I developed pain under my left patella so embarked on VMO strengthening exercises. I tried many but the one I like the most was seated leg extensions with resistance. I figure out the weight I can barely do 3 sets of ten reps with the weak leg and then do the same weight and reps with the strong leg. The weak leg is getting the relatively harder/better workout this way. When doing this, my toes are pointed slightly outward and I can feel the VMO with my fingers to concentrate on its activation. I alternate 10 reps with one leg and then ten with the other without pause until each has done 3 sets. Worked for me...