I'm sure it's already mentioned somewhere that Barber had a very aggressive running style, took a lot of hits and he dished out a lot.
That's what I liked about Bill Parcells, he had a knack for knowing what he could get from a particular player based on the style they played. He always saved Barber for some goal line situations and in the 4th quarter. That's where Barber thrived. His style could get him in the endzone goal to goal and he wore out already fatigued defenses in the 4th quarter. However, Parcells also knew that the kind of energy generated by Barber's running style could only last so long.
The other back, Julius Jones, got about 60% of the carries. He was their starter since halfway through the 2004 season until the end of 2007.
After Parcells left, Jerry Jones resigned Barber in 2008 and gave him starter money. In other words, Jerry made Barber the starter. Barber began to diminish instantly, going from 4.8 yards a carry in the previous two seasons to 3.7. Only once in the next four seasons did he have a better average.
Considering that running style, I wouldn't be surprised at all if it was football related.....but I'm no doctor.