Your heart rate is the best measure of how hard you are exercising (in terms of cardio fitness). Muscles need oxygen to produce the energy required to contract. As exercise intensity increases, muscles contract more frequently and/or quicker and the oxygen requirements of muscles increase. This results in a higher breathing rate to draw oxygen into the lungs, where it passes into the blood stream, and a higher heart rate to pump the oxygenated blood to your muscles.
The American College of Sports Medicine recommends that you accumulate 30 minutes of ‘light’ to ‘easy’ intensity exercise most days to maintain cardiovascular health (i.e. healthy heart, lungs and circulation). ‘Light’ to ‘easy’ intensity exercise means that you are working at between 50% and 70% of your maximum heart rate. Activities that achieve this level of intensity can be as simple as walking to work, gardening, walking your dog or vacuum cleaning your house.
To improve cardiovascular fitness, it is best to train at an intensity of 65% to 85% of your maximum heart rate. At this level of intensity your heart and lungs are working hard but not to the point of exhaustion. At around 80% of maximum heart rate, individuals reach their ‘anaerobic threshold’. This means that the body is unable to deliver oxygen to, and remove waste products from, the muscles as quickly as is required. The muscles then start to fill up with a waste product called lactic acid and a light ‘burning’ sensation is felt in the muscles. Circuit training is a great example of exercise where particular muscle groups are pushed beyond the anaerobic threshold and then given time to recover.
Calculations that estimate maximum heart rate (and subsequently training zones) are an excellent guide to training intensity. However, it must be remembered that everyone is unique and no one has yet managed to create a formula that accurately predicts maximum heart rate for 100% of the population.
If you are interested in monitoring your heart rate during exercise (it is a great way to check that you’re not slacking off!) I would suggest buying a heart rate monitor and check out this webpage for a free heart rate zone calculator: http://www.sarkproducts.com/targetzonecalculator.htm