I was shown this graph in a college PE class on general fitness. I remember the general shape, and that the bottom metric was in reference to the number of days per week that a person exercised. The general point was that if you worked out 1-2 days per week you saw some but little benefit. If you exercised 3-4 days per week, there were dramatic increases. 5 days still got a measurable increase, but 6-7 days were pretty much a waste of "efficient" time. Can you tell me where this came from, what the metric on the left side is, and if it still holds true with today's knowledge.