If you simply look at the most common injury among athletes w/ high mileages in a non-contact sport such as Track & Field.....99% of the time it's some type of tendon related injury (i.e. hamstrings). It's as if when they hit 30, they all of a sudden start pulling out of races w/ a strained hamstring. The tendons just basically wear out like a break pad but our body doesn't repair it as efficiently as we age.