I am sorry to hear this. If your muscles are degenerated you need to be careful not to tear them. Once the tendon is torn partially it should be deformed. Over-stretching and weightlifting can harm your muscles and cause a partial/complete rupture. If complete rupture happens, you will soon seen a terrible hole in your thigh.

Last year I contacted a world-respected doctor in Finland, Sakari Orava. You can read about him here.

He had this to say about my rupture (and note that it all started with your symptoms):

This kind of tear is rare, but we have seen and operated a few. The frequency is 1-2 / 100 ACL operations. In all cases there has been too much passive stretching and too vigorous physiotherapy to the posterior thigh. Usually posterior thigh cannot be trained actively hard for two months after surgery.
You can contact him via email. I can send you his email address if you like.

Wikipedia reference-linkMRI will help.

Some less skilled doctors told me earlier that the tendons had not grown back. Don't listen to this and find better doctors, doctors who have seen this before and know what can be done. My semitendinosus was operated and is working. Same with gracilis, but it's length is only half compared to original and I have a visible hole. These months I am training to compensate for this loss. My doctor says I will able to get back to sports.