Much more slowly. But they can regenerate, given the proper environment (and the removal of whatever caused them to be damaged in the first place).

It's hard to imagine getting severe small fiber damage from kickboxing or other aerobic or anerobic activities, though. Have you had a more complete work-up, such as is found in the Liza Jane spreadhseets--www.lizjane.org--, beyond the nerve conduction studies (which cannot reveal small-fiber dysfunction) to see if there are any more systemic causes?