Each of the sets of twins worked out together for two three-month periods. During one, they ran or cycled for an hour three times a week. During the other, they lifted weights three times a week, also for about an hour. At the end of each three-month session, the twins returned to the lab and the researchers rechecked their aerobic capacities and muscle power. Then they compared how various twins’ legs and lungs had changed during these exertions.
The results were telling. Most of the 84 participants upped their endurance during their three months of jogging and biking, but not all. So, too, leg strength rose in most of the twins after three months at the gym, but stayed relatively puny in a few.
Almost no one, however, responded poorly to both the endurance and strength training. In other words, those volunteers who gained little fitness bang from running substantially pumped up their strength after lifting, and vice versa.
“There were very few recalcitrant non-responders,” says Daniel Green, a professor of exercise science at the University of Western Australia, who conducted the new study with his doctoral students Channa Marsh and Hannah Thomas and others. “Almost all of those who failed to respond to one form of training were capable of gaining benefit by switching to the other.”
Interestingly, the benefits gained could be broad. In some cases, non-responders to aerobic exercise added endurance when they turned to weight training, even though lifting mostly targets muscles.
At the same time, there was little evidence that genes shaped people’s outcomes, since the twins’ responses varied wildly, even among identical pairs.
The upshot of this data is that we should not be discouraged if prolonged running or lifting does not produce the results we hope for. We might, in that case, want to noodle about with different approaches. “There is an optimal exercise strategy for everyone,” Dr. Green says, “but it differs between people,” and will not be determined or limited only by your DNA.