Unilateral vs Bilateral Training: The Bilateral Deficit and Correcting Asymmetries

Unilateral vs bilateral training for hypertrophy is less of a contest than gym talk makes it sound. On current evidence, training one limb at a time and training both together build similar amounts of muscle, as long as the sets are hard and the exercise suits you. What changes is which strength you build, how much weight you can load, and how well you can fix a weaker side.

This article sits inside our exercise selection and biomechanics guide, and it covers one question in depth: when do split squats, single-arm rows and one-arm presses earn a place next to the barbell?

Does unilateral vs bilateral training matter for hypertrophy?

Probably not as much as you think. A 2025 systematic review and meta-analysis pooled 9 studies and found no significant difference in muscle hypertrophy between bilateral and unilateral resistance training (Kassiano et al., 2025). The authors were careful about their own limits: the literature was small and the included studies had a moderate risk of bias.

Strength told a more specific story. Bilateral training produced a larger increase in bilateral strength, while unilateral training produced a larger increase in unilateral strength. The authors concluded that strength gains seem to follow the principle of specificity.

An older trial points the same way. Middle-aged and elderly adults did 12 weeks of knee extension training either bilaterally or unilaterally. Quadriceps cross-sectional area increased in both groups with no significant difference between them, while strength gains favoured the type of exercise each group had practised (Häkkinen et al., 1996). That sample was middle-aged and older, so it is a supporting detail, not proof for young lifters.

What is the bilateral deficit?

The bilateral deficit is the finding that a limb can produce less force when both limbs work together than when it works alone. Add up two single-leg efforts and the total can exceed one two-leg effort.

Here is a simple, purely illustrative picture.

Now the honest part. A review of the research describes the bilateral deficit as an inconsistent phenomenon, with high variability in size and even in whether it appears, and sometimes the opposite (bilateral facilitation) shows up instead (Škarabot et al., 2016). It was more consistent in dynamic than in isometric contractions. The same review says task familiarity has a large influence, and that postural stabilisation demands and the ability to use counterbalances during single-limb actions strongly affect it.

On the cause, the review points to higher-order neural inhibition as a possible mechanism and notes that biomechanical factors seem to matter in explosive efforts. It also says many proposed mechanisms remain largely unknown. So the “neural” explanation is plausible but not settled.

Why single-limb work can still be a smart choice

If hypertrophy is similar, why bother? Because exercise selection is also about what you can actually do well.

• Lighter total load. A split squat with a pair of dumbbells can challenge your quads without you carrying a heavy bar on your back. That is a practical argument, not a proven injury-prevention effect.
• Less equipment needed. A pair of dumbbells and a bench go a long way, which suits a crowded gym or a home setup.
• Balance and trunk demand. Your hips and trunk have to resist tilting and rotating. That can make the exercise feel harder than the load suggests, but it also means balance can limit the set before the muscle does.
• Back-friendly options. If a barbell row or squat flares up your lower back, supported single-limb variations can let you keep training. If pain is ongoing, get it assessed first.

The main downside is the other side of the same coin. When balance, not muscle, ends the set, you are not getting a hard enough stimulus. Use a wall, rack or bench for support so your legs fail first.

Can unilateral training fix muscle imbalances?

It can help, though the research here is thin and mostly from athletes, not everyday lifters.

A 2024 randomized proof-of-concept trial took 30 male university basketball players and gave half of them 10 weeks of unilateral compound training (split squats, Bulgarian split squats, step-ups, single-leg calf raises, plus single-leg jumps), while the other half kept their usual training. The trained group showed a significant reduction in single-leg jump height and power asymmetry (Zhang et al., 2024). A meta-analysis of 8 randomized trials with 157 participants also found that unilateral plyometric training reduced lower limb asymmetry in athletes, though the authors called for further high-quality trials (Sun et al., 2025).

Both are small, athlete-based and jump-focused. They suggest single-limb work is a reasonable tool for asymmetry, but they do not show that it fixes size differences in your biceps or chest.

The most common mistake with side-to-side differences is letting the stronger side set the weight and the reps. With dumbbells that happens quietly, because each arm gets the same load but the weaker arm fails first.

How to correct a lagging side

1. Start each exercise with the weaker side.
2. Match the stronger side to the same reps and the same effort (RIR 1-3).
3. If the weaker side is clearly behind, add one extra set for it. Do not go beyond that, because extra volume you cannot recover from will not help.
4. Re-check every 6-8 weeks. Small left-right differences are very common and often never fully disappear.

For chest-specific side differences, see our guide on fixing an uneven chest with unilateral pressing.

Does training one limb help the other?

There is also a curious effect called cross-education. A meta-analysis of 31 randomized trials (785 subjects) found that unilateral resistance training raised strength in the untrained limb by about 11.9% on average, with larger effects in the lower limb (16.4%) than the upper limb (9.4%), although the studies had a high risk of bias (Manca et al., 2017).

That matters most when one side is injured or in a cast. If you have an injury, ask your doctor or physiotherapist what is safe for you before you train the other side.

Bilateral or unilateral: how to choose per muscle

Situation	Better starting choice	Why
Chasing a bigger barbell squat, bench or row	Bilateral	Strength is specific to the lift you practise
Lower back gets tired before legs do	Unilateral or machine-supported	Lighter spinal load, easier to control
One side is clearly weaker	Unilateral	Each side works on its own
Short on time	Bilateral	More load per set, fewer sets to do
Crowded gym, no free rack	Unilateral with dumbbells	Needs little equipment
Balance fails before muscle	Supported unilateral, or bilateral	Needs a hard enough stimulus

How to add unilateral work to your program

You do not need to rebuild your split. Keep your main bilateral lift for a muscle group and add one single-limb movement after it.

• Legs: squat or leg press first, then a split squat, lunge or step-up. We cover the specifics in Bulgarian split squats and lunges for the legs.
• Back: a barbell or chest-supported row first, then a single-arm row. Our guide to single-arm dumbbell rows, bench versus tripod stance shows how to set up.
• Chest and shoulders: a bilateral press first, then single-arm dumbbell or cable work.

Keep the same weekly hard-set range you would use anyway; adding single-limb work does not mean adding a whole extra block of volume. Count single-limb sets as sets for that muscle, and use the training volume calculator to check your weekly total. Or start from our free leg day plan and swap in the single-leg exercise that fits your gym.

If your spine is a concern, our piece on axial versus non-axial exercises and spinal recovery explains how to spread load without dropping the hard work.

When to see a doctor

Muscle size or strength differences between sides are common. See a doctor or physiotherapist if a gap is large, growing, painful, or began after an injury, or if you notice numbness, tingling or weakness in a limb. Do not try to train through those symptoms.

The bottom line

Unilateral and bilateral training both build muscle, and the 2025 meta-analysis found no significant difference between them for hypertrophy. Strength follows specificity, so keep your main barbell lifts if barbell strength is the goal. Use single-limb exercises where they fix a weak side, spare your back or fit your equipment, and always take the weaker side first. Use the 1RM calculator to track your main lifts while you do it.