I’m not in the kinesiology field, but I dance! And I have horrible natural flexibility, I’ve earned it all myself, so I feel like I can talk about this.
Most important is to listen to your own body: stretching is putting microtears into the muscle fibers so the muscle builds it back longer, so stretching too hard for too long will only do more damage. But this is all a personal length of time—there’s no real magic number. I’ve been told everything from 20 seconds to 10 minutes.
Make sure you’re only strectching when you’re muscles are warm—either through excercise or a hot shower/bath, because then you’re stretching them at their best current flexibility. Cold, muscles are stiffer and are more likely to tear too much.
Personally, I find that just holding a position makes my muscles feel tighter. I plie into them more than purely holding them.
I also like to stretch throughout the day, because my muscles start to tighten within a few hours. Especially after sitting all day at school.
Always stretch (lightly) after your workouts, so the rebuilding muscles rebuild longer.
Also important is how you’re thinking about the stretching. Don’t imagine pulling your muscles, but imagine lengthening and relaxing them into the stretch. Breathing is supposed to help, too. As with everything in dance, half the battle is quality.
Part of flexibility isn’t even the muscle, it’s the mind and your tension. A physiotherapist, Lisa Howell, stresses this in her classes and books (The Perfect Pointe Book): she observed a surgery of an old man, and the doctor showed her how under the anestesia, the man had incredible flexibility, because the brain wasn’t telling the muscles to tighten up. The doctor literally just started plaing with the man’s legs, pulling them up above his head, out into center splits, things that would be impossible if the brain was aware. It’s actually kind of amazing to think about.
Sorry if this is way too much: the key is really to listen to your body, and find what it needs from you to give you what you want. Experiment and find what works for you!