[riggins]

Bodies are like machines, if you use them a lot they will wear out because of over-use.

This is a poor analogy. Bodies can heal (i.e. regenerate tissue). Machines cannot. If this analogy was true, ultra-marathoners would be wearing out their joints rapidly. That's not what is being observed though.

What is true is that you can injure joints by increasing the workload too fast. Joints can strengthen and adapt to increased loads but it happens slowly. So don't go from running 5 miles per outing to running 10 miles. Limit yourself to a gradual increase.

http://journals.plos.org/plosone/article?id=10.1371/journal....

http://content.time.com/time/health/article/0,8599,1948208,0...

[Udo]

There is no question that bodies are in fact machines, there are parts that regenerate and parts that don't. Many extreme sport enthusiasts carry life-long consequences of parts they broke during their activities.

The issue here is more if the oxidative stress put on your components is offset or even completely overshadowed by the biochemical benefits of exercise, and there is indeed reason to believe that a moderate exercise regime - not unsurprisingly - may yield optimal results.

Exercise too little and your body's machinery will start to suffer from use-it-or-lose-it symptoms. Exercise too much and you will cause an inordinate amount of mechanical wear that cannot be regenerated, plus there may be some biochemical stresses as well.

I know "do it in moderation" makes for a shitty headline that interests nobody, but all things being equal that should probably be the take-away here. I agree with you about gradual increase being advisable, but perpetually aiming for your physical limits may not be the most appropriate course of action for individuals who are already fit.

[bootload]

"What is true is that you can injure joints by increasing the workload too fast. Joints can strengthen and adapt to increased loads but it happens slowly. So don't go from running 5 miles per outing to running 10 miles."

Underrated point. This is especially noticeable carrying packs. If you don't train to improve your joint strength (tendons and surrounding tissue, not muscle) you will injure yourself. Slowly you can build up to heavier weights. I now regularly train with 10kg, occasionally 15kg and alternate with 1kg. 2000km this year.