'You Can't Get Big Without Training Legs' — Myth vs Research
Published:
Written by: Hirotsugu YoshimuraReviewed by: Tomonobu Someda
'Skip leg day and your upper body won't grow either,' 'the growth hormone and testosterone from squats and deadlifts build your whole body,' 'legs are your foundation, so without them you won't build a frame.' Leg-day devotion runs deep in gym culture. But is training legs really the key to upper-body growth? Using research on hormones and muscle mass, we separate what's right from what's wrong in this belief.
Let the data settle it.
Do the hormones released by squats also grow your upper body?
What's said
ジムでの通説、フィットネス系YouTube
Big lifts like squats and deadlifts flood your body with growth hormone and testosterone. Those hormones circulate and grow your arms and chest too. So if you skip legs, your upper body won't grow either.
What research says
- The idea that transient post-exercise hormone spikes cause hypertrophy has been refuted by several studies.
- West et al.
- (2010) trained one arm of the same person under a 'low-hormone' condition and the other under a 'high-hormone' condition (with added leg work) for 15 weeks — with no difference in muscle growth or strength, even though hormones clearly rose on the high-hormone side.
- Morton et al.
- (2016; n=49) found post-exercise spikes in testosterone, GH, and IGF-1 did not correlate with gains in muscle size or strength.
- West & Phillips (2012; n=56) reported the same, with testosterone in particular showing no correlation.
'Squat hormones grow your upper body' is largely unsupported in higher-quality research. To be fair, there's one counter-study — Rønnestad et al. (2011; n=9) — where the leg-combined arm grew slightly more. Its design is similar yet its conclusion is opposite, and the sample is tiny. So the honest read is 'refutation is winning, but it's not fully settled.' At minimum, the basis for treating leg training as a prerequisite for upper-body growth is weak.
Are legs your foundation — so without them, no frame?
What's said
ジムでの通説、ボディメイク指導
Legs are the foundation that supports the whole body. Without training them, you won't build a frame, and even a developed upper body will look unbalanced and unstable.
What research says
- While the 'hormones grow your upper body' mechanism fails (see the previous round), the claim that 'skipping legs costs you' stands on separate, quantitative grounds.
- Whole-body MRI by Janssen et al.
- (2000; n=468) shows total skeletal muscle reaches about 38% of body mass in men, and age-related muscle loss occurs mainly in the lower body — the legs are the single largest block of muscle in the body.
- Not training them means leaving a large share of your total muscle mass undeveloped.
- Legs also matter for visual balance and as the foundation for lower-body strength and everyday movement.
'Legs are your foundation' is a bit metaphorical, but directionally reasonable — not because they grow your upper body via hormones, but because the legs themselves are the largest muscle group, and skipping them clearly costs you in total muscle mass, visual balance, and lower-body strength. If 'frame' means whole-body mass and balance, the value of leg training does line up with the research.
Is it okay to train only the upper body and skip legs?
What's said
上半身重視のトレーニー、SNS
If you just want bigger arms and chest, you don't need legs. Focusing on the upper body means it grows more.
What research says
- Upper-body growth itself is largely unaffected by whether you train legs.
- As West et al.
- (2010) and Morton et al.
- (2016) show, hypertrophy of the arms and chest is determined by giving those muscles enough stimulus, volume, and effort — not by systemic hormones from leg training.
- So if your only goal is upper-body appearance, skipping legs won't meaningfully blunt your upper-body gains.
- That said, it's also a choice to give up whole-body mass and balance — because legs are the largest muscle group (Janssen 2000).
If your goal is strictly upper-body development, skipping legs won't hold that development back — 'no legs, no arm gains' is false. But if you value total muscle mass, whole-body balance, and lower-body strength, there's little reason to cut out the largest muscle group. In short: not mandatory, but it depends on your goals.
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Related research
- Are Post-Exercise Anabolic Hormone Elevations Required for Hypertrophy? A 15-Week Within-Subject Trial2010
- Post-Exercise Hormone Elevations and Hypertrophy/Strength in 49 Trained Men: A 12-Week Study2016
- Exercise-Induced Hormone Responses and Their Correlation with Strength/Hypertrophy: A Cohort of 56 Young Men2012
- Does Physiological Elevation of Endogenous Hormones Enhance Training Adaptation? An 11-Week Within-Subject Trial2011
- Distribution of Skeletal Muscle Mass: Whole-Body MRI in 468 Adults2000
Sources
- West DWD, et al. (2010) J Appl Physiol — Elevations in ostensibly anabolic hormones with resistance exercise are not required for greater gains in muscle protein synthesis or hypertrophy
- Morton RW, et al. (2016) J Appl Physiol — Muscle androgen receptor content but not systemic hormones is associated with resistance training-induced skeletal muscle hypertrophy in healthy, young men
- West DWD, Phillips SM (2012) Eur J Appl Physiol — Associations of exercise-induced hormone profiles and gains in strength and hypertrophy in a large cohort after weight training
- Rønnestad BR, Nygaard H, Raastad T (2011) Eur J Appl Physiol — Physiological elevation of endogenous hormones results in superior strength training adaptation
- Janssen I, Heymsfield SB, Wang ZM, Ross R (2000) J Appl Physiol — Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr
Published:
Written by
Hirotsugu YoshimuraFounder of BODYDATA / CEO of INVOLVE
I don't pick things because they "seem good." I check the data first, then test it with my own body.
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Reviewed by: Tomonobu Someda
Content reviewed from the perspective of coaching practice and supplement-industry experience
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