Over at Scientific American, E. Paul Zehr, author of Becoming Batman, explains why nobody could be Batman for very long. Even if you didn't get your back broken by Bane, just the wear and tear of jumping across rooftops and punching criminals would wreak havoc with your body after just a few years. The most Bruce Wayne could hope for is a career of similar length to that of an NFL player.
And here's a simple chart to show just how much Batman would be screwed after a few years:
The basic point of this figure is that very large loading events-getting crushed in Bane's powerful arms-need only occur once to cause significant injury. Other events-repeated impacts during fighting or even the strain of training-accumulate over time so that after many repetitions an injury occurs.
Tissues that are being strained-particularly those with poor blood supplies like tendon, ligament, and cartilage-are not able to fully repair themselves before another big event. Like getting bashed around. Or even just training over and over can create repetitive strain injuries. Injury can happen anywhere on that continuum.
Muscle and bone are biological tissues that have mechanical properties that help explain some of the weird things that seem to happen to us when we are injured. In mechanical terms "stress" means forces applied to a material while "strain" means the change in length or size of the tissue to which the force is applied. Basically repeated stresses lead to strains.