The simplest way to judge the intensity of an aerobic exercise like walking or running by is one's heart rate during the exercise. Heart rate is nothing more than the number of times the heart beats in one minute. Aerobic exercise increases heart rate (and breathing rate) because aerobic exercise uses oxygen which comes from the air. In order to satisfy the increased oxygen demands of aerobic exercise, we have to breathe in more air and the heart has to pump faster to carry oxygen from the lungs to the muscles where it is needed.
Aerobic Exercise: Physical Condition:
Heart rate during exercise is dramatically affected by one's physical condition. Athletes like marathon runners can sustain a tremendously fast run at a fairly low heart rate while a sedentary middle aged man might achieve the same rate during a very slow walk.
Physical Condition Improves With Training
Physical conditioning through regular aerobic exercise whose intensity (speed) is gradually increased over many months leads to improved aerobic capacity. In other words, the marathoner will burn more calories in ten minutes than the middle aged man will even though they both have the same heart rate during that time. So, regular aerobic exercise causes our exercise to become more efficient so that we can burn more calories during the time that we are exercising.
Heart Rate and "Easiness"
Generally, the lower our heart rate is during an exercise, the easier it is to do. The less "miserable" we feel. Obviously a slow walk is easier than a fast run. This is why exercise feels so painful to people who are not physically fit.
Fitness and Easiness:
The fitter we are, the easier it is to burn a lot of calories through exercise. What used to feel like torture becomes enjoyable. This is why fitness and not just calorie burning is a major goal and benefit of regular exercise.
What IS Physical Fitness?
Regular exercise induces fitness in several ways.
1. Stronger heart muscle
2. Stronger breathing muscles
3. Larger muscles
4. More "slow-twitch" aerobic muscle
5. Lower body fat
All these changes make aerobic exercise easier and more efficient (more gain for less pain).
As noted earlier, heart rate is simply the number of times the heart beats in one minute. But how can you tell when your heart beats? Easy…. You feel for an arterial "pulse". Each time the heart beats, it squeezes blood out of the heart and into the circulation. This pushes the blood along and also creates a pressure wave or pulse that we you can feel with your fingertips. And you can feel it anywhere there is a large or medium-sized artery: the neck, the wrist, the ankle, etc. Generally the two best places to feel your pulse are the neck or the wrist. The trick is feeling in exactly the right place. Fortunately, the right place is easy to find as shown in the pictures below.
Once you have found the pulse you can count it. The time-consuming way to measure your heart rate is to count the number of pulses in a whole minute by looking at your watch while you count each pulse. When a minute has passed, voila!, the number of pulses you just counted is your heart rate. But, as I said, that takes a whole minute. A quicker way to check your heart rate is to count the pulses in fifteen seconds or a quarter of a minute. All you need to do then is multiply the number of pulses you counted over that fifteen seconds by four (fifteen seconds is a fourth of a minute) to get you heart rate.
There IS an even easier way to check heart rate: buy a heart rate monitor. Such devices are now sold in most sporting goods stores and they work by essentially "feeling" your pulse for you. The only downside to a heart rate monitor is that they cost money, but most sell for less than $50, some less than $20.