I took me a long while to appreciate the importance of recuperation between workouts. Then I got to know a couple of 40ish Pros.
Basically what I learned was to take a day off after working large muscle groups. Back, chest and legs. Arms, shoulders, abs, I do fine working out the next day. I use the off days for cardio.
Younger guys may not benefit as much, they seem to not think so anyway. But I figure if you are doing 16 set workouts at max load, most people will benefit from a day off. This is the part that's hard to measure from person to person. We all seem to think we are killing the weights at the gym
