(M, 40, 5'9", ~230lbs)
I was recently getting into reasonable (tho still overweight) shape and training to throw the javelin at local club. When I took a very hard throw, left knee basically collapsed

question, good people, is:
How does a heavy athlete (whilst losing weight) train to increase explosive strength, much of which must come from the legs, when his knees are-- and will probably continue to be-- too painful to perform squats, heavily-weighted lunges or any hopping exercises at all....?
All the best
Steve