I don't think normal people ever lost that ability. It's ridiculously easy to pull your whole body up from a dangling position like that if you engage your core and use your legs. Let alone just dangle there with your elbows locked. I mean that's why pull ups are a challenge - because you are consciously not using the muscles in your legs to add momentum. Edit: Looking at the video, the girl is literally just dangling with her arms straight. The first thing to go would be your hand grip, rather than any muscle, and only because those poles usually have a poor surface plus the square/cube law (you'd apply a hell of a lot more pressure on your hands than when you were smaller... even with bigger hands)