I think the reason Cap seems to get stronger every movie is actually made clear in The First Avenger: he has enhanced healing abilities. He can’t get drunk because his body heals too quickly. Not as quickly as Wolverine, but athletes use HGH to get stronger because they allow their muscles to heal more quickly after a workout. Cap is constantly working out, and unlike a normal human, he’s also constantly getting stronger and faster, just about to the limits of what the human form can take. Hell, he doesn’t even have to worry about his muscles becoming too strong for his tendons and ligaments, because those heal faster too.
He was no dummy even before the super-soldier treatment. Remember, he was the one who thought of taking down the flagpole to get the flag and ride back to base with Agent Carter.
Another thing that I remember reading when the Official Handbook of the Marvel Universe came out is that Captain America’s body does not produce lactic acid, so his muscles don’t tire out from exertion. His strength in the movies generally fluctuates depending on the scene and scenario, like described in previous posts, but he also doesn’t get worn down from the exertion, he performs at peak human skill, and he can quickly determine tactics, angles, and leverage. All of this together is what enables him to punch rich a-holes in their armor and beat up Quinjets.
Look, now that they’re all part of Disney, we can just admit it; Cap’s superphysicality, Banner’s Hulk-iness, Dr. Strange’s magic, Thor’s lightning, et cetera, all come from the same source:
Midichlorians
“Search your feelings. You know it to be true.”
Just wait until Disney releases The Avengers Holiday Special centered around Groot returning to his home world for Arbor Day Celebration. I understand that the tap dance number with Falcon, Hawkeye, and Agent Hill is…unique.
I own a lot of those. Each entry has a Strength Level. Captain America is said to be able to “lift (press) 800 pounds”, which is their cut off for human vs. superhuman strength. I assume lift (press) means something equivalent to an Olympic clean and jerk, or at least a Two Hands Anyhow.
Currently the world record for the clean and jerk is 263kg, or a little over 578 pounds. So Cap is significantly stronger than the strongest lifters, even if they out-weigh him by 90 pounds or more (the Handbook says Cap weighs 245, Hossein Razazadeh weighs about 335).