Neither, on the evidence so far: a 2025 meta-analysis of 9 studies found no significant difference in muscle growth between training one limb at a time and training both together4. Strength was different. Each style won on its own kind of test4.
So the choice between a split squat and a back squat is less about how much muscle you build and more about which movements you want to be strong in.
What the terms mean
Unilateral training works one arm or leg at a time: a Bulgarian split squat, a single-leg press, a single-arm dumbbell row. Bilateral training uses both sides together, evenly and at the same time: a back squat, a deadlift, a bench press3.
A meta-analysis pools the results of several studies into one estimate. Results are given as an effect size: how far apart two groups end up, on a common scale where zero means no difference. Specificity is the training principle that you gain most on the kind of movement you practise.
In plain terms: one side or both sides, and the question is whether that choice changes how much muscle grows.
What was studied
Three meta-analyses now address the question, each from a slightly different angle.
| Review | Studies | Who | Main focus |
|---|---|---|---|
| Liao et al. 20223 | 14 | 392 people aged 16 to 26, many team-sport athletes | strength, jumps, sprint, change of direction |
| Kassiano et al. 20254 | 9 (from 703 records) | not stated in the abstract | muscle growth and strength |
| Xin et al. 20265 | 24 randomised trials | not stated in the abstract | strength, jumps, muscle growth, sprint |
In the 2022 review, every included study trained the lower body, for 5 to 12 weeks; nine studies included only men, two only women, and three both3.
What the studies found
Muscle growth: no clear difference. The 2025 review found no significant difference in muscle hypertrophy (growth) between the two styles: effect size −0.21, 95% confidence interval −3.56 to 3.13, p = 0.574. That interval is very wide. It says the data cannot rule much in or out, which is what you expect when only a handful of studies measured muscle. The 2026 review, with 24 trials, also found no significant between-group difference for hypertrophy5.
Strength: each style wins its own test. In the 2025 review, bilateral training gave a larger gain on two-sided strength tests (effect size 0.56, 95% CI 0.16 to 0.96) and unilateral training a larger gain on one-sided tests (0.65, CI 0.37 to 0.93)4. The authors' summary: strength gains "appear to follow the principle of specificity"4. The 2026 review and the 2022 review found the same direction, with smaller effects53.
Strength on each kind of test, unilateral vs bilateral training · effect size
Athletic tests. In young athletes, single-leg training gave a large edge on single-leg jumps (effect size 0.89, CI 0.52 to 1.26)3. Two-leg jumps, sprint speed and change of direction showed no significant difference3. Across all 44 effects in that review, the overall difference was trivial and not significant (0.09)3.
In plain terms: similar muscle, and strength that matches what was practised.
One trial up close
A 12-week trial in 43 recreationally active young women shows what "specificity" looks like on the ground1. They were split into a control group, a unilateral knee-extension group and a bilateral knee-extension group, training twice a week1.
- Thigh muscle thickness increased similarly in both training groups1.
- One-rep max (the most you can lift once) rose similarly in both groups on both one-leg and two-leg tests: unilateral 1RM by 33.3% and 24.6%, bilateral 1RM by 20.3% and 28.5% (unilateral group first)1.
- On a one-leg isometric (static) test, the unilateral group gained more: 21.4% against 10.3%1.
Strength gain after 12 weeks, by training group · %
Unilateral group Bilateral group
The trial also tracked the bilateral deficit: the pattern in which two limbs pushing together produce less than the sum of each limb alone1. After training, the unilateral group showed a bilateral deficit (−6.5%), while the bilateral group showed the opposite, a bilateral facilitation (+5.9%)1. Each group got better at the way it trained.
The nuance: same total, maybe different spots
"No difference in growth" is a statement about the muscle measured, usually the front of the thigh. It does not mean every muscle in the leg grows the same way.
A 6-week trial in 27 young male team-sport players compared a lateral lunge with a half-squat, both on a flywheel machine that overloads the lowering phase2. The lunge group gained more in two inner-thigh muscles (adductor major +11.1%, vastus medialis +12.6%); the squat group gained more in the outer quad and a calf muscle (vastus lateralis +9.9%, lateral gastrocnemius +9.1%)2. That is one small trial with an unusual device, so read it as a hint that exercise choice shifts where growth shows up, not as a rule.
A second nuance is load. The 2022 review notes that during a Bulgarian split squat, a share of the load sits on the rear leg, citing about 15% in one study3. "One leg" is rarely exactly one leg.
In plain terms: the total may match while the pattern differs, which is a reason exercise selection still matters for other goals.
What people in these trials did
These trials describe rather than prescribe. In the 2022 review, 12 of the 14 studies trained twice a week and two trained three times, for 5 to 12 weeks, with repetitions equal between the two groups3. Those who wanted a heavier two-leg lift trained a two-leg lift; the jump and one-leg results went to those who trained one leg.
For building a week around these choices, see how to build your own workout program and how many sets per muscle group per week. For what the squat itself trains, see back squat muscles worked.
The studies at a glance
| Study | Who was studied | Key result | Year |
|---|---|---|---|
| Botton et al., *J Strength Cond Res*1 | 43 young women, 12 weeks, knee extension | Muscle thickness rose similarly; one-leg isometric strength rose more with unilateral training (21.4% vs 10.3%) | 2016 |
| Núñez et al., *PLoS One*2 | 27 young male team-sport players, 6 weeks | Both grew; lunge and squat groups grew more in different thigh muscles | 2018 |
| Liao et al., *Biol Sport*3 | Meta-analysis, 14 studies, 392 people aged 16 to 26 | Single-leg jumps favoured unilateral (0.89); two-leg strength favoured bilateral (0.43) | 2022 |
| Kassiano et al., *Sports Med*4 | Meta-analysis, 9 studies | No significant difference in muscle growth (p = 0.57, very wide interval); strength specific to the style trained | 2025 |
| Xin et al., *BMC Sports Sci Med Rehabil*5 | Meta-analysis, 24 randomised trials | One-side strength favoured unilateral (0.42); two-side strength favoured bilateral (0.28); no significant hypertrophy difference | 2026 |
Limits worth knowing
- Few muscle studies. The 2025 muscle result rests on a small set of studies, and its confidence interval is very wide4.
- Moderate risk of bias across the included studies, in the authors' own rating4.
- Mostly legs. Every study in the 2022 review trained the lower body; whether the results apply to the upper body "remains unclear"3.
- Young people. The 2022 review covered ages 16 to 26 only3.
- Short trials. Five to twelve weeks.
- Abstract only for the 2025 and 2026 meta-analyses and the 2016 and 2018 trials; populations and durations in the 2025 and 2026 meta-analyses are not in their abstracts, so we do not state them. The 2022 review was read in full text.
- Funding. The 2022 review was funded by the Winter Olympics Foundation and the China Institute of Sport Science, with no competing interests declared3.
Latest research
The newest review we found is a 2026 multilevel meta-analysis of 24 randomised trials5. It is larger than the 2025 review and reached the same conclusions. Unilateral training had significant advantages for one-sided strength (Hedges' g 0.42, 95% CI 0.23 to 0.60) and one-sided jumps (0.35, CI 0.14 to 0.56)5. Bilateral training had a small but significant advantage for two-sided strength (0.28, CI 0.02 to 0.54)5. There were no significant differences for muscle hypertrophy, two-sided jumps, sprint or change of direction, and no moderator explained the results5.
The authors conclude the choice "should therefore be guided by specific training goals"5. We read its abstract only, which does not give the number of trials that measured muscle growth.
The short version
- Two meta-analyses, from 2025 and 2026, found no significant difference in muscle growth between one-side and two-side training.
- Strength is specific: you get strongest in the style you practise.
- Single-leg work gave the clearest edge on single-leg jumps in young athletes.
- The muscle evidence is thin and mostly from legs, so "no difference" means "none detected yet".
Educational overview only — not medical advice.
