There’s a reason why you’re not seeing progress as quickly as the guy on the treadmill next to you. According to recent research published in The Journal of the American College of Cardiology, people respond differently to exercise based on genetics, the New York Times reports.
In the study, researchers from the University of Michigan in Ann Arbor and the Norwegian University of Science and Technology bred two groups of rats—one group that would respond well to working out and another that wouldn’t. For several weeks, they had both groups of rats run, noting how far they ran before tiring, and how their endurance increased over time. The males and females that added the most mileage were bred together, while the ones that added the fewest miles were mated.
Once the scientists had rats that represented a high and low response rate to exercise (seven generations of rats), they began the second portion of the experiment. For two months, the rats exercised on tiny treadmills, completing workouts identical in speed and intensity.
In the end, the super rats (bred to respond well to running) increased the distance they could run before tiring by about 40 percent. The rats bred to respond poorly to running were far more resistant to training; they lost about two percent of their endurance during training.
Here’s why: The high-responding rats were developing athletic hearts; cells from their left ventricles showed evidence of structural changes associated with growth and strength. The low-responding rats experienced the opposite; their left ventricles exhibited nearly no physiological adaptation.
All in all, the rats with a particular set of genes responded favorably to running, becoming more fit after a few weeks of running, while rats born with other genes gained little cardiovascular benefits. This “cellular intractability” makes animals weaker; their hearts don’t adapt, so that particular type of exercise (that's the good news: it may be that you'll respond better to a different type of workout) exhausts the body, rather than strengthening it.
So, if you’re logging mileage and it’s not getting any easier or you just can’t seem to run any farther, change things up. Try weight training, or different cardiovascular exercises that can challenge your body in a different way.