Strengthen tendons take time. They are less metabolically active than muscle. I would argue someone with really strong tendons is probably stronger in true raw strength. I practice a Russian martial art called Systema. My chief instructor Vladimir Vasiliev has is freakishly strong and it's because of his tendons. We do tendon strengthen exercises and it's pretty crucial actually. SirSir also had a good suggestion but that's mostly for lengthening and keeping the sinewity (not a word, I know) To strengthen them you would do very slow and calculated movements. Try standing up from prone without using your arms. That is all tendons and ligament strength.