A couple of hours in the gym each week might go a long way. A large new study found strength training was linked to a lower risk of death, but only up to a point.

Researchers led by a team from the Harvard T.H. Chan School of Public Health analysed data from 147,374 people across three long-running prospective cohorts: the Health Professionals Follow-up Study, the Nurses’ Health Study, and the Nurses’ Health Study II.

The study found 90 to 119 minutes of strength training a week was associated with a 13 percent lower risk of dying from any cause over a period of up to 30 years.

Doing more was not linked to extra benefit. The data showed no further reduction in mortality risk for people doing 120 minutes or more of strength training a week.

The participants were all health professionals. Among them, 74 percent met the recommended 150 minutes a week of moderate aerobic exercise, such as running or playing squash, and 46 percent did some regular strength training, such as squats or weightlifting.

The same 90 to 119 minute range for strength training was also linked to a 19 percent lower risk of death from cardiovascular disease and a 27 percent lower risk of death from neurological disease, such as Alzheimer’s.

Aerobic exercise was also tied to lower mortality. People who logged at least 45 MET-hours of aerobic activity a week had a 42 to 47 percent lower risk of dying, regardless of how much strength training they did.

The lowest mortality risk was seen among people who did both. The biggest reduction was among those doing 30 to 44 MET-hours of aerobic exercise a week plus 60 to 119 minutes of strength training. That combination was linked to a 45 percent lower risk of death.

“The mortality benefits of aerobic physical activity are well established,” write the researchers in their published paper. “However, the role of resistance training in reducing all-cause and cause-specific mortality, its dose-response relationship, and its joint effects with aerobic activity remain less clear.”

“Our joint analysis of resistance and aerobic activity showed the lowest mortality risk among participants who engaged in both, consistent with previous studies,” write the researchers. “Importantly, we observed that while either sufficient aerobic or resistance training alone reduced mortality risk, aerobic activity conferred greater benefit.”

Aerobic exercise in the study was measured in MET-hours, or metabolic equivalent of task hours, which compare energy use during activity with energy use at rest. The study says 150 minutes of moderate aerobic exercise is equal to 7.5 MET-hours.

The study does not show that exercise directly caused people to live longer. It was observational and relied on participants self-reporting their exercise habits.

The researchers said they now want to examine how exercise types might affect quality of life and healthspan, defined as how long people remain healthy, mobile and independent.

“Our findings on different dose-response relationships between long-term resistance training with all-cause and cause-specific mortality suggest that different amounts of resistance training may be needed to optimize benefits across outcomes,” write the researchers.

The research has been published in The British Journal of Sports Medicine.

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