I have bow leg.i am a road runner, normally run around 30km per week.i realise that my left leg will be slightly bower than my right leg.what cause it?

Genu Varum: Bowing of the legs is usually a developmental positioning of the knees of unknown origin. It can be associated with diseases that effect bones such as rickett's and blount's disease. The most common cause is "we don't know". Asymmetry of the bowing is usually very slight if at all. For a runner the biggest concern is if excessive more pressure is directed to the medial compartment of knee.