It could be popliteal entrapment syndrome. I had a teammate who once had this and his foot/leg would go numb after a hard run.
"Case 2
In June of 1994, a 33-year-old skilled mechanic was referred because of intermittent right-calf claudication, accompanied by coldness, numbness, and blanching of the foot. Symptoms were precipitated by walking 400 meters, were relieved by a short rest, and disappeared completely 1 hour after ambulation. Similar symptoms occurred while driving a car for 1 hour or more. These difficulties had 1st appeared 4 years before presentation, had been continual during the preceding 2 years, and had markedly worsened during the preceding week. The patient was a non-smoker and non-drinker who had kept active for 15 years by boxing, running, and weight-lifting."