If you want to maximize your exercise and fitness efforts, then using your heart rate as a gauge for intensity is a key factor. If you really want to get scientific, then get your VO2 Max tested by a local specialist so you can find out at which heart rates you burn the most fat, carbs, build up lactic acid, etc.
The most commonly used formula for finding your target heart rate is the Karvonen Formula, which is:
1. Find your MAX heart rate, which is: 220-(your age) = MHR
2. Calculate heart rate reserve: MHR-Resting heart rate = HRR
3. Determine TARGET HEART RATE during exercise, let’s say 75%(.75): HRRx.75 +RHR = THR
Your Target Heart Rate (THR) will vary depending on goal, activity and fitness level.