Same person, same effort, same eight weeks — one leg pressed, the other extended. The quad regions that grew were almost completely opposite.
Both exercises produced strong growth. But in opposite regions of the same muscle. The question ‘which is better?’ has no answer — because the question is wrong.
Twenty-eight trained lifters each trained one leg with leg press, the other with leg extension. Same person. Same effort. Same eight weeks. The front of the quad — the muscle that gives your leg that straight-on pop — grew 8.9% with extensions but only 4.7% with leg press.
The outer quad — the sweep you see from the side — went the other direction. 8.7% growth with leg press, but only 2.9% with extensions.
Both exercises produced strong growth. But in opposite regions of the same muscle. Not different people responding differently. The same person's quad grew in different places depending on which exercise that leg performed.
The question "which is better?" has no answer. Because the question is wrong. Both exercises are needed. They build different parts of the same quad.
Burke's 2025 study found that leg press and leg extension grow opposite quad regions in the same person — and that pattern extends to the calves, where one common exercise barely activates the biggest calf muscle. Four independent research teams have confirmed the principle across multiple muscle groups.
- The pattern extends to calves — the exercise most gyms rely on barely activates the biggest calf muscle, and years of doing it may explain more than genetics
- Three large research reviews covering glutes, chest, and regional growth confirm that exercise selection drives where muscles develop
- One study found the opposite quad pattern in beginners — the split-quad effect is strongest for lifters with a training base
- For complete development of any multi-head muscle, Burke's data shows a single exercise isn't enough — both press and extension are needed for the full quad
What It Costs to Pick Just One
That paradox has a price you can calculate right now.
If leg press is the only quad exercise in the program — and for many lifters trained on the "just squat" advice, it is — the front quad gets about 53% of the growth it could be getting. That's roughly half the potential for the muscle that makes your quads look developed from the front.
The advice that enabled this gap is the most repeated line in every gym, every forum, every training video. "Compounds cover everything." Burke's data puts a number on what that belief costs. The compound movement builds the outer quad beautifully. But it leaves the front quad at half speed.
And the exercise that fills the gap? The one many lifters skip every single leg day.
The Exercise You Were Told to Avoid
Leg extensions have been called "bad for your knees" for at least two decades. The advice has been so persistent that an entire generation of lifters walks past the extension machine without a second thought.
Burke's data found that the exercise most lifters avoid grew the front quad nearly twice as fast as leg press. The muscle region that compounds can't fully reach responds to exactly the exercise the gym has been telling people to skip.
And the knee concern? Physiotherapy clinics use leg extensions for knee rehabilitation. The exercise avoided for safety in the gym is prescribed for recovery in the clinic.
Two ironies. The exercise avoided for safety is the one the front quad needs. The exercise lifters fear is the one clinicians trust. But quads are only half the story Burke's study tells.
Burke's team measured how much the calf muscles actually fire during leg press. Barely at all.
The main calf muscle — the one that grew significantly with straight-leg raises — fired at just 9% of its maximum output during leg press. The other two calf muscles hit 24% and 28%.
This matters. If the calves were getting meaningful work from leg press, the straight-leg advantage could just be extra training exposure rather than exercise choice. They weren't getting meaningful work. The calf finding stands on its own.
Your Calves Might Not Be Genetic
Burke's team didn't just compare quad exercises. They compared calf exercises too. And the finding hits a different nerve entirely.
Seated calf raises (bent knee) versus standing calf raises (straight leg). The main calf muscle — the one that gives your calf its visible shape — grew significantly more with straight-leg calf raises. How confident is that finding? 99.1%.
The seated calf raise machine bolted to the floor of nearly every gym barely targeted the biggest calf muscle.
For anyone who has been doing seated calf raises for years and blaming genetics for calves that won't grow, Burke's data offers a different explanation. Maybe the genetic ceiling hasn't been reached. Maybe the exercise just never tested it.
That reframe changes the calf conversation. The question shifts from "are calves genetic?" to "have I actually trained them with the right exercise?" If this pattern holds for quads and calves within one study, does it hold for other muscles too?
Four Research Teams, One Principle
Four independent research teams answered yes.
Hip thrusts, squats, and deadlifts each grow different parts of the glute — the same split Burke found in the quad. That finding held across 12 studies and 318 participants [1].
The chest told the same story. Flat bench favored the lower portion. Incline favored the upper [2]. Twenty-three studies, same pattern.
And the effect is exercise-driven, not simply a function of which part of the muscle stretches more — confirmed across another 12 studies [3].
Quads, calves, glutes, chest. Four muscle groups, four independent research teams, convergent findings. Exercise selection determines the regional growth map across every muscle group with multiple heads. But is the pattern as airtight as it looks?
The seated calf raise machine bolted to the floor of nearly every gym barely targeted the biggest calf muscle. Maybe it’s not genetics. Maybe it’s the exercise.
Where the Evidence Gets Messy
Earp and colleagues published a 2023 study that found the opposite pattern for the front quad — but in untrained participants. Their multi-joint movement actually favored front quad growth. Burke used trained lifters. Earp used beginners.
That matters — the split-quad effect may depend on training experience. For someone just starting out, the regional difference might not apply yet. Burke's finding is strongest for lifters who already have a training base.
One of Burke's own calf findings didn't hold up. The smaller calf muscle initially appeared to favor straight-leg raises, but when the researchers ran the numbers a different way, that advantage disappeared. That specific finding is genuinely uncertain.
The chest evidence carries a caveat too — it measured muscle activation during exercise, not actual growth over time. A strong signal in one session doesn't guarantee what happens after eight weeks of training.
Burke's quad and main calf findings measure real thickness changes over eight weeks. The satellite evidence is convergent but not identical in quality. The strong evidence feels stronger when the weaker evidence is visible beside it.
A Map for Every Muscle
Burke's study started as a comparison between two leg exercises. It ends as a lens for the entire program.
The principle that emerged: for complete development of any muscle with multiple heads, exercise selection that targets each region separately appears necessary. Burke's data showed the front quad responded to extensions, not press. The outer quad responded to press, not extensions. The main calf muscle responded to straight-leg work, not seated.
The broader application: if this pattern holds for quads, calves, glutes, and chest — and the converging evidence suggests it does — then a single exercise per muscle, no matter how heavy, doesn't guarantee that every part of that muscle grows.
Burke's team studied 28 trained adults, averaging 20.7 years old, over eight weeks at two sessions per week. Each participant trained one leg with each exercise — a design that removes the noise of individual genetic variation. The assessors who measured muscle thickness and the statistician who analyzed the data were both blinded to which leg received which exercise.
The data says what it says. Exercise selection changes where muscle grows. The lifter who was looking for a winner between leg press and leg extension found something more useful.
Both exercises are needed. They always were. The remaining question isn't about legs — it's about every muscle group in the program.
The findings point to a question worth asking about any training program: for each muscle group, does the current exercise selection actually cover every region?
Burke's team found the answer was 'no' for two different muscle groups in the same study. One exercise reached one region. A different exercise reached another. Neither covered everything.
The broader application: any muscle with multiple heads or regions may have the same gap hiding in the program. Not 'which exercise is best' but 'which regions is each exercise actually reaching?' Burke's data showed the answer isn't always what the program assumes.
What other research found
What this means for you
Your outer quad is probably fine. Compounds cover that region well — Burke's data confirmed it. The gap is the front quad, the muscle that gives your leg definition from straight-on.
In Burke's study, the leg that only did press grew the front quad at roughly half the rate of the leg that did extensions. Same person. Same effort. Same eight weeks. The exercise the compound-only program skips is the one that specific quad region responds to.
Burke tested four sets, twice a week. Eight weeks was enough to produce measurable differences in the muscle that compounds miss.
Burke's team found that straight-leg calf raises grew the main calf muscle significantly more than seated raises. That finding held firm across multiple statistical checks. Pilot data showed the main calf muscle fired at just 9% of its maximum during leg press — it was barely being asked to work.
If the only calf exercise in the program has been the seated machine, and calves haven't grown, Burke's data offers a different hypothesis. Maybe the genetic ceiling hasn't been reached. Maybe the exercise just never tested it.
That's not a verdict on genetics. It's an invitation to test the alternative. Burke's participants trained twice a week for eight weeks — enough time to see whether a different exercise tells a different story.
Burke studied trained lifters who had been lifting consistently for at least a year. A separate study by Earp and colleagues (2023) found the opposite quad pattern in untrained participants — their multi-joint exercise actually favored front quad growth.
That means the split-quad effect may depend on training experience. For someone still building a training base, the regional optimization from Burke's data may not apply yet.
The regional growth lens becomes more relevant once a consistent training foundation exists. Burke's data is strongest for lifters who already have that base — and that honesty about who this applies to is part of what makes the finding trustworthy.
Before you change anything
Burke's team studied 28 resistance-trained adults (20 men, 8 women), averaging 20.7 years old, training at least three times per week for at least one year.
The exercises tested were specific: incline leg press, leg extension, straight-leg toe press, and seated calf raise. Results may not transfer to squats, lunges, hack squats, or other leg press variations — those exercises have different biomechanics that weren't tested.
The study included both men and women but didn't have enough female participants (8 of 28) to analyze sex differences separately. The authors see no logical reason results would differ between sexes, but that's unconfirmed.
Key limitations: Sample was specific to young, resistance-trained individuals; findings cannot necessarily be generalized to adolescents, older adults and untrained individuals (though authors note there does not seem to be a logical rationale as to why results would differ between populations). Only assessed MT of one of the vasti muscles (VL); cannot determine how exercise selection may affect the other vasti muscles (vastus medialis, vastus intermedius).
Burke used a within-subject design — each participant trained one leg with each exercise. Same genetics, same recovery, same diet, same sleep. That eliminates the biggest source of noise in training studies.
The assessors who measured muscle thickness and the statistician who analyzed the data were both blinded to which leg did which exercise. The Bayesian statistical approach provides probabilities rather than pass/fail verdicts, giving readers a scale of confidence rather than a binary answer.
The quad and main calf muscle findings are supported by three independent research reviews across different muscle groups. The smaller outer calf muscle finding remains equivocal — it reversed direction in sensitivity analysis.
Burke's team answered the question for quads and calves. The pattern held for glutes and chest in the satellite evidence. But most training programs include more than four muscle groups.
The question that remains is whether every multi-head muscle follows the same regional growth rules — and whether a single study's evidence for each one is as clean as what Burke found here.
What This Study Found
All findings from this paper, in plain language.
- Leg extensions grew the front quad (rectus femoris) 8. 9% over eight weeks, compared to 4.7% with leg press. The front quad responded nearly twice as well to the single-joint exercise.
- Leg press grew the outer quad (vastus lateralis) 8. 7%, compared to 2.9% with extensions. The outer quad responded about three times as well to the multi-joint exercise.
- Straight-leg calf raises grew the main calf muscle (medial gastrocnemius) significantly more than seated calf raises. The researchers were 99.1% confident in this finding, and it held firm in sensitivity analysis (92.5%).
- The smaller outer calf muscle (lateral gastrocnemius) initially appeared to favor straight-leg raises (90.4% probability), but a sensitivity analysis reversed this finding entirely (27.4% probability). The data on this specific muscle is genuinely uncertain.
- The deep calf muscle (soleus) showed no meaningful difference between seated and straight-leg exercises. A slight lean toward seated raises (71.6% main, 85.9% sensitivity) was not strong enough to draw conclusions.
- The standardized effect sizes across all comparisons were modest in magnitude. The growth percentages tell a clear story, but the absolute effect sizes are moderate — the differences are real and consistent but not dramatic.
- Sensitivity analyses controlling for body weight, sex, and body fat percentage were consistent with the primary results. The WAMBS checklist (a Bayesian quality audit) identified no concerns. Nutritional intake remained stable across the study.
- Exercise selection influences regional hypertrophy of the lower body muscles, and evidence of those differences is measurable within eight weeks of training.
- Pilot EMG testing showed the calf muscles barely fired during leg press — the main calf muscle activated at just 9% of maximum, the smaller outer calf at 24%, and the deep calf at 28%. This rules out leg press as a meaningful confounding variable for the calf results.
- A combination of both multi-joint (leg press) and single-joint (leg extension) exercises appears necessary for complete quad development. For calves, straight-leg work appears to promote development of the main calf muscle and the deep calf muscle.