It results from the metatarsals -- the small bones behind the toes in the ball of the foot -- being compressed together and thereby compressing the nerves through that part of the foot.

The result is a sensation like overheating of the sole of the foot, especially in the ball area. Sometimes it feel like your sock is all scrunched up under the ball of the foot.

There are a number of causes, ranging from improper shoe fit (width, size, even the shape of the shoes where the ball of the foot sits) through to pedal cleat placement.

I don't recall anyone posting here or other forums, or involved in randonneuring, having knee problems resulting from fore-aft cleat issues; toe-in/toe-out issue, yes, but not the other. I could understand Achille tendon and calf strains with the cleat too far forward, however.