Exercise intensity is the key to your success with exercise. Keeping the intensity at just the right level for your fitness level is how you make the program work for you.
Muscle strength and growth is the bodies adaptive response to the stress of exercise. We sometimes forget that the reason for exercise is to elicit an adaptive response -  to get stronger, loss weight and look better. Therefore, we must apply the proper training intensity to get the results needed and then we must progress to the next level of intensity to continue to get results.