I appreciate the feedback. Which structures? I have not seen any research showing this exercise to be dangerous. All exercises can cause injury, if they are performed well past the bodies capacity, but I do not believe there is anything inherently dangerous about knee extensions.

I too am a physiotherapist and I want people to move and exercise. If you want specific guidance to maximize results or are concerned about exercising with pain, I can help, but I don't think this exercise requires seeking advice.