Our expert says:
It could be any one of a dozen things. The knee is complex, and injuries can be caused by many different things. I suspect that given that he's just started training, his likely cause is overdoing it. That happens often - joints and muscles that are unaccustomed to training are suddenly asked to contract and take body weight thousands of times and they get injured as a result.
My advice is that he reduces the training volume by about 50%, just for a week, and see how that goes. It may be all it takes. Then build up by 15% per week from there. But to go from zero to vigourous is bound to cause some problems, so I suspect this is all it is.
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