Most avid runners have their 'song': "Chariots of Fire", "Don't Stop Believing", "Freefalling". But is it the terribly clichéd lyrics or the beat itself that provides inspiration? The Yamaha BodiBeat is part MP3 player, part heart-rate monitor, and part metronome for your workout. The gadget serves as a personal trainer and DJ by selecting songs with the beats per minute necessary to keep you going. Armed with a setlist of Journey and Sublime, and in search of a nine-minute mile, we tested the device on the hills of San Francisco.