Resting heart rate
Your resting heart rate is a simple way to assess the cardiac capacity to be an indicator that changes as it progresses to the level of training and demonstrate objectively that walks into your best shape.
A trained heart will have one beat per minute less than another trained all, this represents an efficiency level of their work and a training adapted to the signal that has been performing.
This is a result of larger heart chambers to achieve a greater stroke volume with each beat. Analogously may think of a car with lower power to achieve the same speed for another most powerful car has to be at the expense of an increased number of rotations, instead, the one with fewer rotations can be the same as or more potent!
Now up to you to "car wants to be."