Locking in old age can be contributed to Osteoarthritis leading to formation of loose bodies or degenerative meniscus tear. Ideally xray for both knee joints should be done to rule out loose bodies. If required then sos mri of the knee joint.

Hello dear I have gone through your case very carefully however you have not mentioned your dad's age anywhere which is of utmost importance. Your age being 23 i am assuming he must be around 50. Now if he has been a regular bicycle rider, muscle weakness as a cause for knee complaints is out of question. And since there is no pain and swelling , arthritis also seems less likely from here. I would have said it looks like a meniscus injury but having it in both knees is unlikely again. I would want to see his xrays for sure since strong muscles in a person like your father can mask the symptoms of Osteoarthritis. Get Xray Both Knees AP standing and lateral views Regards Dr Vipin Gupta

Loccking can be due to osteoarthritis /osteophytes in the knee or degen meniscal tear. Ideally knee examination is needed . He should start with walks/knee exercises.

Its mostly due to meniscal injury. Can be sometime due to loose bodies Needs evaluation for further treatment

Knee locking is mostly due to meniscal injury, but as per your history there is no history of any injury or trauma. Basically he might be having osteoarthritis of knees, which is an age related degeneration process. So you need to see an ortho dr and get his xrays and clinical examination done.