I alternate concentration on my arms, belly and legs in that order. That is, I push with my arms till they are tired, then fall back to core till they are tired and fall back to legs.
But I feel aches around the scapula region and above-cavicle region which go away only if I skip the machine for two days. It seems fatiguing to those muscles too. I also get muscle aches around lower back if I am not careful.
Am I putting myself in harm by actively using my arms? Should I use arms only for support? Is there a better warmup for the upper body?