Muscles stretched: Hamstring of the working leg, hip flexor of the standing leg, supporting and surrounding muscles (when working leg is stretched to the extreme)
There are several ways to do this stretch. Doing it with a partner is the most effective way, but it is extremely important to communicate with your partner while stretching. See below for variations.
1. Begin with your back against the wall or barre. Make sure you are stable and supported. lift one leg to your extension to the front.
2. Partner: Hold the leg at your friend's highest extension without pain or discomfort.
3. Dancer: Gently, (with decent force but not as hard as you can) press the leg down towards the ground, as if you are trying to put the foot back down on the ground.
4. Partner: Keep the leg where it is, resisting the pressure your dancer is applying. This should not be extremely difficult.
5. Dancer: release the leg, imagining all the muscles relaxing and going loose, like a "limp spaghetti noodle."
6. Partner: as the dancer releases the leg, press the leg into a higher extension.
Repeat as many times as necessary. Generally it is most effective done 3-5 times, but it depends on the dancer's flexibility.
This exercise can also be done with a theraband, yoga strap, or stretching strap of some kind.
It can also be done laying on the floor, or even on the barre or a Swedish Stretcher. See the video for complete breakdown of how to use different equipment.
A dancer with a lot of hamstring flexibility may find themselves unable to battement or developpe their leg to the front higher than a certain amount because of several factors.
1. They may have a hip structure that does not allow for over 90 degrees of extension, even though they can battement or hold their leg higher than that.
2. They may have tension in their ankles, back, or neck (which is most common) which tightens the muscles surrounding the sciatic nerve. When the nerve is unable to "slide around" to accommodate a high extension, the muscles will tighten to protect the nerve from being damaged. Lisa Howell breaks this down here.