Bilateral vs Unilateral Lifting
New research comparing unilateral and bilateral resistance training shows no real winner for hypertrophy, but a strong specificity effect for strength gains.
Many people do both. But if you had to choose, what would happen if you did bilateral lifts like squats and leg presses, or split squats and single‑leg presses to get stronger and bigger? This systematic review and meta-analysis suggest that for muscle size, it does not really matter, but for strength, it very much depends on how you plan to use that strength in the real world or in your sport. Let’s unpack it.
What This Study Did
This paper pooled data from nine randomised resistance training studies (total n = 200) that directly compared unilateral (one limb at a time) versus bilateral (both limbs together) exercises over at least three weeks. Participants were generally healthy, with seven male-only and two female-only samples, spanning untrained adults to resistance-trained athletes, mostly in lower-body protocols like squats, leg press, knee extension, and their unilateral counterparts. The authors focused on changes in muscle size (via ultrasound or DXA) and dynamic strength (1–5RM) and used robust meta-analytic methods that account for multiple outcomes per study and small sample sizes.
Aim and Methods
The main question was whether training unilaterally versus bilaterally leads to different muscle growth and different strength gains, and whether any differences are large enough to matter in practice. To answer this, the authors searched major databases up to December 2023, screened 703 records, and retained nine trials that directly compared unilateral and bilateral resistance training with matched programs and dynamic strength tests that matched how people trained (e.g., squat 1RM tested after squat training). They calculated standardised effect sizes for between-group differences and ran robust variance meta-analyses for three outcomes: hypertrophy, bilateral strength, and unilateral strength.
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Results
For muscle hypertrophy, there was no detectable difference between unilateral and bilateral training: the standardised effect size was essentially zero when using pre-test SD, and the change-score model was similarly non-significant but statistically unstable due to only two hypertrophy studies. Bilateral training produced clearly greater gains in bilateral dynamic strength, indicating a small-to-moderate advantage when testing with bilateral lifts such as squat or leg press 1RM. In contrast, unilateral training produced clearly greater gains in unilateral strength, with effects consistently favouring the limb-specific training condition.
Strength and hypertrophy
Overall, strength adaptations followed the principle of specificity: people got stronger most in the way they trained, with bilateral training better for bilateral tests and unilateral training better for unilateral tests. The data did not support the idea that unilateral training gives you “free” extra bilateral strength or that bilateral training inherently limits limb-specific strength; instead, each method mainly improved its own test mode. For hypertrophy, both unilateral and bilateral approaches appeared similarly effective in the limited data available. This fits with the broader view that a range of exercises and loading schemes can produce similar muscle growth when volume and effort are comparable. Having said that, only two studies here measured muscle size directly, and the risk of bias was rated as “some concerns” in all nine trials.
Practical Takeaways
For lifters whose main priority is bilateral strength (e.g., barbell squat, bilateral leg press, bench press), bilateral exercises should be the backbone of the program, because they produce superior gains in bilateral 1–5RM strength compared with unilateral-focused training. For athletes and individuals who care more about single-leg or single-arm force production (e.g., cutting, sprinting, step-ups, unilateral pressing), unilateral exercises deserve priority, as they led to larger gains in unilateral strength tests. If the goal is muscle size rather than a specific strength test, both unilateral and bilateral options are reasonable tools. Exercise selection can be driven by time efficiency, equipment, joint comfort, asymmetry management, and personal preference, recognising that current evidence on hypertrophy is limited and longer, better-controlled trials could refine these conclusions.
Reference
Kassiano W, Nunes JP, Costa B, Ribeiro AS, Loenneke JP, Cyrino ES. Comparison of Muscle Growth and Dynamic Strength Adaptations Induced by Unilateral and Bilateral Resistance Training: A Systematic Review and Meta-analysis. Sports Med. 2025 Apr;55(4):923-936. doi: 10.1007/s40279-024-02169-z. Epub 2025 Jan 10. PMID: 39794667.
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