Sometimes, in order to give you a boost to start running or to give you the last ounces of energy for the final leg of your brutal workout, the only thing you need is some music. Dr. Coastas Karageorghis, preeminent expert on the psychophysical and ergogenic effects of music at London's Brunel University, who has created custom-made playlists for many US athletes for the Olympics this year, told Time magazine that music can elevate positive effects of exercise, like vigor and enthusiasm, and alleviate negative ones, like fatigue and tension. Music can also reduce the perception of effort.

Karageorghis gives the following guidelines for a perfect playlist:

Choose songs with high-tempo beats

Upbeat music increases energy in the part of the brain that makes you more excited, the ascending reticular activating system. The optimal range is 120 to 140 beats per minute. You can also find a song that matches the exact heart rhythm that you want to achieve. For example, if you want your heart to beat 130 beats per minute, you should choose a song that progressively matches that.  For ways on how to determine a song's beat per minute, try looking here.

Stick with songs you know

A song's cultural significance is part of what makes them motivational. If you associate a song with a motivational moment in a movie or in your life, it will in turn motivate you more. We tend to prefer songs that we have heard before, so you should stick with songs that are in your library.

Don't play the same music over and over again

The motivational effect of a song will decrease the more you hear it, so don't be afraid to revamp the playlist every few weeks. For people who are less excited about music selection for working out, it might be a good idea to have a long playlist of uptempo songs and then to hit shuffle on the iPod before starting. You may run the risk of hearing a few songs over and over again, but hopefully the other songs that you have not heard in a while will balance that out.

Digitally alter songs

When making playlists for other Olympic athletes, Karageorghis sometimes will digitally alter songs to make athletes work even harder. Increasing a temp up to four beats per minute is indistinguishable to the human ear, so the technique is great for a short burst of energy.

Don't forget the lyrics

Lyrics can hold extreme weight for an athlete. It is important to choose songs with motivational words as well. For example, Michael Phelps likes to listen to Lil Wayne's "I'm Me" before a race, which is well-known for its affirmative words.

Starter Playlist:

Karageorghis and some Time magazine staff give their own preferred songs for exercise playlists. We at Medical Daily have added some of our favorites as well, and maybe you already have some on your workout playlist or your music library. In no particular order:

Feel free to add other songs that energize your workouts in the comments.