Your heart rate is your personal workout metric
Heart rate is a useful tool if you want to learn about your sports performance. When you train with heart rate, you'll be able to monitor and control the intensity of your workouts.
While your heart rate gives you an accurate indication of your effort, it's good to remember that there are many factors that influence it. These factors include for example:
Athletes with an extensive background in aerobic training have more efficient heart muscles. The capacity of their left ventricles has increased and the ventricular muscles have become stronger, leading to an increased stroke volume. This increased stroke volume can be observed as a lower resting heart rate as well as a lower training heart rate.
As temperature increases, so does the need to cool down the body, and blood flow is directed closer to the surface of the skin. The accelerated circulation requires the heart to beat faster, which means that your heart rate goes up. When the surrounding air cools down, circulation in peripheral parts of the body decreases so the heart has less work to do concerning circulation. This causes your heart rate to decrease.
When dehydrated, the amount of plasma in the blood decreases and the heart is forced to pump faster than normally to provide enough oxygen and nutrients to muscles in peripheral parts of the body and to maintain an adequate body temperature. This is why your heart rate tends to go up when you're dehydrated.
Measuring your heart rate with Polar
Polar offers two ways to measure your heart rate. One is to use a heart rate monitor with a chest strap that measures the heart's ECG. In this case, measuring heart rate means measuring the electrical activity of the heart.
The other way to measure heart rate is to optically measure the pulse on the arteries with e.g. a wrist-based heart rate monitor. In this case we're actually measuring pulse, which is caused by heart beats, i.e. heart rate.