The law of diminishing returns, in the sports and fitness context, is not particularly controversial. When you’re a neophyte, small amounts of training produce big gains. When you’re a hardened vet, huge training loads are needed to squeeze out tiny gains. Eventually, and inevitably, you hit a ceiling. We’re so used to this pattern that we seldom stop to ask a very basic question: Why?
A major new review paper from Jeremy Loenneke’s research group at the University of Mississippi, published in Sports Medicine, takes on this question in the context of strength training. What, if anything, prevents us from continuing to get bigger and stronger indefinitely, assuming we’re willing to keep upping the training dose? Despite how obvious the question seems, the answers remain elusive. There are several theories, though, which offer some insight about how to push your own plateau a little higher.
The most obvious explanation for training plateaus is that you let the workouts get too easy. A few years ago, I wrote about a surprisingly effective ultra-minimalist once-a-week strength training routine. People who followed it made substantial gains in the first year or so of training, but only marginal gains thereafter. That seemed like a perfect illustration of failing to progress workout volume and difficulty. But the authors of that paper pushed back against this assumption. Data from powerlifters, they pointed out, showed similar plateau effects on a similar timeframe, even though the powerlifters were presumably following much more rigorous and sophisticated training programs.
Loenneke and his colleagues suggest four different mechanisms that might constrain maximal muscle growth. The first is that training eventually makes your muscle cells less responsive to the signals that usually trigger muscle growth. New muscle proteins are synthesized in response to various triggers including food, hormones, and the mechanical stress that strength training imposes on muscle fibers. There are various lines of evidence suggesting that well-trained people produce less new muscle protein in response to a given trigger than untrained people. No one’s entirely sure why, but the end result is that it gets progressively harder to ramp up the production of new muscle as you get fitter.
The other half of this equation is how quickly the muscle proteins you already have are being broken down. The general thinking is that this is less of a factor than how quickly you’re synthesizing new muscle protein, but one clear finding is that you break down protein at an elevated rate when you’re in caloric deficit. This doesn’t seem like a fundamental barrier, but it does suggest that getting enough calories in might be an increasing challenge as you get bigger and bigger.
The third possibility is that there’s a fundamental limit on how big muscle cells are allowed to get relative to their nuclei. The “myonuclear domain hypothesis” posits that each nucleus can only synthesize enough muscle protein to support a given cell volume. Unlike most cells in the body, muscle cells can have more than one nucleus, which enables them to get bigger with training. But the creation of new muscle cell nuclei is a complicated and poorly understood process, so it may eventually put a cap on how big a given muscle cell can get. There are also some other homeostatic mechanisms that may act to keep muscle cell size within a tight range: myostatin, for example, is a growth factor that hinders muscle growth, and whose resting levels get higher in strength-trained people.
The fourth and final mechanism is anabolic resistance, which is the gradual diminution of response to muscle-building triggers with age. This one sounds a lot like the first mechanism: less muscle-growth bang for your stimulus buck. But the difference is that it’s caused by the mere passage of time, rather than in response to prolonged strength training. There are various possible mechanisms: epigenetic changes, lower levels of sex hormones like testosterone and estrogen, insulin resistance, and so on. The idea here is that if you could magically stay 21 forever, you’d be able to keep getting bigger and stronger semi-indefinitely, but the passage of time means that the gradual accumulation of training gains is always competing with the inexorable creep of anabolic resistance.
So which is it? You’ve probably figured out by now that no one is really sure, and the answer is likely a mix of these and other factors. More research is needed. Still, it’s an interesting question, because understanding what general factors limit muscle growth might shed some light on why those limits differ so much among individuals. And it does suggest a practical takeaway: to raise your ultimate ceiling, hit the weights when you’re young, before anabolic resistance kicks in. Even if you slack off in middle age, there’s evidence that the extra cell nuclei you form when packing on muscle will stick around if you detrain and the muscle cells shrink. These dormant nuclei will make it easier to add muscle again later, a form of muscle memory that might help you defy the tyranny of age. And it’s not an all-or-nothing proposition: anabolic resistance is a long, slow process, so those of us no longer in the first flush of youth are still better able to put on muscle today than we will be tomorrow.
In practice, I’m pretty sure that most training plateaus, whether in muscle size, marathon time, or other fitness goals, don’t actually reflect some immutable biological law. We slip into comfortable routines, repeating the same workouts even though our bodies have already adapted to them. We settle for incremental goals instead of dreaming of quantum leaps. We get hurt or stressed out at work or distracted by other priorities. But there’s also some biology at work in the law of diminishing returns. Understanding that biology might eventually help us break through plateaus—or at least accept them gracefully.
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