@annie1 Hi Annie. I'm glad to see you here in the community. I think what you are describing is fatigue of weak muscles. Muscles work in opposition to each other. An easy example to understand is to think about your arm. The biceps bends your elbow, and the triceps on the back of the arm helps coordination that movement like softly pressing the brakes, so you can slow and stop the movement of the bending arm, and also coordinate the fine tuning, so your arm doesn't loose control. All over your body, muscles oppose each other for coordination and pull against their attachment points on the skeleton.

If you have a situation with a weakened muscle and you're working to strengthen it, it will wear out, but the muscle that opposes it may be a lot stronger. It sounds like a weak muscle tries to relax, and the stronger muscle kicks in when the weak neighbor gives up a bit, so it goes back and forth loosing a bit of coordination. I'd be curious if your physical therapist gives you and explanation like that.

FYI, you can take collagen as a supplement, and there is lots of collagen in meat because it is in the connective tissue that holds it together. I don't know what advantage there is to injecting collagen, but that would be a question to ask your doctor.

Jennifer