Strength training: how much is linked to longer life, and why more may not be better

Official advice on aerobic exercise comes with a number attached: 150 minutes a week. Advice on strength training does not. Adults are told to work their muscles on two days a week and left to guess whether that means ten minutes with a resistance band or two hours in a weights room.

Most do neither. A survey analysis of nearly 400,000 Americans found that three in five U.S. adults do not engage in any muscle-strengthening exercise. For those wondering how much is worth doing, a 2022 review by Haruki Momma and colleagues at Tohoku University in Japan supplied the first pooled estimate, and its answer was smaller than gym culture would suggest. The largest drop in the risk of dying went with about half an hour to an hour a week.

What counts as strength training

Researchers use the term muscle-strengthening activity for exercise that makes muscles work against resistance: lifting weights, using machines or bands, and exercises that use the body’s own weight, such as push-ups and squats. The studies in the Tohoku review asked about exercise of this deliberate kind. They focused on training with weights and callisthenics, but not on muscle-strengthening activities such as carrying heavy loads and heavy gardening.

That narrows what the evidence covers. A builder or a keen gardener may work their muscles hard every day, and nothing in these studies speaks to whether that counts.

What guidelines say about how much strength training

The World Health Organization’s 2020 guidelines say that additional health benefits come from muscle-strengthening activities at moderate or greater intensity on 2 or more days a week, for adults of all ages. The recommendation is graded as strong.

The guideline stops at days. Its authors looked for evidence that more training brings more benefit, a pattern known as a dose-response, and reported that there was no evidence to support a dose-response association with higher volumes of muscle-strengthening activities. At the time, too few studies had recorded minutes.

What the evidence shows: 30 to 60 minutes of strength training a week

Momma’s team searched for long-term studies that had asked healthy adults about strength exercise and then followed them for deaths and disease. Sixteen studies met the eligibility criteria, most of them American, with the largest following nearly 480,000 participants and the longest running for 25 years. All of them took account of how much aerobic exercise each person did, so the results describe strength training over and above walking, running or cycling.

Compared with doing none, any muscle-strengthening exercise went with a 15% lower rate of death from all causes, a 17% lower rate of cardiovascular disease, 12% less cancer and 17% less diabetes.

The dose is where the review broke new ground. Six of the studies had recorded weekly minutes. When the reviewers plotted risk against time, the curve bent. The lowest risk of death was observed at 40 min/week of muscle-strengthening activities, about 17% below that of non-exercisers. Past that point the line turned back up, and by about 140 minutes a week the advantage had disappeared. Heart disease and cancer followed the same J shape, with the best results at around 30 to 60 minutes.

Diabetes behaved differently. Risk fell steeply up to an hour a week and then kept edging down, with no upturn.

The authors drew a cautious inference. “These results suggest that optimal doses of muscle-strengthening activities for the prevention of all-cause death, CVD and total cancer may exist,” they wrote, using the abbreviation for cardiovascular disease.

Is more strength training worse for longevity?

The upturn at higher volumes is the most surprising part of the picture, and the least secure.

It did not originate with the Japanese review. In 2017 Masamitsu Kamada and colleagues at Harvard studied older women from the Women’s Health Study who were free of cardiovascular disease, diabetes and cancer, almost 29,000 of them, and followed them for twelve years. Women who did up to an hour of strength training a week had death rates more than a quarter lower than women who did none. At one to two and a half hours the advantage shrank to about a fifth. At two and a half hours or more there was no advantage at all. Kamada’s team concluded that a moderate amount of time in strength training seemed beneficial for longevity, independent of aerobic activity, and that any risk from larger amounts needed more study.

A second pooled analysis, by Prathiyankara Shailendra and colleagues at the University of South Australia, reached a similar shape by a separate route. A maximum risk reduction of 27% was observed at around 60 minutes per week of resistance training, and mortality risk reductions diminished at higher volumes.

Three things argue against reading this as proof that heavy training is harmful. Very few people in these cohorts did large amounts, so the estimates at the top end are imprecise; in the women’s study the result for the highest group was compatible with anything from a clear benefit to a clear harm. Those who report many hours may also differ in ways the questionnaires did not capture. And the reviews offer no established mechanism by which more training would do harm. The review’s own summary is that the influence of high volumes “is unclear.”

What strength training adds to aerobic exercise

The two kinds of exercise appear to add up. In the Tohoku review, those who did both had death rates about 40% lower than those who did neither.

A large American study published the same year shows the steps. Jessica Gorzelitz and colleagues at the US National Cancer Institute followed 99,713 adults who had described their exercise habits. After accounting for aerobic activity, weightlifting was associated with a 9% lower risk of death from any cause. Adults who met the aerobic guideline but never lifted had death rates about a third lower than those who did neither. Adults who met the guideline and also lifted weights once or twice a week had rates about 40% lower. The authors’ conclusion was that adults who met recommended amounts of both types of exercise appeared to gain additional benefit.

What trials of strength training show

None of the studies above was an experiment. For that, the evidence is about function, and it is much firmer. A Cochrane review by Chiung-ju Liu and Nancy Latham gathered randomized trials of progressive resistance training, in which the load is increased as a person gets stronger, in older adults. One hundred and twenty one trials with 6700 participants were included, most of them training two or three times a week.

The training did what it was designed to do. It produced a large gain in muscle strength, a modest gain in walking speed and a clear improvement in the ability to get up from a chair. Aches and soreness were common where trials bothered to record them, and the reviewers noted that serious adverse events were rare.

These trials lasted weeks or months and did not count deaths. The case that strength training extends life therefore rests on joining two kinds of evidence: trials showing that it improves strength and mobility, and cohort studies showing that people who do it live longer.

The weak point in the strength training evidence: who lifts weights

Momma and colleagues graded their own evidence harshly. They report that the grading of the certainty of the evidence was generally very low, and they list the reasons. The meta-analysis included only a small number of studies. Thirteen studies used self-reporting methods to measure muscle-strengthening activities, and self-reports of exercise are unreliable. Because so few studies were available, the reviewers could not test for publication bias, the tendency for striking results to reach print more readily than dull ones, and they warn that the pooled estimates in this study might have been overestimated.

The deeper problem is who lifts weights. In the American survey, the adults who met the strength guideline were younger, better off, better educated and in better health to begin with. The cohort studies adjusted for many of these differences, and their authors acknowledge that observational studies of this kind are potentially influenced by residual, unknown and unmeasured confounding factors. Confounding means that something other than the exercise, such as income or underlying health, explains part of the difference.

Most of the data also come from one country and from people who chose their own routines.

How the strength training results sit with the two-days-a-week guideline

The numbers fit the guideline better than might be expected. Two sessions of fifteen to thirty minutes fall inside the range where risk was lowest, which suggests the WHO’s advice on days and the review’s finding on minutes describe much the same amount of exercise.

What the review adds is reassurance about the lower end. In these studies, the difference between doing nothing and doing a little was large, and the difference between a little and a lot was small or absent.

Whether that pattern reflects biology or the kind of people who do a little strength training is a question these cohort studies cannot answer.