Re: Arthritis from intense workouts?
I would pose a related but different question:
Are people who do not learn proper movement techniques and who do not keep their joints in shape through functional exercise more likely to develop arthritis as they age?
As Brian points out, nutrition and sub-optimal joint movement are important in the development of arthritis. And as Robin points out, the health risks of being a couch potato are worse than the increased risk of arthritis (if any) anyway--I would trade achy joints in place of metabolic syndrome any day. But there is also an argument that proper exercise and movement will in fact delay or prevent arthritis.