Tom had been wanting to get back into boxing. The sport was his passion in college, leading him to mount a speed bag in his place in hopes to return to it. He even held onto his old gloves and luck jockstrap. He had kept lifting since graduation, maintaining good muscle and staying in shape to return to the ring.
He managed to find a boxing team with a captain that was huge, someone with an intensity Tom hoped to achieve. The captain agreed to let Tom join the team, only if Tom would help him train in his personal time. Tom was glad to be able to start working out again, hoping that the captain saw his potential and he'd gleam some extra training.
Tom started meeting the captain out during the week, wearing boxing mitts for the captain to hit against. The captain had even given him a dogtag that said "punching bag", a constant reminder of his role. The training was brutal, each hit knocking Tom back and back. Tom felt his body struggling to keep up, the captain's power quickly overwhelming him. His arms were shot by the time they finished.
Slowly he grew used to it, his body adjusting and growing stronger. He endured the beating for longer, something changing about him that helped keep his ground. He didn't know what was happening at first, his body packing on pounds and slowly growing larger and larger. His arms and chest filled out, growing thicker as they built with mass to keep up with the captain.
It wasn't until Tom saw himself in the mirror one morning did he realize how much had changed. He grabbed his belly in shock, a thick sack hanging from his front that weighed him down. His muscle had lost its definition, thicker and covered with padding that could take a beating. He had grown enough to take the captain's punches, his extra mass rooting him to his place. He had grown into the captain's own personal punching bag, and couldn't wait to throw back his weight.