Brie and Nikki Bella are identical twins who entertain folks as a professional wrestling tag team for the WWE. Nikki says lifting weights works to trim her down and stay strong. Brie says staying fit takes a mix — such as a 14-minute barre routine on busy days and two hours at the gym when there's time.

The Bella twins are an example of what researchers from Australia found when they looked at how 30 sets of identical twins responded to endurance (aerobics) and resistance (strength) training.

Their study, published in the Journal of Physiology, reveals that the response to exercise is highly individual, even for identical twins.

That helps explain why some people say "Sweating to the oldies" builds endurance and muscle, while others find it leaves them as untrained as when they started their ongoing routine — and why others find that strength training builds muscle and endurance, but aerobics don't. 

If you're frustrated by how slowly you're getting into shape, the study also found that almost everyone can improve fitness with the right exercise program.

"Low-responders to one mode may be 'rescued' by switching to an alternate mode of exercise," say the researchers.

So, if after three months of aerobic or strength training you aren't much more fit than when you started, try switching.

But we suggest you don't abandon either workout style completely. Choose your core workout (three-plus days a week) and add in one or two days of the complementary style to avoid boredom and achieve maximum agility, strength, and cardiovascular fitness.