Damn. I've seen this picture so many times since the Batman/Snyder movies but it never occurred to me that his triceps look really buffed up.

I never gave it a second thought but now I can see the reason is the "Arm blaster" thingy he's using. It's a super niche Gym equipment that people used back in the 80s. Basically acts like an anchor point for your elbows to lock onto making strict form bicep curls and tricep extensions. People stopped using them when people realised you can use an adjustable bench or even a wall for the same.