You know what makes me feel like a total badass when I'm sitting around with a hot cocoa on a snowy winter afternoon? Being able to say, "Oh yeah, that weather's terrible. I ran five miles in it this morning."

Spend enough time on the roads, though (or the bike, or the cardio machines, or a few too many, such as perhaps more than zero, Zumba classes) and those in the know might tell you there's a sinister downside to your expanding lung power: cardio sabotages your strength training.

Could it be true? Do you really have to choose between strength and cardio? Can't you have the best of both worlds—a lifter's strength and a marathoner's body fat percentage (or at least her bad weather bragging rights)? Is it time to give up cardio for good?

That may be a good idea. Let me explain why.

Endurance athletes, for example marathoners, need lots of mitochondria in their muscles. These are the parts of muscle cells that burn calories, literally giving you energy to work. They'll also have a little bit of hypertrophy (gains in size) of slower Type 1 muscle fibers, but their Type 2 fibers either remain the same, or sometimes even shrink during training.

Strength athletes' adaptations are the opposite: a huge increase in Type 2 fibers, with a decrease in mitochondrial density. Their capacity to turn calories and oxygen into energy may actually be lower than your average couch potato. So if you plan to only ever lift weights, you may want to skip the cardio to max out those very specific gains.

But not all athletes are so specialized. If you play a sport (or multiple sports) where endurance is important, you may not want to sacrifice your mitochondria just to gain a little more muscle. Plenty of exercise protocols now include a combination of strength and endurance. How does that fare?

There have been dozens of studies that compared strength and endurance training. In 2012, a group of scientists reviewed the evidence from 21 of them. They found that on average, adding cardio to your strength training does decrease some of the benefit you get out of strength training. It's important to note that it doesn't take away gains you've made; it just means your gains will come a little slower than if you skipped the cardio entirely. The effect is also specific to the muscles involved: if you run or cycle, only your lower body will experience the trade-offs. Your upper body's strength or power won't be affected.

While adding cardio slows gains in explosive power, it doesn't have a huge effect on strength or on gains in muscle size. Athletes who did a combination of strength and endurance were roughly as good as the athletes who only did strength training. Both groups made far better gains than the endurance-only athletes.

But the amount of endurance exercise matters, too. The more minutes per day, and days per week of endurance training, the more your power, muscle size, and strength gains suffer.

On the flip side, endurance athletes who added strength training were actually able to increase their endurance a little bit. So the take-home here is that a strength plus cardio combo is actually a good bet for many athletes.

The same analysis dipped into a very interesting related question: does it matter what kind of endurance exercise you're doing? The studies compared running with cycling and found that runners lost more body fat than cyclists, but also had smaller gains in the strength-related parameters. It's not clear why that is: maybe it's muscle damage from the eccentric contractions in running, or maybe it's just because the motion of cycling is more similar to the strength tests used to measure the outcome.

Bottom line: skip the cardio if you need the biggest possible gains in explosive power, but don't care at all about endurance or losing fat. Athletes who are interested in strength and muscle size, or who need some amount of endurance, can add endurance exercise without any serious losses in strength. Preliminary results say that cycling is better than running in this case, but stay tuned for more research to figure out whether this effect is real and where other sports fall on the continuum.