I've seen the figure 60% of your body weight for the amount of weight you are moving in a standard push up. I've also seen 75% but the 60% is the one I've found most on the net. (I.e. at 80 kg BW I'm moving 48/60 kg in a push up.)

Anyone know where these figures come from and which is more reliable?

Also, does anyone have similar figures for

curls

chin ups

lunges

squats?

I know the last two may seem to be your whole bodyweight but of course, you're not actually lifting your legs...