I have been running a lot lately in 40 degree weather and today I m not sure what happened but I can t run on my left knee but can walk. Stair s are difficult and when I rest my thigh on an edge the whole left side of the knee cap and patella tendon goes numb.
Hi, You've likely caused a strain to the ligaments or tendons in the patellar region.  Though it's possible that the cooler temperatures may have played a role, 40 degrees is not that cold a temperature if you're a seasoned runner.  If you are someone who is just starting to run regularly (and depending on your usual running climate/temperature) maybe the 40-degree temperature was a bit more than you should've handled.  Resting the affected leg/knee at this point along with ice wraps for the first 24-48 hrs. Is what I would recommend.  You can then, switch to warm compresses several times daily after that time.  If the knee continues to be stiff or excessively painful after a week of conservative treatment then, you should see a doctor in order to get imaging of that area to be sure nothing is torn or otherwise abnormal. Hope I have answered your query. Let me know if I can assist you further.