You're bundling up for a chilly morning run. Or about to climb on the elliptical for a high-energy workout. Or warming up before a weightlifting session.

What's the first thing you reach for?

Your earbuds, naturally.

Research shows that listening to music that fits the cadence of what you're doing — running, cycling, aerobics — makes you work harder.

"The metronome aspect, the synchronization of movement to music, is the most important," said Carl Foster, director of the Human Performance Laboratory at the University of Wisconsin-La Crosse.

The idea of synchronizing movement to a beat is nothing new, he pointed out: In Roman galleys, the drumbeat drove the pace of the rowers. "But there is also the distraction and arousal that music brings," Foster said. They both matter, but it's unclear how much. "There's definitely more buried in music that affects us. But we don't know exactly how to tease it out."

Music is so important to Katie Haggerty, group fitness brand manager and instructor at Twin Cities-based Life Time Fitness, that she spends "a lot more time than I probably should" picking it out. "I lose a lot of sleep at night worrying about what music to use."

There's a very thin line between good and bad exercise music, she said. Tunes that are too slow drag down the class, but choose ones that are too fast "and the people end up running around like chipmunks."

So, how to pick the "right" music?

To make a workout mix based on tempo — or BPM, for beats per minute — various websites, including www.songbpm.com, can help you determine the tempo of your favorite music to see whether it fits your intended activity. Or you can go to sites such as www.motiontraxx.com that offer playlists at a certain BPM for running and cycling as well as other activities. Other sites include www.workoutmusic.com and www.powermusic.com.

"Music is positive energy," said Deekron "The Fitness DJ" Krikorian, who produces fitness playlists for Motion Traxx. "So when I put together playlists, I look for intensity, positive feeling and cohesiveness."

If he finds a song that feels right in terms of mood and intensity but has the wrong tempo, he might manipulate the BPM to fit the type of exercise intended.

"The beat becomes very important any time there is repetitive movement," Krikorian said. "Our instincts tell us to move to the beat. Our feet tell us to move to the beat."

Running on beat

The ideal cadence for running is a hotly debated topic in the running world, and variations in stride length mean that finding your ideal tempo could take a bit of experimentation. Some sources say an eight-minute mile corresponds to a BPM of 170; others go up to 200. Some suggest that the ideal running cadence is in the 170s to 180s. And some studies show that faster may be better for injury prevention.

If that sounds like too much work, try a group fitness class; cycling, step and aerobics instructors have been leveraging the power of the beat for years.

Ingrid Nelson, a cycling instructor who packs her tempo-driven classes, said intensity, style and cadence are all important when putting together her playlists.

"I like a lot of '90s hip-hop and usually stay in the range of 95 to 105 BPM," Nelson said, aligning the beat to the cyclers' revolutions per minute. But she might go as low as 80 or as high as 120 BPM for hills and sprints, respectively. When drills are aligned with the beat, she said, participants "connect with music" and "relax into the pulse."

As for other fitness activities such as step aerobics, the tempo hovers around 130 BPM, said Harold Sanco, a group fitness director.

"You have to pick music that is both safe and effective," he said. "If you are going too fast, you risk injury and you're not working out effectively because you are not getting the full range of motion."

But Sanco said music is important beyond tempo and genre; it also helps put participants in a lighter mood.

"Music can make people happy no matter what their day has been like," he said. "It entertains and educates."

Beyond the beat

In her classes for Life Time, Haggerty looks beyond just the tempo of the music.

"I also try to look for a message," she said. "I don't tell people I'm doing that. The message is more subliminal. But I like to find songs that say something [inspiring]. I like to find songs that push people."

The first and last songs of a session are particularly important, she said.

"The first song has to grab them; it has to take command of the room," she said. "The last song is important because people are trying to get out of the room. They don't want to stay for stretching. So you have to hook them into staying. Sometimes I use a singalong that gets people to stretch without even knowing that's what they're doing."

Rachel Goldberg, who co-owns a cycle studio in Washington D.C., uses the phrasing of the music to get the most out of her rides. "When you marry your body's movements with the music, it's a more holistic experience," she said. "You start flowing with the music." If there is a chorus or other recurring crescendos in the music, Goldberg might use those to increase the intensity.

"The music becomes your North Star — it guides you."

It also distracts you — something many of us have relied on during a long treadmill workout. Cedric Bryant, chief science officer for the American Council on Exercise, said this is the aspect of music that resonates the most with him. "I enjoy using music as a distraction," he said, adding that music can keep you going no matter how tired you are.

Distraction, whether it be music or even a comedy show, can be helpful in a workout — at least in the beginning, Foster said. That's where the importance of the beat and arousal come in. "After about 20 minutes or so, 'Larry the Cable Guy' is not enough to keep us going," said Foster, who used comedy in one of his studies. "We need more than a joke to carry us."

Staff writer Jeff Strickler contributed to this report.