How do you train a client with a history of quadricep tendonitis & patellar tendonitis? What exercises are safe, which avoidwhy?

I have not worked with too many clients with this issue, but I have been experiencing it in myself recently. In clients, I look for imbalanced through the femur & patella and choose strength & rolling exercises accordingly. In women, the Q angle makes them prone to have some lateral deviation in the patella (which is happening in my case). I have been rolling IT bands, vastus lateralis, and strengthening my VMO like crazy. My favorite VMO strengthener is footwork on the Pilates reformer with a small ball or cushion in between the knees- I do full range, and short range near the foot bar- works like a charm!

Most clients I have had with this issue came to me wearing a patellar stabalizing strap and don’t have nearly the issue during cardio; however, that is of course a band-aid solution in my opinion. I prefer to address it in my training.

I would strengthen their hamstrings, abductors and adductors isometrically.
I would also have them gently roll out thier quads and suggest swimming as an option

I would have them do upper body and “core” work in positions that would not induce irritation.

I would avoid squats, lunges, any jerky motions, lateral motions, until they are stabilized

I wouldn’t not do squats or lunges. Maintaining strength in the glutes are critical to alleviating destructive forces on the knees. You can reduce the range of Morton, ensure the squat is starting with a partial sitting motion then extend the hips from there. No need to go deep our full squats, but we squat to sit, stand, perform daily activities, so we have to continue to strengthen those muscles performing those motions. Lateral motions could strengthen those stabilizers that are the hip abductors and adductors.