Working out on a mini-seesaw has been proven to activate the core muscles. The proof is this picture. And now, I present to you a long lecture on the importance of working out shirtless with an obvious erection:
Work out shirtless with an obvious erection so voyeurs can stare at your muscles and bulge.
This concludes my lecture. If you still have questions, consider the following addendum:
If you must wear a shirt, wear it only briefly, then take it off slowly. If at the start of your exercise session, your penis is not hard, stare at your own body (thinking of yourself as exhibitionist and voyeur) until your penis complies with hardness. If your gym does not allow you to work out shirtless and with an erection, start a change.org petition. And in the meantime, wear a low tank top with the sides cut out. And wear shorts with no underwear. Cock ring optional. Or exercise outside by your own rules, AKA my rules.