Can Muscles Get Bigger Without Getting Stronger? What a New Study Found
Quick answer: Yes, and a trial by Villalba and colleagues, published 11 June 2026 in Health Science Reports, caught it happening. Older adults who trained by tensing opposing muscles against each other, with no weight at all, grew their quadriceps by roughly 10% in eight weeks while their peak knee extension torque went down about 10%.
Every gym has the lifter whose arms look carved and whose bench is unremarkable, and the one built like a filing cabinet who pulls double bodyweight.
Size and strength usually travel together. Usually. This study is a clean demonstration that they can come apart, and it points at exactly which one your logbook is measuring.
What the researchers actually did
Thirty-four older adults, all already active but none with structured lifting experience, were split three ways.
The traditional group trained on leg extension and leg curl machines at about 70% of their one rep max. Five sets of ten on each, two seconds down and two seconds up, 90 seconds rest.
The co-contraction group sat on an ordinary chair and did nothing else. On cue they contracted their quadriceps and hamstrings against each other as hard as they could for four seconds, rested four seconds, and repeated. Five sets of ten, with no machine, band or weight involved. The only resistance was one muscle group fighting the other.
The control group carried on as normal.
Both training protocols were matched for sets, reps, rest, and time under tension, so the one real difference was whether an external load was involved. Before and after, the researchers measured knee extension and flexion torque on an isokinetic dynamometer, muscle thickness by ultrasound at three points along the thigh, plus a 30-second sit-to-stand test and instrumented walking.
What did they find?
The strength result splits cleanly along the load line.
Machines built strength. Tensing against yourself lost it.
- Before
- After 8 weeks
View the data
| Group | Co-contraction | Machine training | Control |
|---|---|---|---|
| Before | 106 N·m | 138 N·m | 131 N·m |
| After 8 weeks | 95.3 N·m | 161 N·m | 141 N·m |
The machine group gained about 17%. The co-contraction group lost about 10%. Note that the control group also moved, which the authors flag themselves as a reason to be careful with any single measurement.
Now the muscle.
Both training groups grew muscle, including the one lifting nothing
- Before
- After 8 weeks
View the data
| Group | Co-contraction | Machine training | Control |
|---|---|---|---|
| Before | 0.7 cm | 0.8 cm | 0.9 cm |
| After 8 weeks | 0.8 cm | 1 cm | 0.9 cm |
In the paper's own summary, that is roughly 15% growth with machines and roughly 10% with co-contraction. The control group moved 1% to 2%, which sits inside the 1.83% measurement error of the ultrasound operator, so it is noise rather than growth.
So one group got measurably bigger quadriceps and measurably weaker knee extension over the same eight weeks.
Why did that happen?
The researchers offer two explanations, and they are careful to label both as proposed rather than measured.
Strength is partly a skill, and they practised the wrong one. Co-contraction means switching your quadriceps and hamstrings on at the same time. Peak knee extension torque means switching your quadriceps on hard while switching your hamstrings off. Spend eight weeks rehearsing the first and you get better at it, which makes you worse at the second. Your brain has been trained to bring the brakes along every time it presses the accelerator.
Growth does not need external load, only a hard enough local environment. With nothing to lift, mechanical tension was low. But sustained maximal contraction of opposing muscles traps blood and metabolites in the thigh and raises pressure inside the muscle. The authors compare it to blood flow restriction training, where light loads build muscle because of what accumulates in the tissue rather than what sits on the bar.
That combination is the whole finding: enough stimulus to grow the tissue, the wrong practice to express it as force.
What this study doesn't prove
The participants were older, untrained adults, not lifters. Thirty-four people aged 60 to 85, none with structured training experience. Beginners respond to almost anything. Nothing here says a trained 30-year-old would react the same way.
It is small, and the control group moved too. Roughly a dozen people per arm. The control group's strength and walking scores also shifted, which the authors put down to familiarity with the tests. When an untrained group improves, individual numbers deserve caution.
The strength loss was task-specific. Isometric torque fell. Isokinetic torque, measured through movement, showed no difference between groups. The study caught a change in one specific test, not a general collapse in leg strength.
Muscle thickness is a proxy. Ultrasound measures the distance between two tissue borders at one spot on your thigh. It is a good surrogate for growth, but it is not a body scan and it is not a photograph of you.
Eight weeks is short, and effort could not be checked. With no external load, there was no way to verify that participants were genuinely giving a maximal effort each set. The authors say so plainly and recommend electromyography in future work.
Funding was clean. The trial was supported by Brazil's CNPq and CAPES, both public research bodies, with no role in design or analysis. The authors declare no conflicts of interest.
How to test this in your own training
The useful part is not co-contraction. It is the realisation that your log measures one of the two things you care about.
- Keep logging load and reps. That is your strength trace, and it is the one that responds fastest to a change in programme.
- Add a size trace, monthly. A tape measure around the same arm and thigh spot, or progress photos in the same light. Once a month is plenty.
- Run one block and compare the two lines. Six to eight weeks. Note whether they move together or split apart.
- Read a split honestly. Size up, strength flat usually means technique or fatigue, not a failed programme. Strength up, size flat usually means you got better at the lift. Neither is a reason to change everything.
- Check the estimate, not just the top set. Feed your best set into the one rep max calculator so a rep-range change does not look like progress that is not there.
| Question | What your logbook answers | What it cannot answer |
|---|---|---|
| Am I getting stronger? | Directly, every session | |
| Am I getting bigger? | Only by inference | Needs a tape or a photo |
| Is my technique improving? | Partly, via rep quality notes | |
| Which one is actually stalling? | Only if you track both |
Keep logging the way you already do
If you write your sets in Apple Notes or a paper journal, you already have the strength half of this. What you probably do not have is the shape of it over three months.
Gym Note Plus reads the text you already type and turns it into that shape.
- Type
leg extension 60kg 10,10,8and it becomes structured, charted sets. - Per-exercise strength trends and estimated one rep max are tracked automatically.
- Bodyweight and measurements sit alongside the lifting numbers, so a split between size and strength is visible instead of hypothetical.
- Old notes import, so the history you already wrote comes with you.
Want to see it on your own text first? Paste your last leg session into the free notes to workout translator.
Muscle size and strength are two different measurements that usually agree. When they stop agreeing, only the person tracking both will notice.
Frequently Asked Questions
What did the 2026 co-contraction study find about muscle size and strength?
Villalba and colleagues, publishing in Health Science Reports on 11 June 2026, found that eight weeks of maximal co-contraction training in older adults increased rectus femoris thickness by about 10% while isometric knee extension torque fell by about 10%. A machine-trained comparison group gained roughly 15% thickness and 17% strength.
Can your muscles get bigger without getting stronger?
Yes. Muscle growth and force production rely on partly separate mechanisms. Growth responds to mechanical tension and metabolic stress in the tissue, while strength also depends on how well your nervous system recruits that tissue for a specific movement. Training that grows muscle without practising the movement can raise size while leaving strength flat or lower.
Does co-contraction training build muscle?
In this trial it did. Older adults who tensed their quadriceps and hamstrings against each other, with no equipment, grew measurable muscle thickness over eight weeks. They did not gain strength. Co-contraction training has only been studied in small groups of untrained older adults, so treat it as a fallback when equipment is unavailable, not as a replacement for lifting.
Why did the co-contraction group get weaker?
The researchers propose that the participants trained a competing skill. Co-contraction rehearses switching the quadriceps and hamstrings on together, whereas peak knee extension requires the hamstrings to relax. Eight weeks of practising the first pattern likely interfered with the second. Note that this appeared in the isometric test only, not the isokinetic one.
Should I stop trusting my workout log?
No. A workout log measures strength well, and strength is the variable that responds fastest to a change in programme. It just does not measure size. Add a monthly tape measure or progress photo alongside your logged sets and you cover both, which is what makes a divergence like this one visible.
Final Takeaways
- Size and strength are separate measurements. They usually move together, and this trial is a clean case of them moving apart.
- Growth does not strictly require external load. Tension and metabolic stress inside the muscle were enough to grow it, even with nothing to lift.
- Strength is partly a rehearsed skill. Practise a pattern that fights the test and the test score goes down.
- The participants were untrained older adults. Useful, but not evidence about what happens to a trained lifter.
- Track both, or you will only ever see half of it. Reps and load in your log, a tape measure once a month, and the two lines together tell you what one line cannot.
Josh Ibbotson
Josh is the creator of Gym Note Plus, building tools that make workout tracking as simple as taking notes.
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