Undoing evolution

I did a body pump (weight lifting) class tonight. I've been doing this class for a year and a half, but lately I've been finding the chest presses harder, because when my hands are at the top, I'm doing something weird (possibly twisting them) which means that the piece of my hand between my thumb and my first finger gets tired.

Anyway, at my instructors suggestion, today I did chest presses with my thumb going the same way as my fingers over the bar.

So basically, I was lying on my back holding a bar with 10 kilograms of weight just using my fingers (and no opposable thumb). Luckily, it did not end in tragedy, although I'm not sure if I'll continue doing this -it made it harder to keep my wrists straight.