Your maximum heart rate depends on your age and is the fastest speed your heart can beat when you're exercising. As with your target heart rate, it’s an estimate. Your maximum can be calculated by using the number 220 and deducting your age. So, if you’re 42, you take away 42 from 220 to arrive at 178bpm (beats per minute).
Check your pulse regularly when you’re working out. If you feel dizzy or you are struggling to breathe or talk, that's a sign you are probably doing too much. Slow down if you experience these symptoms, as you may be doing more harm than good to your heart.