I had a miserable time in my 20’s when I started doing fitness seriously – meaning, as a job. I was down for the count a few times with terrible knee tendonitis from over training and not being aware of what movements I was doing that were causing the injuries. I’ve spent many hours with great physical therapists who I’m grateful to as well as orthopedists to help get me straight (ok, pun intended). My body is a bit of an exception to the norm since I have certain structural abnormalities in my skeletal system that put me at greater rish, but everyone who works out regularly is at risk of knee tendonitis since we use that joint more than most.
Here’s a great little primer I found on Real Age to help people who have had knee injuries or are setting themselves up for one by using poor form while exercising, wearing high heels, and/or not having proper arch support. Click here to read it: http://www.realage.com/chronic-pain-management/bad-habits-knees-knee-injuries-2
Remember it’s always a good idea to talk to a knowledgeable trainer (like me who stresses form and biomechanics) if you’re having some mild discomfort, a physical therapist (who can diagnose and treat the problem) or an orthopedic specialist if it’s severe.