A corporal is a low-ranking noncommissioned officer in the armed forces. Corporal is a term that relates to the human body. "Corporal tunnel syndrome" sounds like carpal tunnel syndrome which is a condition characterized by numbness and tingling in the hand and arm caused by a pinched nerve in the wrist.  A junior NCO is a type of military officer that has not purused a commission. The joke is using the word corporal in different ways.
The Junior NCO had sore wrists . The doctor said it was corporal tunnel syndrome .