What is the relationship between heart rate and fitness level?

The relationship between heart rate and fitness level is variable for individuals. The ability to push more power at a lower heart rate indicates that you are more fit. Your heart is a muscle and when it grows stronger through endurance training, it is able to pump more forcefully, pushing more volume of blood through the muscle per beat. This is a positive training effect, as the body improves efficiency, by being able to deliver the same amount of oxygen, transported by red blood cells, to working muscles with fewer heart beats. Another positive indicator of fitness is being able to work aerobically for longer periods of time at higher intensities. This is referred to as increasing lactate threshold. Keep in mind that hydration, nutrition and rest play a major role in rate of perceived exertion and heart rate. So you will not necessarily see lower heart rates on subsequent field tests, in fact you might see higher heart rates as reflective of your increased lactate threshold.
Cardiovascular training and an overall endurance exercise regimen will decrease resting heart rate and heart rate recovery time will be quicker which implies a strong heart. The heart is a muscle the more you strengthen your heart it will become more efficient by increasing the stroke volume(The amount of blood that is pumped out with each contraction of a ventricle). Remember that proper nutrition and hydration play a key factor in keeping heart healthy.Stay consistent in your  training you only have one heart so handle with care.
A normal resting HR is about 70-80 bpm for the average person. When your resting HR is high, your heart is working harder to pump oxygenated blood throughout your body. When you become more physically fit, your heart is able to work more efficiently at getting oxygen to working muscles and you will recover more quickly even after exertion. Heart Rate zones are used to determine a person's cardiorespiratory level.  You ultimately want to increase your cardiorespiratory endurance, which inturn will help decrease your resting HR and place less stress on your heart.
The normal heart rate range is 60 to 100, with below 60 known as bradycardia and above 100 known as tachycardia. People who are conditioned or athletic are going to have a resting heart rate closer, if not lower, to 60. The heart is a muscle, so the better condition of the heart the lower the heart rate. The lower the heart rate will make it a little harder to achieve a higher heart rate zone which means the level of fitness is greater than someone that achieves their heart rate zone faster. However, some medications will also lower your heart rate. Check with your physician before any exercise programs are started.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.