Your heart rate is a function of how often your heart beats in a certain amount of time. Most heart measurements record beats per minute; the more beats there are in a minute, the harder your heart is working to pump blood through your body. Since the cardiovascular system is responsible for transporting oxygen and nutrients throughout the body via the bloodstream, heart rate is an indication of the needs of your body during different types of exercise.
The Cardiovascular System
Muscles use cellular respiration to reset after a contraction. This process converts glucose, glycogen, amino acids and fat into an energy transport molecule called ATP using oxygen. When your blood provides enough oxygen for this conversion, your body is undergoing aerobic respiration. It is possible to go beyond the limits of your cardiovascular system. Anaerobic respiration occurs when there isn't enough oxygen for the reaction, converting glucose directly into ATP and producing lactic acid, which causes fatigue.
When you do an exercise that doesn't leave you feeling winded, it's a good bet that your heart rate is also low. Yoga, walking and jogging, and rock climbing exercises often involve this type of low-intensity cardiovascular response. When your heart rate is low, your muscles are getting all the oxygen they need to keep performing the current task. These exercises will usually elevate the heart rate above the normal resting rate.
High intensity activities like sprinting, swimming and playing football involve several muscle groups working at a high rate, which elevates your heart rate. Your muscles need more oxygen at this level of activity, and your heart compensates by pumping more blood to transport this extra oxygen provided by your lungs. These types of activities fatigue the body much quicker, especially if the activity pushes past your aerobic capacity. Your heart rate during anaerobic exercise is the maximum it can be, since your heart can't transport oxygen-rich blood any faster than it already is.
Choosing an Exercise
Low-intensity and high-intensity workouts are both valuable to your cardiovascular health. Your heart rate is an indication of which exercises make your body work harder, but even aerobic exercise helps build a good base of overall fitness and health while strengthening specific muscles. Determining whether an exercise is high or low intensity can be done using the heart rate; check your pulse during your next workout to see how hard your heart has to work to keep up with your energy demands.
Max Roman Dilthey is a science, health and culture writer currently pursuing a master's of sustainability science. Based in Massachusetts, he blogs about cycling at MaxTheCyclist.com.