Can you actually be too flexible? It depends. Some people are just naturally bendier than others. Flexibility, in general, comes down to the range of motion of your joints and the elasticity of your muscles and connective tissues. Your muscles and some connective tissues are malleable and can lengthen and become more flexible if you stretch regularly. So it's not necessarily a bad thing to be, say, a yogi who is becoming more lithe, if you've been working at your flexibility safely.
On the other hand, joints are stabilized by tendons and ligaments, both of which are not very elastic and can become injured if you overstretch them. If you try to force tendons and ligaments beyond their range of motion, the joint has less and less support and becomes unstable, which can lead to injury. One sign that you're putting too much stress on a joint while stretching (and, in turn, potentially overstretching the surrounding support) is if there is any sensation—pain, tightness—inside the joint itself.
Some people have naturally hypermobile joints—which may put them at a long-term risk of arthritic changes due to wear and tear on the cartilage. If you're hyperextended, it's important to strength train to build up the muscles surrounding your joints, in order to stabilize them.
In rare cases, hypermobility can be a sign of Ehlers-Danlos syndrome, a group of inherited disorders that affect the connective tissues. The most severe forms of Ehlers-Danlos syndrome can weaken the aorta and the arteries to the kidneys and spleen—so if you're experiencing symptoms (others include translucent or stretchy skin or skin that bruises easily or doesn't seem to heal properly after a cut), you should get checked out by an MD.
Health’s medical editor, Roshini Rajapaksa, MD, is assistant professor of medicine at the NYU School of Medicine.