The NY Times blog, Fifth Down cited a study by the American Journal of Sports Medicine suggesting certain prior injuries are especially tough on players at particular positions. For example, old shoulder damage, even after it's healed, often limits the effectiveness of offensive and defensive linemen. Meniscus tears in the knee are particularly hard on defensive backs, according to the study.

We've known some of this already - that a running back that relies on quick cuts might be hurt worse by a 12-month old ACL repair than a power back who runs strictly north-south. But the study looked at the orthopedic histories of 5,000 college players, so it's a decent sample.