Did your buddy stick with just bird-dogs for his back? I messed mine up carrying a big toolbox down some stairs. My doc told me to add side planks and glute bridges into my routine. Those really helped me get stronger in the right spots. I make a point to brace my stomach now before I pick up anything awkward. It saved me when I had to move a thick glass tabletop last month.

Isn't mold growth usually a bit slower than two days in most cases?

What do you do when the concrete kicks off way too quick? @lily_black44 is right about wetting it down, but that's not always an option. Aluminum floats are lighter and cooler to the touch, which helps. But have you messed with different edgers or trowels too? I found that a combination of tools works best when the heat is brutal.