Just as your muscles become stronger with exercise, so does your heart, making it more efficient. This allows you to exercise with less effort, and for longer periods of time. As you exercise more, you will notice that your resting heart rate has decreased, which is an indication of a stronger heart.
For those of you who do not have a heart rate monitor, and are serious about working out and getting results, I would highly recommend that you invest in one. Not only will it constantly monitor your heart rate, but it will also tell you how many calories you have burned during your workout.
There is a misconception that the harder we work out, and the higher we get our heart rate, the faster we will lose weight. In most cases this is not true. If you are exercising to0 hard and your heart rate is too high, all you are burning is sugar, rather than fat. All of our bodies are different and it is best if you can be tested at your local gym to find out what heart rate workout zones are best for your body. These zones are usually based off of percentages of your maximum heart rate. Your maximum heart rate is generally the fastest your heart can beat for one minute. There are a couple mathematical formulas out there for determining your maximum heart rate. One such example is 220 minus your age. Well I am 31, so my maximum heart rate would be 189. I know that not all people who are 31 will have the same maximum heart rate, but it is at least a generic guide.