This routine lasted for six months, by which time the people lifting weights had almost all gained strength and improved various markers of their health, even if they had lifted only once a week.

But then, after the months of supervised lifting, the exercisers abruptly were on their own. The researchers explained that they could no longer have access to the university facilities and provided them with information about low-cost, suitable gyms in the area. But any subsequent training would be at their own volition.

The researchers waited six months and then contacted the volunteers to see who was still lifting and how often. They repeated those interviews after an additional six months.

They found, to their surprise, that a year after the formal study had ended, almost half of the volunteers still were lifting weights at least once a week.

“We had estimated a rate of 30 percent,” says Tiia Kekalainen, a project researcher at the University of Jyvaskyla who led the psychological study with the senior author, Simon Walker, and others.

Also surprising, the researchers discovered little direct correlation between muscle and motivation. The people who had gained the most strength or muscle mass during the study were not necessarily those most likely to stick to the training.

Instead, it was those who had come to feel most competent in the gym. If someone’s self-efficacy, which is a measure of confidence, had risen substantially during the study, he or she usually kept lifting.