Two years ago I decided it was time for a new challenge, so I picked up a running habit. This was something I had been meaning to do for some time, but which I had always put off out of fear for losing my hard-earned strength. Because everyone knows cardio and strength don't go together, right? Long story short: after running bi-weekly for almost a year, I could still deadlift my old PR. Hm. Interesting. Like many strength and muscle building enthusiasts, I had always treated endurance training like the devil. But how justified is this reputation?

"We're talking slower growth here, not a complete stop."

Let’s first have a look at the bad. I remember asking a physiologist about muscle size and endurance some years ago, and he gave me a rather simple answer: in a large muscle oxygen needs to travel farther, so it will naturally be poor at enduring. That made sense. He further backed his claim by explaining that, on a biological level, the processes which develop either endurance or strength tend to compete for the podium - with endurance adaptations generally winning the contest. Hence the 'running makes you skinny' dogma.

If we look at more practical research of later years, these ideas are confirmed: adding cardio to a strength training routine has consistently been shown to have a negative effect on strength and muscle gain. And this negative effect gets worse as cardio frequency and intensity go up, or when the cardio activity is of higher impact (think: running in comparison to cycling). Important to note, however: we're talking slower growth here, not a complete stop.

On to the good. There are quite a few benefits to having a solid aerobic capacity ('to be fit'). Not only is it good for your heart and nervous system - it's nice to be alive - but it also enhances your recovery both within and in between strength sessions. It's this aerobic capacity that helps supply energy for your next set in the gym, while the increased blood flow aids in general recovery. So, if you're fit, you'll feel better, sleep better, relax easier, and be able to train harder and longer. Please note that I speak of 'being fit' though, which is not to be confused with posting impressive long distance running times.

"Concurrent training goes a long way"

As stated earlier, there definitely is an interference effect between strength and cardio training, but it barely shows its head as long as you're not taking either end of the spectrum to an extreme. Yes, in order to grow your muscles to their maximal size, you need to squeeze every last drop out of your growth potential. And to run that super fast marathon you need to keep your body weight down. But are you bordering at this level of expertise? Considering the amount of crossfit athletes that I see squatting twice their body weight while having traps up to their ears, I'd say that concurrent training goes a long way. A good question still is, where is the safe zone?

As a general rule of thumb: if endurance training is not really perceived as a serious training stimulus, it is unlikely to hurt muscle size and strength. To give you something a little more quantifiable: adding a couple of 20-30 minute cardio sessions to your weekly routine, with your heart rate in the low zones (120-130bpm) should not be a problem. And the added health and recovery benefits might actually benefit your strength training.

If you’re still worried about losing gains: there’s shown to be less interference between strength and cardio work when you separate your sessions, but we're already majoring in the minors here. The take-home message I'm trying to give you is: if you're not crazily big, and not on a hardcore conditioning routine, it's probably not worth it to lose sleep over lost gains.