Good question, one I have been pondering myself. I am doing gym work to build my leg and hip strength after my lay off. I believe I am a stronger rider than I was before, due to the weights and stationary bike work I believe. I have no idea what the experts would prescribe, my program is designed by my physio for my rehabilitation. I don't know what the particular exercises are called, I do ones for my quads, one leg at time, 3x30 reps at 14kg. Hamstrings, one at a time 3x30 reps at 21 kg, hip adductors 3x30 reps at 25kg, hip abductors 3x30 reps at 20kgs. Not bike specific but it seems to work. I also use the cross trainer ski type machine, 20 minutes in my fat burning zone, five minute periods alternating forwards and backwards. I also do hilly intervals on the stationary bike for 20 minutes again keeping my HR at less than 130 and rpm at 90-100, currently I need level 12 for this. FInally I do hip/core exercises which is like pilates or yoga, very slow, controlled movements designed to promote control rather than strength. Again not to help my cycling as such but to make be 'normal'.