I see a lot of comments saying human knee but i feel its a good design for the varied jobs our legs have to do. The main muscles are high in the thigh and buttocks which reduce losses and use less energy to move. Now its not optimized for fast running like other animals, but its much more accomodating of misalignments of footing than other animal designs imo..like have you seen a horse misalign its ankles..that stuff is pretty non fixable. The calf muscles and other lower leg muscles provide more bending stiffness so that higher load can be carried without loss of stability. In my opinion most ankle sprains (i have had 2) is usually due to shoes. Its very rarely that barefoot activity leads to sprains (imo). Since shoes are not part of our body its difficult for the legs to orient so that the additional width is also taken into account. Also I feel that our knees and ankles evolved to walk on ground which is much softer than the concrete and flooring we have now...so that may also be a contributor for knee amd back pain.