In the late Roman Empire there was constant difficulty in raising sufficient troops. Like the high standards of the AIF in WWI, the Roman limits to military service changed with need. Emperor Valens passed laws stopping land owners substituting homeless men for able bodied men. But there were also laws passed to stop draft dodging. Pat Southern writes:

First of all, it was declared illegal for recruits to cut off their fingers or thumbs. The repitition of such laws show they [emperors] were unable to stop the practice.

Valentinian passed laws apparently that would burn at the stake draft dodgers who mutilated themselves. Theodosius dealt with it by allowing thumbless soldiers to still serve.