For any physiological system to improve it must be systematically made to perform more work than it is

accustomed to performing

Explanation:

This is because the principle of overload.

According to the principle of overload, physical exercise only causes beneficial adaptations if it responds to stimuli that exceed the training threshold.

This principle is directly related to the Threshold Law: by increasing our effort tolerance threshold, we must also increase the workload.

This can be achieved by performing the following tasks:

* Increasing training frequency (for example, spending three to four days per week).

* Increasing the volume (spend an hour and a half to two hours each training session).

* Increasing the density, that is, the relationship between the duration of the effort and the duration of the recovery or rest pause (for example, increase the number of speed series to be performed from six to eight and decrease the recovery time between series 5 to 4 minutes).

* Increasing the intensity of the stimulus (for example, reduce the time to perform each series in swimming or increase the weight in a squat exercise).