder pads. Holding these handles will help ensure your back remains against the pad and that your shoulders and chest don't roll inwards.
Begin lowering the lever apparatus or sled by flexing the hips and bending the knees. Allow sled to descend until you've hit the desired depth, which for most will be until the hips are parallel with or slightly lower than the knees. Those who are more flexible can descend until the thighs hit the calves. You should still be holding a big breath and bracing your abdominals.
To initiate the raising portion of the exercise begin pushing through your heels to fully extend your knees and hips. At the top of the movement, often referred to as the lockout, legs are straight but not hyper-extended. Some choose to breath between each rep while others prefer to breathe out during the ascent; experiment and see which feels most comfortable and natural to you.
The knees should be pointed in the same direction as your toes throughout the entire rep. At the end of the set ensure the safety pins are in-place before stepping off the machine.
This exercise can be performed using straight sets, pre-exhaust sets, drop sets, rest-pause sets, supersets, trisets, giant sets, paused reps, partial reps, forced reps, or slow negatives.
As with any exercise, the two most important components are high-quality form and progression. Progression can take a variety of forms (e.g. more weight, sets, or reps, decreased rest period, improved rep quality, etc...) but strive to improve every time you walk into the gym.
Hack Squat Form Tips
Switch Up Your Stance - To emphasize glute involvement take a wider stance and/or place your feet slightly higher on the sled or platform. To emphasize quadriceps involvement take a narrower stance and/or place your feet slightly lower on the sled or platform.
Regardless of what stance tweaks you make, aim to perform full range-of-motion reps with knees pointing in the same direction as your toes.
Avoid Half Reps - The hack squat machine provides maximum benefit if performed using full range-of-motion reps. Half or quarter reps place additional stress on the knees, reinforces poor movement patterns, and doesn't provide as much stimulus to the target and supporting muscle groups.
Use One Leg - If you're having trouble getting a good quad contraction or if one quad is lagging in size or strength compared to the other, perform the leg press with one leg at a time. This will help improve the mind-muscle connection during the exercise as well as even out any imbalance that may have developed between quadriceps.
Furthermore, single leg hack squats will recruit additional stabilizing muscles - gluteus minimus and medius, the quadratus lumborum (deep lower back), and the obliques (abdominals).  To perform a one-legged variation simply cross one leg above the knee over the leg touching the platform; perform the desired number of reps and then switch legs.
Pause at the Bottom - Pausing at the bottom of the rep increases the intensity by extending the duration of the set and time under tension as well as stretches the adductors (inner thigh muscles) and hips.
Stay on the Padded Support - Throughout the entire movement doesn't let your glutes and back come off the padded support. Coming off the padded support changes the movement pattern, shortens the range of motion of the rep, and doesn't provide as much stimulus to the target and supporting muscle groups.
References1) "Quadriceps." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 21 Oct. 2015.
2) "Sled Hack Squat." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 21 Oct. 2015.
3) "Sled Single Leg Hack Squat." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 21 Oct. 2015.