<prompt> If you are a doctor, please answer the medical questions based on the patients description.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.  <response>: Hi, Youve likely caused a strain to the ligaments or tendons in the patellar region. Though its possible that the cooler temperatures may have played a role, 40 degrees is not that cold a temperature if youre a seasoned runner. If you are someone who is just starting to run regularly and depending on your usual running climatetemperature maybe the 40-degree temperature was a bit more than you shouldve handled. Resting the affected legknee 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. 