Over stretching is NOT a myth. There are a couple of different ways to look at this.
First is over stretching causing microtrauma during the stretch due to forcing a stretch too hard, too far, and too fast. Possibly contributing to the cumulative trauma cycle and causing compensation patterns due to the body’s protective mechanisms.
Second is the length tension principle. There is a point in a muscles length when it is the strongest due to optimal overlapping of the actin and myosin. If a muscle is lengthened to an extreme there will most likely be strength deficits at certain points openning that person up to an injury. It is possible to strengthen extreme ranges of motion and very important if a person is hyperflexible.
Yes, you can over-stretch. For example, one of the dangers of conducting PNF stretching can be overstretching to the point of injury if you are not very careful since this technique basically overcomes your body’s natural stretch reflex.
I hope this helps.
Over stretching is just like over training. You shouldn’t stretch beyond a slightly uncomfortable tightness. You shouldn’t stretch beyond a joints safe range of motion. You shouldn’t stretch “cold”. There is a point at which more stretching will not yield further gains in ROM without tissue damage. There is a a limit in how long you want your muscles and tendons to be stretched to for performance and safety.
The number of people who intentionally over stretch is very small though. The vast majority of people do not stretch enough or properly.