A Popular Question asked Pete Egoscue (Courtesy of The Egoscue Method of Health Through Motion by Pete Egoscue)
Is there a point of diminishing returns when it comes to exercise and movement?  I have heard that a workout lasting longer than 30 minutes is a waste of time and energy.

Pete: First of all, just moving --any old movement--isn't enough.  I'm talking about design movement.  Where people go wrong is that they decide "Okay, I've got to get back in shape."  They start running or hitting the basketball court after years of being on the sidelines.  By that point, the necessary functions are on "hold."  To run or play basketball, the body starts compensating, using muscles and joints that weren't intended for the job.  Pretty soon there's pain. 

As for 30 minutes of exercise, it's ridiculous to think that the body has an internal stopwatch ticking away, and precisely at 30 minutes--not 29 or 31 minutes--says "Wait, that's enough."  But the body does, in fact, know when it's had enough.  It will tell you.  More and more effort will be required to run or row or operate the StairMaster until the motion stops being pleasurable.

I'm not saying that it's time to stop when you begin sweating, working at it, and growing short of breath.  There are plenty of pleasurable activities that require work, sweat, and wind--sexual intercourse to name one of them.  My contention is that by deliberately pushing an exercise past the pain threshold, we are simply engaging in a form of masochism which does not yield any real fitness benefits, and may in fact be injurious.