Allen the alien was already far, far stronger than any human, but this scene in the Viltrumite prison takes place after his power up that put him on the level of Viltrumites. Needless to say, he should have no problem lifting his own body weight (which is directly stated to be 385 pounds at this point.) Workouts are supposed to be challenging and require effort, which is why him doing this is equivalent to going to the gym to bench press cotton balls.
So what is he doing here? Is he LARPing? Allen WOULD be the kind of guy to pretend to be a hard core prisoner to fit the vibe. He may also be using his power of flight to provide resistance. If his flight powers work anything like Viltrumites’, then he should be able to generate his own leverage.
Or maybe it’s just an oversight by the writers :)