Our body shape just isn't very well adjusted to walking upright. A few hundred million years of tetrapod evolution to make us wonderfully well adapted to getting around on all fours, but no, that's just not good enough. We're all super special and need to stand up, so biomechanics be damned.
All of which is a long and convoluted way of saying: ow.
My body is not happy today. I'm used to having a bad back, but today it's been joined by dull achey pains all up the right hand side of my body. My foot is sore, as is my calf, my hip, my shoulder and my forearm. I'm not sure what I could have been doing to upset my body so much, but it feels like the sort of pain from awkward motions that the body wasn't designed for. Grumble.
My sympathy and admiration to anyone reading this who has to put up with pain on a day to day basis.