No, not in the trial that tested it. In 2026, people who added leg sets to their arm training grew their arms as much as people who trained arms alone: 0.19 cm versus 0.18 cm of upper-arm thickness over 6 weeks4.
The worry is that muscles compete for a limited pool of resources, so growing big legs would leave less for the arms. Below is how that was tested, what an older line of research found about the opposite idea, and where the evidence stops.
What was studied
The key study is a randomised trial published in 20264. Randomised means chance, not the participants, decided who went into which group. One hundred and five untrained people were split three ways:
- Arms only (36 people): a one-arm curl, called elbow flexion, with a light load taken to failure (the point where no more reps are possible).
- Arms plus legs (33 people): the same arm work, plus 4 extra sets of knee extension (straightening the knee against a load) in each leg, also to failure.
- Control (36 people): no exercise, with time matched to the training groups.
The training groups completed 18 supervised sessions over 6 weeks4. The arm work was 2 sets at 30% of 1RM, taken to failure. 1RM, or one-rep max, is the heaviest load a person can lift once. The leg sets used a load people could lift about 20 to 30 times.
Before and after, the researchers measured muscle thickness on the front of the upper arm at two sites, and the 1RM of the trained arm4. Muscle thickness is how deep a muscle measures, a standard stand-in for its size.
| What | Arms only | Arms plus legs | Control |
|---|---|---|---|
| People | 36 | 33 | 36 |
| Sessions | 18 over 6 weeks | 18 over 6 weeks | none |
| Arm sets per session | 2 at 30% of 1RM | 2 at 30% of 1RM | none |
| Extra leg sets per session | none | 4 per leg | none |
In plain terms: two groups did the same arm training. One also trained a large amount of leg muscle in the same sessions.
What the study found
Both training groups gained arm size and strength over 6 weeks4. The extra leg work did not slow the arms down:
- Upper-arm thickness: +0.19 cm with arms plus legs, +0.18 cm with arms only. In the authors' words, the leg exercise "did not attenuate the muscle growth in the anterior upper arm"4.
- Arm 1RM: +0.25 kg with arms plus legs, +0.32 kg with arms only. The changes "did not differ between training groups"4.
Change in front upper-arm thickness over 6 weeks · cm
The authors used a Bayesian analysis, a statistical method that weighs how well the data fit competing ideas (here, "legs reduce arm growth" against "no difference"). Their conclusion: "there was no evidence for competition of adaptations in muscle size and strength under uncontrolled nutritional conditions"4.
In plain terms: training a large amount of extra muscle in the same session did not take growth away from the arms.
The older question: does leg work help arms grow?
For years the question ran the other way. In these studies, blood hormone levels went up for a short time after heavy leg sessions. Researchers asked whether that brief change could help other muscles, such as the arms, grow more. Three trials tested it by training one arm with and without leg work.
- 2001, 16 untrained young men, 9 weeks. One group trained one arm only; the other trained one arm and then legs1. Hormone levels rose more in the leg group, and isometric arm strength (force against an immovable resistance) rose 37% in its trained arm, against 9% (not significant) in the arm-only group's trained arm. Functional strength rose about the same in both trained arms (20% and 19%). And the leg group started 20% to 25% weaker, and the authors warned that caution "must be taken with the interpretation".
- 2010, 12 healthy young men, 15 weeks. Each man trained one arm with curls alone, and the other arm with the same curls followed immediately by a high volume of leg exercise, on separate days2. Hormone levels went up after the leg sessions, as planned. Arm muscle cross-sectional area rose 12% without leg work and 10% with it, with no difference between arms. Strength gains did not differ either.
- 2011, 9 people, 11 weeks. Each trained one arm after leg exercises and the other arm without3. Both arms grew and got stronger, but the arm trained after legs gained more 1RM and more size at its widest part. The authors attributed it to the higher hormone levels.
The results split, the trials were small, and the longest of the three found no effect. None of them found that leg work reduced arm growth.
Does training other muscles change arm growth? How the evidence built up
- 2001Hansen et al.16 men: more arm strength with leg work, but the groups started unequal.
- 2010West et al.12 men, 15 weeks: arm growth the same with or without leg work (+12% vs +10%).
- 2011Rønnestad et al.9 people: the arm trained after legs gained more strength and size.
- 2026Kataoka et al.105 people: extra leg work did not reduce arm growth or strength gains.
In plain terms: whether leg training helps arms is unsettled, but none of these trials, old or new, found that it hurts them.
What the result does and does not mean
The 2026 authors were careful about what they did not test4:
- Nutrition was not controlled. People ate as they normally did. The study cannot say what happens when food intake is limited, and the authors name that as an open question.
- The volume was modest. The extra leg work was 4 sets per leg per session. The authors also flag "greater training volume" as an open question.
- The people were untrained. Trained lifters were not tested.
- Light loads, one arm exercise. Heavier loads and full programs were not studied.
The trial also does not say what the legs themselves did. Only arm outcomes were reported in the summary we read.
A common misreading is "muscles never compete". The study supports a narrower claim: in untrained people eating normally, a moderate amount of leg work did not reduce arm gains over 6 weeks.
In plain terms: there is no sign that training legs costs you arm growth in the conditions tested. The edge cases remain open.
What this means for a program
The studies describe, rather than prescribe. What they suggest for how training is split:
- Training arms and legs in the same session did not reduce arm growth in the 2026 trial4 or in the 2010 trial2.
- Full-body sessions and splits both train several muscles each week. The trade-offs are covered in full body vs upper/lower vs push/pull/legs.
- Weekly sets per muscle are a separate variable from what else is trained that day. See how many sets per muscle group per week.
A separate question, whether cardio slows muscle gains, has its own article on this site.
What the studies found
| Study | Who was studied | Key result | Year |
|---|---|---|---|
| Hansen et al., *Scand J Med Sci Sports*1 | 16 untrained young men, 9 weeks, arm only vs arm plus legs | Isometric arm strength +37% with legs vs +9% without; groups started unequal | 2001 |
| West et al., *J Appl Physiol*2 | 12 healthy young men, 15 weeks, one arm each way | Arm muscle area +12% without legs, +10% with legs; no difference | 2010 |
| Rønnestad et al., *Eur J Appl Physiol*3 | 9 people, 11 weeks, one arm each way | Arm trained after legs gained more strength and size | 2011 |
| Kataoka et al., *J Strength Cond Res*4 | 105 untrained people, randomised, 6 weeks | Arm thickness +0.19 cm with legs vs +0.18 cm without; strength did not differ | 2026 |
Limits worth knowing
- Abstracts only. The 2026 paper is not open access; we read its abstract. It gives no confidence ranges, no ages or sex split, and no numbers for the control group. We also read only the abstracts of the three older trials.
- Untrained people, 6 weeks. The 2026 result may not hold for trained lifters or longer programs4.
- Nutrition not controlled. Named by the authors as a limit4.
- Small older trials. Nine to 16 people each, and the 2001 groups started at different strength levels1.
- Not measured. Leg outcomes in the 2026 summary, and any effect of very high total training volume.
Latest research
The newest study we opened is the 2026 trial itself, published online in September 20264. It is the first we found built to test competition directly, with a non-exercise control and more than 100 people. We found no newer study on the question. The authors' own next steps, higher volumes and limited food intake, are the tests to watch for. When one appears, this page will be updated and its date will change.
The short version
- Adding leg sets did not reduce arm growth in a 2026 trial of 105 untrained people (0.19 vs 0.18 cm).
- Arm strength gains did not differ either.
- Older trials asked whether leg work helps arms; the results split, and none found harm.
- Untested: trained lifters, very high volumes and limited food intake.
Educational overview only — not medical advice.
