Having trouble getting fit? Blame your parents.

Are you one of those people that seems to never see results from exercise — despite how hard or how often you work out? Well, a new study published in The Journal of the American College of Cardiology shows that your stubborn fitness could be due to your genes.

Scientists from the University of Michigan in Ann Arbor and the Norwegian University of Science and Technology in Trondheim studied two groups of carefully bred rats, some who responded markedly to exercise and some who didn't respond much at all. After undergoing two months of identical training programs on tiny treadmills, the rats who had been bred to respond well to exercise had increased the distance they could run by 40 percent, while the other rats actually lost 2 percent of their endurance.

Similarly, the high-responding rats saw physiological changes, like structural adaptations in the heart consistent with growth and strength, while the other rats showed almost no physiological change. If hearts can't adapt, workouts can drain bodies, explaining the loss of endurance.

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While humans possess the same genes that were found to be operating differently in the two groups of rats, it's impossible for scientists to say whether they would react in the same way to exercise. However, if two months of regular cardio workouts aren't producing results, you might want try something else.