Our expert says:
It is probably caused by weakness in the knees as a result of the time you have spent inactive in the last while. It's quite a tricky stage, when you come back to sport after a long layoff, because the muscles, joints and ligaments are all weaknened by inactivity and then you try to put them through stresses that they were previously capable of dealing with. So the key here might be that you have to back off very slightly on the training, perhaps you are just going too much in terms of racing. 21 km is a tough ask, and if you are not 100% strong, then it will most definitely show up any knee weaknesses. And the downhill would bring that out most, so it does seem reasonable. So my advice is that you strengthen your knees, either in the gym, or through running, but reducing the distances for a while, until your joints and muscles have adapted a little better. And then it should start to become less of a problem.
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