A company by the name of Valencell plans to redo the heart rate monitor and put it in something you're almost certainly already using while your work out: earbuds.

A sensor called PerformTek Precision Biometrics will be licensed to some manufacturers this year to make fitness headphones. The sensor itself will track heart rate, distance, calories, speed, cadence, and VO2 max.  VO2 max is supposed to be the most accurate way to measure the efficiency of your work out (usually it's done through breathing in a tube while exercising, which in turns measures your oxygen intake). The earbuds are supposed to be accurate within a 7% margin. You'll be able to see all of the numbers it measures on your smartphone.