Training is what strengthens connective tissue. I don't think you can strengthen a tendon. They are what they are. But the end points you can strengthen. Resistance exercise (weights) is what does it best. IME supplements do nothing for that. However, glucosamine sulphate has worked for many people to keep cartilage in good working order. Chondroitin
AFAIK has never been shown to do anything. Glucosamine hydrochloride has also been shown to provide little if any benefit.
http://www.wynnpharm.com/images/rand...ial%20AR07.pdf