Weight Training Load Doesn’t Affect Muscle Mass or Strength

A new study suggests that, if sets are taken close to failure, the amount of weight on the bar does not determine muscle growth. However, individual differences in muscle-building ability appear to be real [1].

How should we train?

Muscle mass and strength are among the strongest predictors of longevity, but the best way to gain them is a matter of ongoing debate. In a study published in The Journal of Physiology, a team of scientists from McMaster University, with collaborators from Queen’s University and Liverpool John Moores University, set out to test the long-standing question of whether larger weights and fewer repetitions are superior to smaller weights and more repetitions for building muscle.

The scientists recruited 20 healthy, recreationally active but untrained young men and ran an ingeniously designed experiment: each participant trained both arms and both legs, but one arm and one leg were randomly assigned to high-load training (HL), and the other arm/leg did low-load training (LL).

HL was set at 8-12 repetitions at about 70%-80% 1RM (one-repetition maximum, the heaviest weight a person can lift for a single repetition with proper form). LL was set at 20-25 repetitions at about 30%-40% 1RM. Crucially, every set was taken to volitional fatigue, the inability to complete another repetition with good form (“close to failure”), so the overall effort was matched even though the weights differed. The baseline 1RM values were 18 ± 4 kg for bicep curls (about 40 pounds) and 40 ± 14 kg (almost 90 pounds) for knee extensions.

The weight is largely irrelevant

After 10 weeks of 3 supervised sessions per week, the researchers assessed muscle growth with several readouts: DXA to measure arm and leg lean mass, ultrasound to measure muscle cross-sectional area and thickness, and biopsies (from the vastus lateralis) to quantify type I and type II muscle-fiber growth. Muscle strength was also tested; in cohort studies, muscle strength often predicts mortality more strongly than muscle mass [2]. Across all these outcomes, the HL and LL conditions were statistically indistinguishable.

This does not mean, however, that all the participants performed at the same level. In fact, variability in responses between participants was quite high, even when accounting for baseline differences. In other words, some people were just better at gaining muscle mass and strength than others. The researchers tried to control for dietary differences, including by providing every participant with enough protein to support muscle gain. They hypothesize that this variability in individual muscle-building capacity stems mostly from intrinsic causes, such as genetic differences.

Complicating things even further, individual rankings for muscle mass gain and for muscle strength gain did not match. In other words, some participants were big “muscle size responders” without being big “strength responders,” and vice versa.

While it is tempting to interpret the results through the lens of total “volume load” (sets × reps × weight), this does not seem to cleanly reflect outcomes here, at least for bicep curls, in which HL produced a significantly higher volume load than LL, yet the hypertrophy readouts were still similar. This suggests that, under the study’s conditions (sets taken to volitional fatigue), volume load alone is not a reliable predictor of hypertrophy.

Muscle building gets slower with time

To probe the mechanism, the authors measured myofibrillar protein synthesis (MyoPS), the rate at which muscle builds new contractile proteins (actin and myosin). Because MyoPS was assessed on a time course (baseline, week 1, and week 10), they could report a clear dynamic: MyoPS rose early in training but was much smaller by week 10, signifying that it was attenuated with training, both in high-load and low-load limbs.

In contrast, the main hypertrophy and strength outcomes were measured only at two points: baseline and the end of week 10. Therefore, the study could compare net changes and HL/LL differences, but it could not determine whether those size/strength changes followed an “early spike then slowing” trajectory due to the lack of intermediate time points.

The authors suggest that early training may elevate MyoPS more due to novelty and muscle damage. Later, as the muscle adapts and damage decreases, the MyoPS response gets smaller even with progressive overload. Only vastus lateralis muscles (leg muscles), but not biceps, were biopsied, making a direct MyoPS comparison between arms and legs impossible.

While providing interesting and potentially actionable insights, the study also had limitations, including a small sample size, short duration, and possible “cross-education” effects (when training one limb affects the other), although such effects are thought to be minimal [3]. The study’s conclusions are also conditional on training to true fatigue, which is not trivial in real life.

Literature

[1] Lees, M. J., Mcleod, J. C., Morton, R. W., Currier, B. S., Fliss, M. D., McKellar, S. R., … & Phillips, S. M. (2025). Resistance training load does not determine resistance training‐induced hypertrophy across upper and lower limbs in healthy young males The Journal of Physiology.

[2] Newman, A. B., Kupelian, V., Visser, M., Simonsick, E. M., Goodpaster, B. H., Kritchevsky, S. B., … & Harris, T. B. (2006). Strength, but not muscle mass, is associated with mortality in the health, aging and body composition study cohort. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences, 61(1), 72-77.

[3] Song, J. S., Yamada, Y., Kataoka, R., Hammert, W. B., Kang, A., Spitz, R. W., … & Loenneke, J. P. (2024). Does Unilateral High‐Load Resistance Training Influence Strength Change in the Contralateral Arm Also Undergoing High‐Load Training?candinavian Journal of Medicine & Science in Sports, 34(12), e14772.