Two legs pressing together should produce twice the force of one leg pressing alone. Usually they don't. Lift with both limbs at once and the combined output is often lower than the sum of what each limb can produce by itself — a finding called the bilateral deficit, and it's the first clue that "both limbs together" and "more total work" aren't the same thing.
That's the wrong axis to argue from anyway. The real programming question isn't whether unilateral or bilateral training is better in general — it's which specific strength quality you're trying to build, because the research shows each mode builds a different one.
What's the difference between unilateral and bilateral training?
Unilateral training is resistance training in which one limb produces force independently per repetition; bilateral training is resistance training in which both limbs produce force together against a shared load. That single mechanical difference — independent force versus shared force — is what drives every other difference between the two.
In a bilateral lift like a back squat, deadlift, or barbell bench press, the load is fixed to one bar, so the two limbs share the work. If one side is weaker or fatigues first, the stronger side can quietly take up the slack without the lift failing — the total number on the bar can look fine while one side is doing more of the job. Barbell vs dumbbell training covers this same mechanism from the equipment side: a barbell is bilateral because the bar connects both hands' output into one path, even when the grip is wide.
In a unilateral lift like a Bulgarian split squat, single-arm row, single-leg Romanian deadlift, or single-arm overhead press, that compensation isn't available. Each limb has to produce its own force or the rep on that side doesn't happen. That removes the stronger side's ability to hide a weaker one, at the cost of extra demand on balance and stabiliser muscles that a bilateral version doesn't require.
Does unilateral or bilateral training build more muscle?
No — the best available evidence shows no significant hypertrophy difference between unilateral and bilateral training when volume and effort are matched. Kassiano et al. (2025) reviewed 9 randomised controlled trials, drawn from 703 screened studies, and found an effect size of -0.21 (95% CI -3.56 to 3.13) for muscle growth between the two modes, a result too small and too uncertain to call a real difference.
That result makes physiological sense. Muscle growth responds to mechanical tension, metabolic stress, and enough total volume per week — how many sets per muscle group covers those volume thresholds directly — and none of those depend on whether the other limb happened to be working at the same time. A quad taken to near failure in a leg press doesn't grow differently to a quad taken to near failure in a Bulgarian split squat, provided the working sets and effort are comparable.
Where the two modes do diverge for hypertrophy is practical, not physiological. Bilateral lifts usually let you handle more absolute weight per set, which can make hitting a target training frequency per muscle group easier in less time. Unilateral lifts take longer to accumulate the same total volume because each side is trained separately, but they let a lifter push a smaller, more stable load through a fuller range when a joint or a bilateral lift's setup gets in the way.
Which builds more strength, unilateral or bilateral training?
Strength gains are specific to the mode you train in: bilateral training builds bilateral strength best, and unilateral training builds unilateral strength best. Kassiano et al.'s (2025) same review found a bilateral-strength effect size of 0.56 favouring bilateral training, and a unilateral-strength effect size of -0.65 favouring unilateral training — the two modes don't transfer to each other's strength test nearly as well as either transfers to itself.
That specificity isn't new. Howard and Enoka's classic 1991 study tested maximal one-limb and two-limb force in untrained subjects, cyclists, and competitive weight lifters. Untrained subjects showed the expected bilateral deficit — less combined force with two limbs than the sum of each limb alone. Weight lifters showed the opposite: bilateral facilitation, producing more force with both limbs together than either limb could manage on its own. The same two limbs, trained three different ways, behaved three different ways under the exact same test.
For a self-coached lifter, that means the strength test you care about should decide the training mode you prioritise. A lifter chasing a bigger barbell back squat needs to squat on a barbell; adding split squats will build real single-leg strength, but it won't move the bilateral squat number nearly as efficiently as bilateral squatting does. Progressive overload training program design should account for that specificity rather than assuming any leg work counts the same toward a bilateral goal.
Does unilateral training improve athletic performance more than bilateral training?
Unilateral training produces a clear advantage for unilateral strength and single-leg jump performance in team-sport athletes, but no significant overall advantage for sprint speed or agility. Wu et al. (2026) analysed 15 randomised controlled trials across 355 basketball, soccer, rugby, and ice hockey athletes and found the following outcomes.
| Outcome | Unilateral training advantage? | Effect size (g) | p-value |
|---|---|---|---|
| Unilateral strength | Yes | 0.68 | 0.007 |
| Bilateral strength | No | -0.10 | 0.56 |
| Unilateral countermovement jump | Yes | 0.37 | 0.025 |
| Unilateral horizontal jump | Yes | 0.45 | 0.03 |
| Sprint | No | -0.22 | 0.12 |
| Agility | No | -0.40 | 0.08 |
The sport-specific detail matters more than the overall averages. Basketball athletes in the same review showed a significant unilateral-training advantage for both agility (g=-0.77, p=0.04) and sprint performance (g=-0.37, p=0.04), even though the pooled result across all sports found no overall sprint or agility advantage. Most explosive sporting actions — a sprint stride, a cutting step, a single-leg jump off — are unilateral by nature, which is one reason athlete strength training programs for team and field sports lean on single-leg and single-arm work rather than treating bilateral lifts as the whole answer.
What is the bilateral deficit, and does it matter for your program?
The bilateral deficit is the finding that total force produced by two limbs together is often lower than the sum of each limb's force tested alone — typically 5-25% lower, depending on the test method, the population, and the muscle group involved (Škarabot, Cronin, Strojnik & Avela, 2016). It's the reason "both limbs at once" doesn't automatically mean "maximum combined output."
The deficit isn't purely a fixed neural ceiling, either. Simoneau-Buessinger et al. (2015) tested bilateral force output under two dynamometer setups: a locked configuration, where the bilateral index came out at 90.7% of the summed unilateral force, and an open configuration that let the body adjust its posture more freely, where the index rose to 103.2% — no deficit at all. Their conclusion was direct:
"Maximal bilateral contractions yielded lower force than the unilateral condition." — Simoneau-Buessinger et al. (2015), PLoS One
But only in the locked setup. That gap between 90.7% and 103.2% under the same muscles and the same people shows some of what gets labelled a "neural" bilateral deficit is really about body positioning and equipment constraints, not a hard ceiling on how the nervous system can fire both sides at once. Howard and Enoka's (1991) finding that trained weight lifters show bilateral facilitation rather than a deficit points the same way — the deficit shrinks or reverses with the right training history.
The practical takeaway is not to treat your bilateral numbers as a predictor of your unilateral capacity, or the reverse. A lifter who squats 180kg bilaterally has no guarantee of a 90kg-per-leg Bulgarian split squat, and that gap isn't a sign anything is wrong — it's two different qualities, trained two different ways, showing up as two different numbers.
How should a self-coached lifter program unilateral and bilateral work together?
Use bilateral lifts as the primary driver for raw strength and unilateral lifts as targeted work for single-limb strength, side-to-side imbalance, and sport-specific transfer — not as substitutes for each other. The research above gives a specific reason to include each: bilateral training for the strength test that's actually bilateral, unilateral training for everything a bilateral number can't tell you.
| Situation | Emphasis | Why |
|---|---|---|
| Chasing a bilateral 1RM (squat, bench, deadlift) | Bilateral primary | Strength transfer is specific to training mode (Kassiano et al., 2025) |
| Field or team sport with single-leg actions | Unilateral accessory work | Sprinting, cutting, and single-leg jumps are unilateral movements (Wu et al., 2026) |
| Noticing one side lagging in training logs | Unilateral, logged per side | Removes the stronger side's ability to compensate mid-rep |
| Returning from a one-sided injury | Unilateral for the affected side | Trains the healthy limb fully without forcing symmetric bilateral loading |
| Bilateral lift has stalled | Add unilateral accessory for the limiting side | A masked weak side can cap a bilateral lift without showing up as an obvious fault |
Programming this well depends on actually being able to see a side difference, which a bilateral lift's single logged number can't show you. IronLedger logs load, reps, RPE, and RIR per set for every exercise, and tracks each one — a barbell back squat and a Bulgarian split squat, or a conventional deadlift and a single-leg Romanian deadlift — as its own line with its own personal record and Epley-estimated 1RM. Logging the split squat separately, rather than folding it into general "leg day" notes, is what actually surfaces a side that's fallen behind instead of leaving it to a feeling in the gym.
That same exercise-level separation is why compound vs isolation exercise choice and unilateral-vs-bilateral choice are independent decisions, not one combined judgment call — a program can run a bilateral compound lift and a unilateral accessory for the same muscle group in the same session, each pulling its own weight for a different reason.
Frequently asked questions
No. Kassiano et al.'s (2025) meta-analysis of 9 randomised trials found no significant hypertrophy difference between unilateral and bilateral training when volume and effort are matched.
Sources and references
- Wu et al., 2026 — "A systematic review and meta-analysis of 15 randomised controlled trials across 355 team-sport athletes found unilateral training significantly improved unilateral strength (g=0.68, p=0.007) and unilateral jump performance, with no significant overall advantage for sprint or agility." Biology of Sport (2026).
- Kassiano, Nunes, Costa, Ribeiro, Loenneke & Cyrino, 2025 — "A systematic review and meta-analysis of 9 randomised controlled trials found no significant difference in muscle hypertrophy between unilateral and bilateral resistance training (ES -0.21, 95% CI -3.56 to 3.13), while bilateral training produced superior bilateral strength gains (ES 0.56) and unilateral training produced superior unilateral strength gains (ES -0.65)." Sports Medicine, published online 10 January 2025.
- Škarabot, Cronin, Strojnik & Avela, 2016 — "The bilateral deficit typically ranges from 5% to 25%, depending on test method, population, and muscle group." European Journal of Applied Physiology 116(11-12), 2057-2084 (2016).
- Simoneau-Buessinger et al., 2015 — "Maximal bilateral contractions yielded lower force than the unilateral condition in a locked dynamometer configuration (bilateral index 90.7%), but the deficit disappeared in an open configuration (bilateral index 103.2%)." PLoS One (2015).
- Howard & Enoka, 1991 — "Untrained subjects showed a bilateral deficit, while competitive weight lifters showed the opposite: bilateral facilitation, producing more combined force with two limbs than the sum of their one-limb maximums." Journal of Applied Physiology 70(1):306-316 (1991).
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