Picking the perfect playlist makes you feel really alive, doesn't it? The beat pumps you up. Listening to a rock star makes you feel like a rock star. Hit the roads with great tunes and the miles will fly by. But what if you put on that same playlist at the gym? New research shows it may have the opposite effect, deadening your performance.

That's the news from a study of CrossFit athletes out of a gym in Basel, Switzerland. It doesn't contradict the huge body of research about music being good for performance; rather, it shines a light on the fact that those studies rarely looked at anything like CrossFit. And while we still don't have good data on Olympic lifting, knowing the pros and cons of music can help guide your own experimentation.

As anthropologists tell it, people have been working to music for as long as people have been doing work: think of Romans or Vikings rowing in ships, military cadences, slaves' work songs. Scientists see several ways music helps: it distracts you from the pain or boredom of your task, it gives you a rhythm to follow, and it just plain makes you happy.

Research bears this out. One study found that sprinters did better at the 60-meter dash after listening to the Rocky theme than after standing around silently for an awkward minute. In another, participants performed better on a rowing-machine test when they listened to Beethoven's Seventh rather than silence—and they were fastest during the faster-tempo parts of the symphony. Meanwhile, USA Track & Field classifies devices that can play music as illegal "assistance" that may not be used by anyone competing for prizes. Renowned sports psychologist Costas Karagheorgis has likened music to a legal performance-enhancing drug.

Karagheorgis penned a massive review of the evidence surrounding music and athletics in 2012; both part 1 and part 2 are freely available. Some studies were inconclusive, but many showed that performance improves with music (24 out of 32, if you're looking at music played during the task. He also considered pre- and post-workout music.) I should mention here that Karagheorgis has told the world what his favorite workout playlist is, and it includes Flo Rida, Ke$ha, and a lot of Michael Jackson. His tips on creating your own optimal playlist can be found here.

So why did CrossFitters' performance plummet with tunes? Unlike some studies, we can't blame artificial conditions or weird synthesized music; the researchers set up in the athletes' usual box and had the thirteen subjects exercise either to silence or to one of the gym's most popular playlists: a collection of five AC/DC tunes. (If you must know, they were Shoot to Thrill, Rock N Roll Damnation, Guns for Hire, Cold Hearted Man, and Back in Black). The test was the Cindy workout: in 20 minutes, they were to perform as many rounds as possible of 5 pull-ups, 10 push-ups, and 15 air squats.

The researchers expected to see participants' scores go up with music, but instead they dropped: the average score was 460 total reps when AC/DC was cheering them on, down from 498 in silence.

Why? It may be that music's strengths for endurance training are weaknesses when it comes to certain self-paced workouts: music distracts you from what you're doing, taking up attention and brain power that you need to focus. Pacing yourself is tough, and you do require some brainpower for the complex movements involved in “Cindy”—complex, that is, compared to running or cycling, where you're just putting one foot in front of the other. Attention is a two-edged sword. It's thought to benefit elite athletes, who need to monitor exactly how their body feels during a race or competition—the better to squeeze out every last drop of performance. Beginning or casual exercisers, on the other hand, benefit from dissociative techniques that take their mind off any unpleasant feelings.

The bottom line: music has its ups and its downs. Here's a handy chart:

When to use music:

• When you want to keep your mind off what you are doing.
• For aerobic, endurance exercise, like running or cycling.
• When you're trying to keep a rhythm. Choose music that matches the rhythm you want, and do your reps or footsteps in time with the music.
• Before a competition, to improve your energy level.

When to keep quiet:

• When you need to focus on what you are doing.
• When you need to pace yourself by effort.
• When you're doing anaerobic work and want to achieve maximal effort (though you can still sneak in some music during your warm-up for the mood boost).