Knees pain in 35 age (no sport activity)

I have pain in both Knees last 6 years. My age is 35. (1) Pain during the descending the Stairs. (Note, only little bit pain during Climbing the stairs). (2) I can not FAST RUN since last 6 years due to Knee pain. (3) Pain during small jump also. Few days ago, I took X-ray of AP, Lateral view & Skyline view but no problem found. It means no PATELLA issue. I am also taking Calcium, Vit-D3 supplement since last 6 years (Few months I forgot medicine). Uric acid level is proper. No any sport activity since last 15 years. Physiotherapy completed (Static Quadriceps, Hamstring with Weight Cuff-1 KG) but no result. One doctor said it may be Muscle week. How to check Muscle is week or not of Knee (I suppose, Dexa scan may not say Knee muscle strength, Dexa is for other part of body bone). Shall Sonography/MRI tell me strength of Knee-Muscle ??  I am living alone at home last 10 years & I have bad habit of masturbation due to which all Calcium, Vit-D3 are losing by SEMEN every 2-3 days.