New York Times reports that researchers found that among three groups of volunteers: one exercised two times a week, one exercised four times a week, and one exercised six times a week, there is no notable difference in fitness gained. However, the group that exercised six times a week shows more sign of stress and expend less energy during inactive time.
I have personally observed similar result among my patients. People who come to see me are usually very active in all aspects of their lives. Many are physically active but experience fatigue which they cannot explain. From my experience, I find they spend very little time in recovery. They usually move on to new activities just when one activity stops.
People need to keep in mind that every activity, especially intense physical activities, is a trauma to the body. It is the recovery, or in medical terms, the compensation, that makes people stronger and fitter. Physical activity is a key component to good health. However, if done without adequate recovery time, the stress can accumulate and result in functional health problems. If the physical stress accumulate over a long period of time, there may even result in physical injuries.
https://well.blogs.nytimes.com/2013/02/13/why-four-workouts-a-week-may-be-better-than-six/