I suffered from compartment syndrome in that muscle (the one that runs down the side of the shin bone). Basically the muscle sits in a sack but in occasions the sack is too small for the muscle. So when you use it the muscle gets bigger with no where to go (think of blowing a balloon up in a pipe) and you lose the up and down motion of the foot. Mine was pretty bad (couldn't walk a few hundred meters properly), but it took doctors years to identify. If it happens often and you don't suffer from shin splints, maybe get the doc to check for that.
A quick surgery of slicing the side of your legs open and problem solvered. Also chicks dig scars, so well worth it
