Teen muscle growth: 23 studies, different comparisons
In short
Across 23 studies with 1,443 under-18s, lifting had a small hypertrophy effect versus no-exercise controls (d 0.22). Against other activities, the overall effect was d 0.61 and postpubertal youth led at d 1.50. Sex effects were inconsistent.
The comparison group changes the answer about teen muscle growth. Komm and colleagues' 2026 review pooled 23 controlled studies with 1,443 under-18s. Hypertrophy was small versus no-exercise controls: d 0.22 (95% CI 0.06–0.37). Against other activities it was d 0.61 (0.03–1.20). Cohen's d is a standardized group difference, not a percentage or kilograms gained.
What do passive and active controls mean?
Passive controls do no exercise; active controls do another activity. The abstract does not specify those activities. These pooled comparisons do not show that another activity suppresses growth: they are separate contrasts, not a head-to-head test of control types.
| Stage | Versus passive controls | Versus active controls |
|---|---|---|
| Prepubertal | 0.16 (-0.04 to 0.36) | Smaller, not significant; value not given |
| Peripubertal | 0.30 (0.01–0.58) | Smaller, not significant; value not given |
| Postpubertal | 0.01 (-0.31 to 0.33) | 1.50 (0.46–2.54) |
Do puberty and sex settle the question?
Maturation subgroups differed against active controls (p=0.0227), with the largest effect after puberty. That pattern did not carry over to passive controls. Against active controls, males had a large effect (d 1.26), females a trivial one (d -0.12). Yet the authors concluded sex did not consistently influence outcomes; the female estimate cannot establish that girls cannot grow muscle.
What should parents and coaches track?
- Log exercises, repetitions, load and technique feedback.
- Review load progression with a coach while keeping technique consistent.
- Explain that strength may improve before visible size does.
The nine-week study in boys also separated force gains from muscle thickness. Use fairlift logs to discuss progress; the Muscle Index compares adults' relative strength and should not judge a growing teen's development.
The review combined randomized and non-randomized studies, using direct imaging or indirect instrumented measures. The abstract cannot predict an individual's growth or an optimal program. Youth lifting should be supervised and technique-first.
Frequently asked questions
Can teenagers build muscle by lifting?
A meta-analysis of 23 studies in under-18s found a small hypertrophy effect versus no-exercise controls (d 0.22). This group estimate does not guarantee an individual's gains.
Are gains largest after puberty?
Postpubertal youth had the largest effect against active controls (d 1.50). Against passive controls their effect was d 0.01, so the comparison must accompany the claim.
Does lifting fail to build muscle in teenage girls?
The female estimate against active controls was d -0.12, with a wide 95% CI of -1.05 to 0.81. The authors found no consistent sex influence, so this does not rule out muscle growth in girls.
Should teens use the Muscle Index to assess development?
The Muscle Index compares adults' relative strength and should not assess a growing teenager's development. Track technique and load progression under supervision instead.
Source: PubMed (published )