Realistically it's not about endurance... I mean even the strongest human can still break their bones if they land wrong. The problem with us, though, is that there are so many wires running beneath our shells, especially in less protected area like the arms and legs, that a break can often rip some of them.

Even then it takes a while for it to properly register in our systems 'you can no longer move these limbs because there's a break in the connection'. Hence why he was able to continue sparring even with it broken.