Page 28, Heracles invented dumbbells.
Or so I thought but upon googling the origin of dumbbells, they were actually invented in Ancient Greece, some say around 2000 years ago. Though Heracles in this manga was born around 4000 years ago (2000BC) so he's still using them 2000 years early.
Furthermore, the first dumbbells were club-shaped unlike modern dumbbells so Heracles is likely getting divine inspiration for his creation so that he can see into the future.
TLDR: Heracles got strong by asking the gods how people in the future did their bodybuilding and ended up building exercise equipment.