Some friends and I were driving home after day-trip recently, and my iPod was plugged into the car stereo. For miles, we listened to the mellow, obscure, alternative-ish music that's popular on college radio stations.

And then ABBA's Dancing Queen came blasting through the speakers. I clicked past it, only to hit Justin Timberlake's Sexy Back, Kelly Clarkson's Since You Been Gone, and Rihanna's SOS.

My iPod had zeroed in on my workout playlist, revealing the music that motivates me through grueling workouts. I gravitate to dance-y top 40 tunes because they keep me going on the elliptical trainer.

My friends had also created workout-specific playlists. A cyclist in the group swears by The Who and Led Zeppelin for long, arduous rides. (He promised that he keeps the volume very, very low, but I still think it's hazardous to ride a bike and wear headphones.) A jogger -- a serious guy who never listens to the radio -- downloaded a bunch of workout playlists from a fitness site and now runs to Beyonce and the Black-Eyed Peas.

Music and fitness websites are excellent places to discover great workout mixes. iTunes offers all kinds of pre-assembled mixes, ranging from "Tracks to Make You Sweat" and "Alt Rock Workout" to Nike mixes created by trainers. Fitness Magazine listed their Top 100 workout songs, and Women's Health has a huge collection of workout playlists. Some time back, I pulled a bunch of fun songs off Lime's Dance off the Duck mix.

It's also possible to fine-tune your playlist to suit a specific workout. According to an article in the Seattle Post-Intelligencer, tempo is an important thing to consider. Music with a tempo between 120 and 160 beats per minute corresponds to a person's heart rate during a workout which can help set or maintain a pace.

For that reason, tunes on the slower end of that spectrum (think Rihanna's Umbrella or Salt -N- Pepa's Push It) might be better-suited to a moderate workout, while songs with a faster beat (like The Killer's Mr. Brightside, Green Day's American Idiot, ABBA's Dancing Queen) can help increase your intensity.

If you want a power-based workout, such as strength-training, consider heavy metal, rap or arena-rock. Experts say that loud, aggressive sounds might help push you through the exhausting repetitions and sets. Just remember to keep the volume low to prevent hearing loss.

Loud, heavy metal may not be your thing. But if it works, your friends might be surprised to discover Metallica on your MP3 player.