Originally posted by jrodslam
Your muscles can get tired or sore. Doesnt have much to do with breathing. Its the strain and exertion of the muscles that probably tired him out.
A side effect of his armor's strength enhancing effect is a vastly greater muscle efficiency than an ordinary human. While armored, his muscles generate considerably less lactic acids than those of ordinary humans, granting him superhuman levels of stamina in all activities. He can exert himself at peak capacity for several days before fatigue begins to impair him. Colossus' armored form is also invulnerable to rusting under Earthly conditions. Colossus has demonstrated no need to breathe while he is in his armored state and there's no known limit as to how long he can remain in this form. Colossus also has no limit with his strength, as long as he ages works out in his normal form his steel form will continue to grow stronger.
So the comics lied he shouldn't have been tired after lifting the sub.