According to your description, most likely you may have a Patellofemoral Syndrome, which basically is an unbalanced muscle pull that produced excessive pressure or leverage forces on the knee joint surfaces. The pain typically is located behind, underneath the kneecap and often shows during activities that require prolonged knee flexion and forceful contraction of the quadriceps (e.g, during squats, ascending/descending stairs or pendants). Pain may be worsen by sitting with the knee flexed for a long period of time, such as while watching a movie, hence leading to the terms "theatre sign" and "movie-goer's knee." The causes are diverse: Overuse (repetitive use or activity), overloading, and misuse of the patellofemoral joint. The suggested strategy should be: strengthening and stretching the main muscle of the thigh: quadriceps, to restore the muscle balance around the knee joint, for this is indicated physical therapy for 6 to 8 weeks, using ice packs at the end of the exercise routine to decrease pain and swelling. The use of soft knee braces helps to control patella position and restrict full knee flexion which is good to ease the pain. Also, and very important, you have to stretch the hamstring muscles and the activities or sports that require frequent squatting must be avoided until the pain is under control. Regarding of the numbness on the left side of your foot , maybe a nerve root impingement and/or inflammation might be causing neurologic symptoms in the area that is supplied by the affected nerve root, in your case the left foot. I suggest an evaluation by an orthopedic surgeon if the numbness progresses or persists.
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