DC's Superman may strut around in a skin-tight blue costume, but does he spend any time pumping iron to maintain that figure? The Man of Steel is clearly proud of his physique, wearing a costume originally inspired by circus strongmen, but does he have to pour the work into his body, or does his Kryptonian DNA give him free muscles?
Kal-El would be just a normal guy on Krypton thanks to its red sun and the intense gravity of his former home planet. But once on Earth, Clark Kent rose above mere mortals, displaying unfathomable strength and awesome powers. Earth's yellow sun charges Superman’s cells to the extent that his strength is basically unlimited. The hero can move planets and has even held black holes in the palm of his hand. His Silver Age exploits are so obscene as to be almost a farce. For example, he nonchalantly nudged the sun a little closer to Earth, and that was just a regular day for him.
Superman’s Kryptonian DNA gifted him super-strength that has an almost unimaginable limit. But does he need to do 1 billion daily pushups to keep his strength up? His powers come from the yellow sun our planet orbits that grants him not only super strength but, depending on the comics era, all sorts of other powers. Clark never needed to practice deep-breathing techniques in order to use his super breath, though he did need some practice flying when his powers first developed back in Smallville. Despite this, Kal-El does have limits, needing to sleep every five days or so in order to maintain his sanity.
But as for his strength, he very rarely needs to train. There have been some snapshots in previous comics of training equipment that Superman reportedly uses, but no suggestion this is a regular activity or something that significantly alters his strength levels. In Scott Lobdell and Kenneth Rocafort’s Superman #13 during the New 52 run, Superman attempts to measure the upper limits of his power by bench-pressing the Earth’s weight on a specialized machine in Dr. Shay Veritas’ lab. Superman did enough reps that he actually broke a sweat, but reacts in surprise, unable to remember the last time he perspired. This suggests how difficult it is for him to exert himself in the way a normal person would when exercising.
The Man of Steel once took over for Atlas holding up the Earth for a number of days, so it’s hard for him to find a strenuous enough activity to generate a real workout. One could argue he gets exercise each time he uses his powers, which would explain his existing physique. But there are only rare instances when Superman actually trains his body to increase his capabilities, most often when he has to fight a fellow sun-fueled Kryptonian like General Zod, in which any edge counts.
Ultimately, Superman's incredible power comes from the solar radiation absorbed by every cell in his body. In Grant Morrison and Frank Quitely’s All-Star Superman, Superman lifts 200 quintillion tons with one hand - a feat he managed because his cells have been overcharged by the sun. Crucially, this upgrade comes with no noticeable change in his physique. Similarly, Superman Prime - the ultimate version of the hero - doesn't seem to be in much better physical shape than Kal-El, while his cousin Supergirl possess almost identical powers despite physical differences. While his superheroic exertion likely takes care of his impressive abs, Superman gains his powers from the sun rather than exercise.