Hack Squat vs Back Squat: What the Research Shows

The hack squat is a machine squat in which you lift a loaded sled along fixed, angled rails while a pad supports your back and shoulders. The back squat is a free-weight squat with a barbell on your upper back. Both train the quads hard, and lifters often ask which one deserves a place in the program.

The honest answer is that the evidence is thin. We found one study that compares the two lifts directly, and it measured trunk muscles rather than leg growth. So this article reports what that study and two broader reviews found, marks where we are reasoning rather than citing, and shows how to keep a clean record if you train both.

Is the hack squat or the back squat better?

Neither is better on the evidence we found. No trial has compared the two for muscle growth or strength, and the single direct study measured trunk muscle activity. The practical differences come down to loading, trunk demand, and which lift you can progress steadily.

That does not make the choice arbitrary. The hack squat takes balance and bar path out of the lift, and the back squat leaves them in. Those are real differences in what each set asks of you, even though no study has shown they change how much muscle you build in the legs.

If you have read our comparison of leg press vs squat, the pattern will look familiar. Machine squats and the barbell squat overlap heavily on quad work. They differ mainly in what carries over to the lift you are trying to improve.

What does the research say about hack squat vs back squat?

One study compares them directly. Clark, Lambert and Hunter (2019) found that back squat trunk muscle activation was greater than hack squat activation, even though lifters moved heavier loads on the machine. The study did not measure the quads, so it cannot rank the lifts for leg growth.

Ten men took part. They first tested their one-rep max (1RM) in both lifts. They then performed three reps at four relative loads in each lift (65, 75, 85 and 95% of maximal system mass) while the researchers recorded surface electromyography (EMG) from trunk muscles. The main results were:

The authors drew a clear conclusion from it:

"Back squat is more effective than HS in activating the muscles of the trunk and therefore, arguably more effective in developing trunk strength and stability for dynamic athletic performance." (Clark et al., 2019)

Three limits matter. The sample was ten men. The measure was EMG during a few reps, which describes how active a muscle was, not whether it grew. And the trunk was the target, so the study says nothing about quad activation or quad size.

Our reading, which is reasoning and not a finding: the pad supports your trunk and the sled removes balance demands, so you can move more load while the trunk does less. The study's higher machine loads and lower trunk activation are consistent with that idea, but it did not test why.

Do machine squats build as much muscle as free-weight squats?

Probably, going by the broader evidence, though none of it is specific to the hack squat. A 2023 meta-analysis found no difference in hypertrophy between free-weight and machine training. It also found that strength gains were specific to the type of exercise used in training.

Haugen et al. (2023) pooled 13 studies and 1,016 participants (789 men and 219 women). Each study compared free weights against machines for at least six weeks in adults aged 18 to 60. The key results:

• Hypertrophy: no difference between modalities (SMD -0.055, 95% CI -0.397 to 0.287, p = 0.751).
• Dynamic strength: no difference in direct comparison (SMD 0.084).
• Strength in free-weight tests: rose more after free-weight training (SMD -0.210, p = 0.023).
• Strength in machine tests: tended to rise more after machine training (SMD 0.291, p = 0.064), though this was not statistically significant.

The authors summarised it this way: "strength changes are specific to the training modality," and the choice between free weights and machines comes down to individual preferences and goals.

Two cautions. Only five of the 13 studies measured hypertrophy, so that result rests on a smaller pool. And the review pooled many machines and lifts. It did not isolate the hack squat, so applying it here is an inference.

A 2025 trial gives a second, narrower data point. Kassiano et al. split 70 young women into two groups for 10 weeks of training three times a week. The constant group did a 45-degree leg press and a stiff-leg deadlift every session. The varied group did those two in the first session, then hack squat with a prone leg curl in the second and Smith machine squat with a seated leg curl in the third. Thigh muscle thickness rose similarly (+7.8 to 17.7% in the constant group, +7.5 to 19.3% in the varied group), and so did 1RM gains.

That study does not show what the hack squat did on its own, because it was one of several exercises in the varied plan. What it does suggest is that a mix of machine squats can grow the thigh as well as a leg press routine in this population. For the wider machine question, see free weights vs machines and Smith machine vs barbell squat.

What does the evidence mean for strength and trunk training?

If you want a bigger barbell squat, the evidence favours squatting with a barbell. The meta-analysis found strength rose most in the type of test that matched training, and the direct study found the barbell squat loaded the trunk more. Neither result proves the hack squat is a poor strength tool.

Putting the studies side by side:

1. Specificity. Haugen et al. found free-weight strength rose more after free-weight training. A lifter who trains a machine squat should expect the best carry-over to machine tests, and less to a barbell max.
2. Trunk demand. Clark et al. found more trunk activation in the back squat at every load tested. If a strong, stable trunk under a barbell is part of your goal, the back squat gives that practice and the hack squat gives less of it.
3. Load. Clark et al. found higher loads on the hack squat. That means a hack squat number is not comparable with a back squat number, which matters when you log them.

The other side of the argument is also reasonable. When your back, hips or balance limit how hard you can push the legs in a back squat, a supported machine lets the legs be the limiting factor. That is our judgement, not a trial result, and it does not mean the hack squat is safer. If something hurts, get it assessed by a clinician.

For other squat comparisons, our guides to front squat vs back squat and leg extension vs squat cover how bar position and single-joint work change the picture.

Which should you pick?

Pick by the goal of the training block, the equipment you have, and which lift you can progress steadily. The table below is our practical judgement built on the studies above, not a finding from any of them.

Keep the evidence limit in view. A table of suggestions is a starting point to test against your own training log, not a prescription. Our squat stance width article covers one variable that matters on the barbell squat, and machines add their own setup differences between gyms.

How do you program and progress each lift?

Progress each lift on its own history, with effort tracked set by set. The same rep-range methods work on both. Because the two lifts allow different loads, never carry a weight from one to the other as if it were a continuation.

Three approaches fit both lifts:

• Double progression. Pick a rep range, such as 8 to 12. Add reps at the same load until every set reaches the top of the range, then add load and start again. Our double progression guide covers the steps. It suits the hack squat well because machine loads change in small, predictable steps.
• Top set and back-off sets. One hard set, then lighter sets for volume. This fits the back squat when you want heavy practice, and our top sets and back-off sets article explains the structure.
• Reps in reserve. Stop sets at a planned number of reps short of failure and write it down. A back squat set at 2 RIR and a hack squat set at 2 RIR are comparable in effort even though the loads are not. Our RIR guide explains how to rate sets.

A simple weekly pattern, offered as an example and not a tested plan: back squat early in the week for heavy sets, then hack squat later for moderate-rep sets closer to failure. If you do both, remember each set of hack squat adds to the quad work you are already doing.

How do you log hack squat and back squat in IronLedger?

Log them as two separate exercises with load, reps and RPE or RIR on every set, and judge each against its own history. IronLedger is a free strength training app that keeps your full training history with no cutoff, so you can see whether either lift is moving over a block.

What that looks like in practice:

• Per-set logging. Load, reps, RPE in half steps or RIR, and a free-text note on any set.
• Notes for the setup. Hack squat machines differ between gyms in sled angle and platform. Write the machine and your foot placement in the set note, so a number from one machine is not compared with a different setup. IronLedger does not record machine settings for you, so the note is your convention to keep.
• Estimated 1RM per lift. IronLedger uses the Epley formula, weight x (1 + reps / 30), and shows it on the progress page. A 100 kg set of 5 gives an estimate of 116.7 kg. Treat it as a direction of travel for each lift, not a number to compare across lifts. Our guide to estimated one-rep max explains its limits, and the one-rep max calculator runs the same sum on any set.
• Personal records per lift. Records are tracked for each exercise, and a set that beats one is flagged as you log it.
• Mid-session swaps. If the hack squat is taken, you can replace the exercise while the workout is running.

Do not add hack squat kilograms to back squat kilograms. Clark et al. showed the machine allows higher loads, so a combined total hides more than it shows. Your workout history is more useful with each lift on its own line.

What mistakes should you avoid when choosing between them?

Most mistakes come from treating the two lifts as interchangeable or from reading more into the research than it says. Judge each lift by its own trend, and let effort rather than plate count tell you whether a set was hard.

• Comparing loads across the lifts. A bigger hack squat number does not mean a stronger squat. Clark et al. found machine loads were higher at the same relative effort.
• Dropping the barbell and expecting barbell strength. Haugen et al. found strength gains were specific to the training modality. A lower back squat after a long hack squat block fits that pattern, though we cannot say how large the effect is for you.
• Reading EMG as a growth result. Lower trunk activation in the hack squat does not show smaller trunk growth, and higher activation in the back squat does not show bigger legs.
• Citing the meta-analysis as hack squat proof. It covers many machines and lifts. It supports the idea that machine training can grow muscle, not a claim about this one machine.
• Changing the setup and losing the trend. A different machine, foot position or depth is a different lift. Note it so the history stays honest.
• Ignoring pain. Persistent knee, hip or back pain needs assessment, and no exercise comparison can replace that.

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