Free weights may build more sport-relevant adaptations. Here's my case. — Coach Adam James

Coach Adam James — March 2026

Free weights may build more sport-relevant adaptations. Here's my case.

The meta-analyses say there's no difference. I agree — on the metrics they tested. But athleticism is more than a lab score. Here's the full picture, with citations, concessions, and the questions the research hasn't asked yet.

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.

The evidence

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.

Haugen, M.E., et al. (2023). "Free weights vs. machines: a systematic review and meta-analysis." BMC Sports Sci Med Rehabil, 15:103. 13 studies, 1,016 participants.

OBJECTION "That just shows specificity — of course you get better at what you train. It doesn't prove athletic transfer."

RESPONSE Exactly — specificity IS the point. A sprint, a tackle, a jump all require force production through an unstable kinetic chain against gravity. That's kinematically closer to a barbell squat than a leg press. SAID doesn't just apply in the gym — it applies to the transfer between gym and field. If the gym movement is kinematically closer to the sporting action, transfer is higher. That is the logic of the specificity principle, even if the exact size of real-world transfer still has to be demonstrated more directly.

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.

Wirth 2016a (n=78)

+12.2%

Squat group SJ (p ≤ 0.05)

Wirth 2016b (n=120)

+13.8%

Squat group SJ (p < 0.001)

Leg press groups

+3-5%

NOT statistically significant

Between groups

Sig.

Both studies, p ≤ 0.05

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.

Wirth, K., et al. (2016). J Strength Cond Res, 30(5): 1205-1212. n=78.

Wirth, K., et al. (2016). J Human Kinetics, 53: 201-210. PMC5260589. n=120.

Rossi, F.E., Schoenfeld, B.J., et al. (2018). J Sports Med Phys Fitness, 58(3): 263-270.

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.

OBJECTION "The Rossi study didn't find significant differences, and Schoenfeld was a co-author. That contradicts Wirth."

RESPONSE Rossi's results still trended in favour of the squat group, but because the between-group difference was not statistically significant, the safest reading is that the study was suggestive rather than decisive. And the two Wirth studies, with 78 and 120 subjects, both found significant differences. Three studies, same direction, two reaching significance. That's a consistent signal, not a one-off.

Jump transfer across all three studies

Squat group (SJ/CMJ avg) Leg press group

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.

OBJECTION "But the Wirth studies also used untrained subjects over 8 weeks. You can't criticise novice data and then cite it."

RESPONSE Fair point — and I won't dodge it. Here's the distinction: the "no difference" studies show parity in novices and conclude the modalities are equivalent forever. What I'm pointing out is that if significant differences in transfer already appear in novices after just 8 weeks — conditions that should MINIMISE the gap — then the gap will logically widen with training experience and time. The novice hypertrophy data tells us about the floor. The jump transfer data tells us the ceiling hasn't been tested yet.

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.

Haugen 2023 meta-analysis — standardised mean differences across all measured outcomes

OBJECTION "You're just shifting goalposts to untested metrics. That's speculation, not evidence."

RESPONSE I'm not shifting goalposts — I'm pointing out the current goals are in the wrong place. Real sport requires multi-planar agility, reactive change of direction, and force production under unstable conditions. The absence of evidence in those domains isn't evidence of absence — it's evidence that nobody has measured it yet. And if free weights demand more stabilisation and coordination (which the EMG data confirms), then the SAID principle predicts they'll transfer better to tasks requiring those qualities.

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.

Core EMG activation: stable vs unstable conditions — SMD (Batista 2024)

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.

Batista, J.P., et al. (2024). "Core Muscle Activation during Stable and Unstable Exercises: A Systematic Review and Meta-Analysis." Sports, 12(4): 111.

OBJECTION "Higher EMG doesn't mean better performance. You're confusing mechanism with result."

RESPONSE Correct — EMG alone doesn't prove performance transfer. It proves a different training stimulus exists. Combined with SAID and the Wirth jump data, the chain is: higher stabiliser demand (EMG) → specific neural adaptation (SAID) → superior transfer to tasks requiring stabilisation (jump data). That chain is plausible and partly supported, but it should still be treated as an inference rather than a fully proven sequence.

The deeper science

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.

Liu, Z., Chen, J., & Lin, Z. (2025). "Neuromuscular control and biomechanical adaptations in strength training: Implications for improved athletic performance." Molecular & Cellular Biomechanics, 22(5), 1709. doi:10.62617/mcb1709

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.

Liang, C. & Liu, H. (2025). "Effects and mechanisms of resistance training on corticospinal adaptation." Frontiers in Physiology, 16:1569639. doi:10.3389/fphys.2025.1569639

The bottom line

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.

References

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.

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