If you've spent any time in fitness spaces online, you've seen the headlines: "Free weights aren't better than machines — science says so."
And honestly, those headlines aren't lying. The meta-analyses do show equivalence on certain metrics.
But I think they're answering the wrong question.
They're measuring whether free weights build more muscle or produce more force on a controlled test. The answer is no. But that was never what made free weights essential for athletes. The real question is: which modality builds the nervous system, coordination, and stabilisation capacity that transfers to chaotic, three-dimensional sport?
That question has a very different answer. And I can back it up.
What I'll concede up front
I want to start with what the evidence clearly shows, because the strongest position is an honest one.
Hypertrophy is equivalent. When volume and intensity are matched, free weights and machines produce comparable muscle growth. The Haugen et al. 2023 meta-analysis (13 studies, 1,016 participants) found an SMD of -0.055 (p = 0.751) — essentially zero difference. If your only goal is bigger muscles, a leg press builds quads just fine.
Machines build real strength — on the machine. Machine-trained subjects do get stronger. The Haugen meta showed a trend toward machine groups outperforming on machine-based tests (SMD: 0.291, p = 0.064). Machines aren't useless. The issue is transfer, not strength.
Machines have legitimate roles in any programme. Volume accumulation when fatigued, training around injuries, isolating weak points, rehab progressions. No serious S&C coach uses free weights exclusively. My argument is about the foundation, not the entire programme.
Now — with that established — here's where the picture changes.
Where free weights actually pull ahead
1. Strength gains are modality-specific — and sport is "free-weight-shaped"
The Haugen et al. 2023 meta-analysis found that free-weight-trained groups gained significantly more strength when tested on free-weight exercises: SMD = -0.210 (p = 0.023). That's the only statistically significant direct comparison in the entire 1,016-person meta-analysis — and it favours free weights.
This is the SAID principle (Specific Adaptation to Imposed Demands) operating in real data. You adapt to what you train on. And here's the pivot: sporting actions — sprinting, tackling, jumping, changing direction — all happen in unconstrained 3D space against gravity. That's biomechanically far closer to a barbell squat than a leg press. The specificity data doesn't debunk the free-weight case. At minimum, it supports the idea that training adaptations depend heavily on the movement demands you rehearse.
2. The squat transfers 3-4x more to jumping than the leg press
This is the single most important finding in this entire debate. Two studies by Wirth et al. (2016), with 78 and 120 subjects respectively, both found the same thing.
A third study — Rossi et al. 2018, co-authored by Brad Schoenfeld — found the same directional trend (squat CMJ +8.9% vs leg press +0.5%) but the between-group differences did NOT reach statistical significance.
Both groups in all three studies got significantly stronger at their own exercise. But only the squat groups showed better carryover to jumping. The difference wasn't in the muscles they built — it was in how their nervous systems learned to use them.
Worth noting: Neither Wirth study tested both groups on a common strength measure — each group was only tested on their own exercise. So I can't claim the strength gains were identical. What the data does show is that both modalities built significant strength, but the transfer to jumping was dramatically different.
3. The studies are too short and use the wrong populations
A majority of studies comparing modalities last 6-12 weeks and use untrained or recreationally active subjects. Novices improve rapidly on any stimulus — this "novice effect" washes out the specific differences between modalities. The Schwanbeck 2020 study explicitly acknowledged 8 weeks was "insufficient to elicit divergent long-term structural changes." (Haugen 2023 reported approximately 7 untrained and 6 trained study samples — so the short-duration criticism is the stronger one here, not the population point alone.)
Sophisticated coordination, stabiliser strength, and sport-specific transfer compound over months and years — timeframes no RCT has ever tested. Judging free weights vs machines after 8 weeks with beginners is like comparing a bicycle and a sports car over 10 metres from a standstill. Both cover the distance in about the same time. The difference shows up at 200 km/h on a track.
4. Testing metrics are blind to the dimensions where free weights excel
Standard athletic tests in these studies use bilateral countermovement jumps (sagittal plane, stable surface), isometric dynamometry (single axis, zero degrees of freedom), and linear 30m sprints. Only a small portion of the literature has examined broader athletic outcomes such as reactive agility, loaded multi-directional change of direction, dynamic balance, or long-term injury patterns in direct free-weight-versus-machine comparisons. (Schwarz et al. 2019 is a notable exception, including pro-agility and zig-zag COD tests — though its findings on machine-trained subjects warrant direct engagement rather than omission.)
When the research says "no difference," it usually means no difference on a fairly narrow set of outcomes that researchers chose to measure. That is useful, but it is not the same thing as testing the full range of qualities that matter in live sport.
5. Less stable, less constrained training can produce higher stabiliser and core activation
A 2024 systematic review and meta-analysis (Batista et al.) found that unstable conditions increased activation in several core muscles compared with stable conditions. That does not directly prove free weights beat machines, but it does support the broader point that training environments with greater stability demands create a meaningfully different neuromuscular stimulus.
The internal obliques finding (SMD = 1.04) is a large effect size. These are the muscles that resist trunk rotation when someone hits you from the side or when you plant and cut at full speed. That's not a marginal difference — it's a meaningfully different training stimulus.
Supporting mechanisms
Compound training drives neural coordination efficiency
Wavelet-based EMG analysis of 47 competitive athletes over 6 weeks showed coordination efficiency improving from 71.4% to 92.4%. The nervous system needed less energy (wavelet power dropped from 1.35 to 0.98) to produce more coordinated output.
A note on this source: This study examined compound resistance training generally — not free weights vs machines specifically. This source should be treated cautiously and as suggestive rather than definitive. It is cited here for the specific quantitative data on wavelet-based coordination metrics; the core argument does not depend on it.
The Bernstein degrees of freedom problem
Every rep of a barbell squat is a 3D physics problem the brain solves in real time — managing dozens of joints, hundreds of muscles, and thousands of possible trajectories while gravity tries to fold you in half. A Smith machine solves that problem for you. Your muscles still work, but the motor-control demands are reduced. And your brain is what runs the show on the field.
This is Nikolai Bernstein's "degrees of freedom" framework from motor control science. A free weight forces the CNS to manage all the variables — balance in three planes, countering gravity, adjusting to perturbations. Machines collapse this complexity down to 1-2 degrees of freedom by fixing the bar path. That makes machines easier. It also makes them a fundamentally different neural stimulus.
Corticomotor plasticity
TMS (transcranial magnetic stimulation) research shows that resistance training enhances corticospinal excitability, increases motor-evoked potential amplitude, and significantly reduces short-interval cortical inhibition (SICI) — lowering the threshold for rapid movement initiation. Motor skill learning, such as acquiring technique for squats, Olympic lifts, and kettlebell work, produces additional corticospinal adaptations consistent with use-dependent synaptic plasticity. Because free weights typically require more motor control than fixed-path machines, they may provide a richer corticomotor stimulus. That is a reasonable inference from the motor-learning literature, but it has not been cleanly isolated in direct free-weight-versus-machine trials.
Where this leaves us
The meta-analyses are not wrong. For building muscle, machines and free weights are interchangeable. For raw force production on a stable, predictable test, they're comparable. I accept that, and any honest coach should.
But athletes don't play sports on a leg press. They play in three dimensions, against gravity, on unpredictable surfaces, with opponents trying to disrupt everything they do. The SAID principle says your body adapts to the specific demands you impose on it. Free weights impose demands that are kinematically, neurologically, and proprioceptively closer to sport than any guided machine.
The squat-vs-leg-press jump data shows it. The EMG data shows the mechanism. The specificity findings from the largest meta-analysis available suggest that training modality shapes exercise-specific adaptation, which strengthens the case that transfer may differ across modalities as well. And the fact that these differences appear even in novices over just 8 weeks — conditions that should minimise the gap — tells me the full picture, over years with trained athletes, would be even clearer.
Use both. But build the foundation with free weights. The research trends, the proposed mechanisms, and common coaching practice all lean in the same direction, even if the full picture is not yet settled.
Sources
Haugen, M.E., et al. (2023). BMC Sports Sci Med Rehabil, 15:103.
Wirth, K., et al. (2016). J Strength Cond Res, 30(5): 1205-1212.
Wirth, K., et al. (2016). J Human Kinetics, 53: 201-210. PMC5260589.
Rossi, F.E., Schoenfeld, B.J., et al. (2018). J Sports Med Phys Fitness, 58(3): 263-270.
Schwanbeck, S.R., et al. (2020). J Strength Cond Res, 34(7): 1851-1859.
Batista, J.P., et al. (2024). Sports, 12(4): 111.
Liu, Z., Chen, J., & Lin, Z. (2025). Mol Cell Biomech, 22(5): 1709. doi:10.62617/mcb1709
Liang, C. & Liu, H. (2025). Front. Physiol. 16:1569639. doi:10.3389/fphys.2025.1569639
Bernstein, N.A. (1967). The Coordination and Regulation of Movements. Pergamon Press.