On an upright bike, hard pedalling is a whole body exercise, with the swaying side-to-side. Body weight is a force limiter. I’ve found it easier to pedal hard on a recumbent, against the seat back. One of my knees hurts now. It’s a bummer that the upper body doesn’t get exercise.

I don’t know what you mean by “pushing sideways” in this case - that only applies to foot-steered Flevo bikes/trikes.