With constant exercise, the body adapts by improving blood delivery (cardiac output) to the various organs while improving energy utilization & efficiency at any given workload.
Exercise fitness testing is a measure of the body’s adaptation to a given or increasing workload. Using the HR change before, during, and after a period of exercise conditioning you are able to assess improving or deteriorating fitness levels.
Fitness monitoring is done with HR measurement before, during, and especially after the exercise testing session (recovery HR).
Fitness assessment can be measured to assess general well-being, improve exercise fitness levels or overtraining fatigue, or assess ongoing fitness levels while recovering from a medical illness or recovery from an injury during the rehabilitation period.