On the surface, the condition of a mech pilot's body was unrelated to the individual's ability to pilot a mech.
In theory, as long as a mech pilot's head got cut off from his body, he would still be able to interface with a mech!
In the early days of the Age of Mechs, some pioneering mech designers wanted to test if this was actually the case!
The problem was that these research-obsessed scientists and engineers didn't want to wait for a mech pilot to suffer an injury that was so heavy that his or her entire body below the neck would have to be scrapped.
The early-generation mech designers captured mech pilots belonging to an enemy state and cruelly separated their heads and stuck them into containers to keep them alive!
When these body-less heads were mounted into mechs in a twisted attempt to create 'semi-permanently active mechs', the results failed to meet the expectations of the mech designers.