@rr79: Wrong, they were holding his arms before Mantis got onto him. She was the last one in the plan. She bought them a lot more time, but they held him before she was added in. With his level of strength you are implying he should have ragdolled them immediately.
And why exactly would he hold back against Cap? He was at the culmination of the goal he had been working on for years. It makes absolutely no sense for him to hold back.
It doesn't matter if it makes sense or not. Watch the scene, at no time do you see a single bit of effort from Thanos against Cap.
I will have to rewatch the other scene to be sure, but if I am not mistaken they weren't holding him back at all.