Hack Squat.
The barbell hack squat holds the bar behind the legs at arm's length and squats down to parallel. The rear loading position shifts the center of mass forward and demands unusual quad activation, making it one of the few barbell lifts that isolates the quad the way a machine does.
Step by step.
Set a loaded bar on the floor behind your heels. Standing in front of it, take an overhand grip about shoulder width apart with your arms hanging behind you.
Elevating your heels on small plates or wearing squat shoes makes the whole lift easier to balance and gives the bar more room to pass your legs.
Brace your core, lift the bar so it rests against the back of your thighs, and stand tall to start.
Squat down toward parallel, letting your knees travel forward and your ankles bend so the bar clears your calves instead of dragging on them.
Keep the load on your quads and your torso leaning slightly forward, which is normal for this lift because of where the bar sits.
Drive through your feet to extend the knees and return to standing, keeping the bar tracking close behind your legs the whole way.
This is a fundamentally awkward exercise and that's not a reason to skip it. The awkwardness is the mechanism. The bar behind the legs creates forward lean of the torso that's actually the wrong direction for most squats but right for this one. The quads have to extend the knee against gravity with the bar pulling the hips back, which concentrates load on the rectus femoris and vastus group with minimal glute involvement.
Bar path is the tricky part. At the bottom of the rep, the bar needs to clear your calves and hamstrings as it passes by. If you squat straight down, it'll drag against the backs of your legs. The solution is to let the knees travel slightly forward and the ankles dorsiflex freely, which naturally shifts the bar path away from the leg. Heel elevation (either plates under your heels or squat shoes) helps significantly here. It reduces how far back you have to push your hips to stay balanced and gives the bar more clearance.
Grip is also a real constraint. With a shoulder-width overhand grip behind the body, the forearms are in an internally rotated position that's uncomfortable at the bottom. Some people use a mixed grip (one over, one under) to get through the range. Wrist wraps can help if the extended wrist position at the bottom is the limiting factor. Heavy loading isn't common in this lift because grip and the mechanics of the position both limit how much you can realistically do.
The hack squat machine (the 45-degree angled sled) is the modern version and allows much heavier training. The barbell version is a finisher and an educational tool, not a primary strength lift.
If grip is what caps the barbell version, that's the whole argument for the leg press. It loads the quads just as hard with no bar to hold behind you, so you can keep adding weight long after your hands would have quit. The narrow stance hack squat on the machine biases the outer quad, and the sissy squat hits the quads through a deep stretch with no bar at all.
Common questions.
The quadriceps, specifically the rectus femoris and the three vastus muscles. The rear loading position shifts the center of mass forward relative to the hips, reducing hip extensor (glute) involvement compared to a conventional back squat. This makes it one of the more quad-focused barbell exercises available. The hamstrings and glutes assist but are secondary. Some lifters use hack squats specifically to bring up lagging quad development when back squats and leg presses have plateaued.
The machine version uses an angled sled (typically 45 degrees) that guides the movement along a fixed path. This allows much heavier loading, removes the balance and grip constraints of the barbell version, and is more practical for accumulating volume. The barbell hack squat is the original version, predating the machine, and requires grip strength, balance, and a specific technique to clear the bar around the legs. Most gyms now have the machine; the barbell version is worth knowing but rarely the most efficient tool for the goal.
The bar needs to travel close to the back of the legs on the way down, and at full depth it will naturally be behind the calves. If it's hitting your shins, the problem is usually that you're squatting straight down without letting the ankles dorsiflex and the knees travel forward enough to clear the bar path. Heel elevation solves most cases. Even 5-10 lbs under each heel shifts the mechanics enough to open the clearance. If you're already elevated and still hitting the shins, reduce depth until your ankle mobility catches up.
It's one option, but probably not the first tool to reach for. If your quads are a limiting factor in conventional deadlifts, the more direct fix is front squats, leg press, or a higher bar position on your back squat, all of which allow heavier loading and cleaner progressive overload. Hack squats are limited by grip and the mechanical awkwardness of the position. For targeted quad work with high loads, the leg press or machine hack squat gives you more control over progressive overload without the grip ceiling.
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