What 14 of the World’s Top Muscle Scientists Just Agreed On

Resistance training, response heterogeneity, and what it means for the rest of us

When 14 of the world's most prominent muscle scientists gather to present their findings… I think it’s worth writing about what they shared.

Two people walk into the same gym. They follow the same program, lift the same weights wth the same intensity, eat about the same amount of protein, and sleep about the same number of hours. Six months later, one has added meaningful muscle, and the other has barely moved the needle. Why?

How many sets do I need? How much should I rest? How often should I work out? This consensus report gives us solid guidance on why adaptations occur and how we should train to maximize them.

Summary

• True non-responders to resistance training are far rarer than older studies suggested. The most apparent low responders are simply underdosed.

• The biology of muscle growth has turned out to share molecular similarities with cancer cell growth. Go figure. Growing muscle reprograms its metabolism in surprising ways.

• How many sets per muscle per week appears to be the sweet spot for most people? And when does the law of diminishing returns apply?

• Free weights and machines…The choice between them might be far less important than most people think.

• The two molecular processes that act as the central drivers of growth.

• Chronological age is not the barrier to building muscle that most people assume. Older adults respond well when training is dosed appropriately. I won’t bury that lede!!

• Genetic testing for “muscle response” is not yet ready for the clinic, and probably will not be for a long time.

• Of all the extrinsic factors, supplements are the least important driver of how well you respond to training. Consistency, effort, sleep, and food matter far more.

Let’s get into this….

First, the science that the researchers presented… then the practical application. Skip the next section if the science doesn’t interest you.

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What actually changes when we lift

Resistance training increases the cross-sectional area of skeletal muscle through radial growth, meaning the individual muscle fibers get fatter. This is driven primarily by an increase in muscle protein synthesis via mTORC1, the central anabolic switch in the muscle cell. We have known this for years. But some new facts have come to light.

Troy Hornberger’s laboratory has used advanced imaging techniques to show that the radial growth of muscle fibers occurs primarily by increasing the number of myofibrils within each fiber. The myofibrils are the contractile machinery, the long ropes of sarcomeres ( single muscle unit) that actually generate force when you lift. Each fiber is roughly 80 to 85 percent myofibrils by volume, so adding more of them is the dominant way the fiber grows. The old textbooks had assumed that the existing myofibrils just got bigger.

Hornberger also showed that muscle fibers grow longitudinally, meaning they get longer, by adding new sarcomeres along the length of the fiber. This occurs through a process in which the Z-lines, the structural anchors between sarcomeres, split transversely, and a new sarcomere forms in the gap. Interestingly, this longitudinal growth appears to be independent of the mTORC1 pathway, suggesting there is a second growth program inside the muscle cell that we are only beginning to understand. I realize this is technical… but it’s super interesting and very different from what I was taught.

Abigail Mackey’s work has updated another piece of the same textbook. The myonuclei, the nuclei inside the muscle fiber that direct protein synthesis, were thought to be permanent once added during training. Mackey’s recent mouse work suggests this is not accurate. Detraining can lead to the loss of some newly acquired nuclei, which means muscle memory may be more nuanced than the rigid “once added, always there” model we used to teach. Use them or lose them is still valid, I guess.

And then there is the finding from Juha Hulmi’s laboratory.... Metabolic growth of muscle resembles that of a cancer cell. Both take up large amounts of glucose, both upregulate glycolytic enzymes, and both shift toward the so-called Warburg effect, in which glucose is used for biomass production rather than just energy generation. The molecules required to build new myofibrils, ribosomes, and membranes all come from glucose-derived intermediates such as serine and glycine. Hypertrophy, at a metabolic level, is a controlled growth program that borrows from the same playbook as tumor growth.

For Members, we will now discuss the training-programming variables that drive growth—the rest interval and the proximity-to-failure questions many of us have. We will also discuss the weekly volume sweet spot and address the machines-versus-free-weights argument. Lastly, we will review how concurrent (aerobics and resistance) training affects hypertrophy, why older adults are often under-dosed rather than non-responders, the two molecular processes that distinguish high responders from low responders, why commercial genetic tests are not yet worth your money, and a realistic, evidence-based framework for what you can actually expect from a year of consistent training. I will close with a take-home section specifically for readers whose primary focus is building muscle.