The first law of strength training is simple: Lift weights, and your muscles get bigger. That’s the fundamental bargain that gets people through the doors of the gym, and it’s true – up to a point.

But the rise of sophisticated analytical tools powered by artificial intelligence is enabling physiologists to ask more nuanced questions about how our bodies respond to exercise. “We already know that the muscles you train get bigger and stronger,” says Wim Derave, a professor in the Department of Movement and Sport Sciences at Ghent University in Belgium. “But what about the other muscles?”

That’s the riddle Derave and his colleagues tackle in a new study published in Medicine & Science in Sports & Exercise. Their AI-driven analysis demonstrates that the muscles you target in a weight-lifting routine do indeed get bigger, but at the same time the untargeted muscles can actually shrink. The finding sheds new light on what triggers muscle growth, why you need adequate fuel and how to exercise to avoid robbing your triceps to pay your biceps.

Derave’s team recruited 21 volunteers – 11 men and 10 women – to do a 10-week strength-training program targeting specific arm and leg muscles with two or three workouts per week.

Three-dimension MRI scans before and after the training program measured muscle sizes – a process that until recently required enormous amounts of time and computing power even for a single muscle. But recent breakthroughs by University of Virginia biomedical engineer Silvia Blemker and others use AI to speed up the process dramatically, making it practical for the first time to measure multiple muscles simultaneously.

Using Blemker’s technique, Derave was able to measure the size of 30 different muscles, 17 of which were targeted by the strength-training program and 13 of which weren’t. Sure enough, the targeted muscles grew more than the non-targeted ones. More worryingly, eight of the non-targeted muscles decreased in size after 10 weeks of training, with two of them (in the hip and calves) showing statistically significant shrinkage.

The subjects filled out dietary questionnaires periodically to track their food intake. Those whose calorie intake was in the top half of the group managed to maintain the size of their non-targeted muscles, increasing their volume by 0.3 per cent on average. In contrast, those with lower calorie intake lost 1.7 per cent of their non-targeted muscle volume.

The results were a surprise to Derave and his team. One school of thought is that strength training boosts levels of muscle-building hormones, such as testosterone, circulating throughout the body, which would imply that even non-targeted muscles should grow. That theory has fallen out of favour, and the new results add further evidence that hormones aren’t what drive the benefits of strength training.

Still, you might hope that nothing at all would happen to the non-targeted muscles. But muscles are never static, Derave points out. On any given day, about 2 per cent of your muscle is being broken down into its constituent amino acids and then repaired with new building blocks, which means that your entire body is being remodelled every two months or so.

What seems to have happened with the subjects who consumed less food is that there weren’t enough amino acids to go around, so some were reallocated from the breakdown of non-targeted muscles to support the growth of targeted ones.

There are two key takeaways from the study. One is that you need to fuel your workouts properly, or else gains in one part of body will come at the expense of other parts. It’s not clear from the results whether the key is getting sufficient calories or getting sufficient protein. Derave suspects the former, as long you’re above a minimum of about 1.2 grams of protein per kilogram of bodyweight per day, which is close to the average consumption for Canadian adults.

The other takeaway is that diverse workout routines have an advantage over repetitive ones, and multijoint exercises trump single-muscle isolation moves. In other words, there’s no free lunch: The only muscles that get fitter are the ones you use.

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Threads @sweat_science.