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An early morning fitness group exercises in Bryant Park in New York on June 15, 2016.ALEX WROBLEWSKI/The New York Times News Service

There’s a puzzling pattern in the use of personal trainers to supervise strength training, according to exercise scientist James Fisher of Solent University in Britain.

Beginners often hire trainers, he says; so too do serious powerlifters and bodybuilders. “But most of the people in between seem not to use them.”

To understand why, he and his colleagues have been studying why people hire personal trainers, what they get out of it, and most crucially, whether they end up bigger and stronger as a result. The latest results, published in the Journal of Sports Sciences by an international team of researchers including Fisher and Brad Schoenfeld of CUNY Lehman College in New York, bolster the case that supervised workouts do indeed work better – though the specific benefits depend on who you are.

The researchers assigned 45 young men and women, all experienced lifters, to lift weights three times a week for eight weeks, completing all the same exercises. Half the volunteers were supervised by a trainer; the other half worked out alone using the same gym.

Sure enough, the supervised group saw larger increases in muscle size as measured by ultrasound, and also did better in some of the strength tests. They increased the maximum weight they could squat by 13.5 per cent, for example, compared with 7.7 per cent in the unsupervised group.

Notably, just two participants from the supervised group didn’t complete the study, one of them because they caught COVID. In comparison, seven of the unsupervised participants dropped out.

The results support two of the claimed benefits of personal training: that it encourages you to keep showing up to workouts, and that it spurs you to work harder – or better yet, smarter – when you’re there.

But they also pose a riddle. Last year, Fisher led a systematic review of 12 studies comparing supervised with unsupervised strength training. In that analysis, having a personal trainer had little effect on body composition and at best a small influence on strength. Why the difference?

The most likely explanation, Fisher says, is that the subjects in the systematic review were mostly novice strength trainers, in contrast to the experienced lifters in the new study. When you’re just starting out, pretty much anything you do will lead to initial gains, so the details that a trainer might add don’t matter as much.

That assumes you’re showing up in the first place, which is more likely if you’ve agreed to take part in a study. In the real world, of course, motivation and accountability are among the biggest benefits of hiring a trainer for novice exercisers.

For more experienced lifters, on the other hand, the easy gains are long gone. Instead, the details of a well-planned strength-training program become more important, perhaps along with the motivation to squeeze out an extra rep or two.

That means different people will gain different benefits from personal training, says Leigh Graham, a Toronto-based personal trainer who specializes in working with older clients.

Seniors might have health conditions, old injuries, or balance problems that make it hard to follow generic cookie-cutter fitness programs, she says. “These require a greater understanding of the body, and expertise in how to move it safely and effectively.”

More generally, she says, beginners often don’t have a good sense of their own limits, meaning that a trainer’s job is to hold them back to avoid injury – the opposite of the role they might play in helping an experienced exerciser push harder.

For those of us who are neither beginners nor experts, the middle ground can be tricky to navigate. Hiring a personal trainer requires a substantial commitment of both money and time. Whether it’s worthwhile depends on your goals and on what’s currently holding you back. But the new results suggest that, if you do take the plunge, you’ll get measurable results.

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

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