I am a huge fan of heart rate monitors. As I explain to patients, these devices keep you honest when you exercise. You get immediate feedback regarding your heart rate, which is a reflection of how hard you are working. Exercising at the proper rate leads to more efficient exercise and metabolic benefit.
 
Aerobic exercise, also known as cardio or endurance activity, is the cornerstone of most fitness training programs. Aerobic exercise causes you to breathe faster and more deeply, which maximizes the amount of oxygen in your blood. The better your aerobic fitness, the more efficiently your heart, lungs and blood vessels transport oxygen throughout your body — and the easier it is to complete routine physical tasks and rise to unexpected challenges, such as running to your car in the pouring rain. Aerobic exercise includes any physical activity that uses large muscle groups and increases your heart rate. Try walking, jogging, biking, swimming, dancing, water aerobics — even leaf raking, snow shoveling and vacuuming. Aim for at least two hours and 30 minutes a week of moderate aerobic activity or one hour and 15 minutes a week of vigorous aerobic activity — preferably spread throughout the week.

Training in this zone will develop your lactic acid system. In this zone, your individual anaerobic threshold (AT) is found - sometimes referred to the point of deflection (POD). During these heart rates, the amount of fat being utilised as the main source of energy is greatly reduced and glycogen stored in the muscle is predominantly used. One of the by-products of burning this glycogen is lactic acid. There is a point at which the body can no longer remove the lactic acid from the working muscles quickly enough. This is your anaerobic threshold (AT). Through the correct training, it is possible to delay the AT by being able to increase your ability to deal with the lactic acid for a longer period of time or by pushing the AT higher.

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