It's more common than you think...
If you've been training for awhile in the fashion that most Westerners do, which is H-A-R-D-C-O-R-E :p , you're likely starting to burn into some of that precious Yin that nourishes your tendons and ligaments. Jow works great so you can keep training, but the root cause needs to be addressed...
The ligaments are nourished by the Liver Blood, specifically, and the health of the bones is due to the Kidney Yin. It's a natural process (kinda) to burn the yin when you work hard - the Qi has to come from somewhere, and the Jing is the repository. Now, Jing is the essence of Yin, and, correct me if I'm wrong, the great amounts of Yang used in heavy exercise to convert the Jing to Qi can injure the Yang of the Kidneys and Spleen, ultimately reducing the body's ability to create blood (which is a 'spleen' function), thus undernourishing the tendons.
I've heard Liu Wei Di Huang, or 6 Flavors Rehmannia formula is a good one for nourishing the liver and kidney yin, and thus nourishing the tendons. Consult your local herbalist to have the formula modified to suit your individual constitution / presentation.
The point is, you gotta replace that Yin somehow - herbs, adequate rest and diet / supplements, Dao Yin exercise, meditation all help to nourish the Yin.
And then there's those esoteric techniques for bedroom fu... :p
peace
herb ox