Hyper-extended knees - I will help you learn how to build strength and how to hold the legs correctly. Understanding neutral spine, neutral leg, and understanding the support system of your core muscles is vital. Every ballet exercise requires a neutral standing leg. Dancing in pointe shoes, especially, requires a strong neutral leg and perfect alignment.
Explaining a neutral leg, in the first few classes, is as important as explaining a neutral spine, whatever words the teacher uses. (Unless there are no hyper-extended legs in the class, this is necessary).
A neutral leg is: knee above the ankle bone, pelvis above the knees. A line drawn down through a photo of you standing, taken from the side, should reveal a postural plumb line, from the center of the skull, neck, spine (through your natural curves), pelvis, through the knees and ankles, or just in front of the ankles, depending where you rested your weight.
With over-straight legs, the knees stretch back behind this line. While this curving out at the back of the knees is an admired line in a dancer's gesture leg, it causes many problems in a standing leg.
First you tackle the feeling of the legs being bent, when held in a straight position. However, in this straight position it is much easier to hold the turnout and have the pelvis in a neutral position where you can build strength in your center.
When the knees rest back in hyper-extension, the thighs turn in, and often the pelvis tucks under. Now you have compromised the safety of your low back. (In this position you may develop bulky thighs and bulky hip muscles). From here you destabilize your balance. You can compensate for this distortion in many ways in soft shoes, to a degree.
When you get into pointe shoes, all kinds of problems will show up, if they haven't already.
If you are a few years into training, it's extra work, it's reprogramming neural pathways. But most dancers do this all the time as they progress through their training and careers.
There is also a lot of talk about overdeveloping the quads or inside knee muscles, or some other deformity, because of holding the leg straight. Not so. If your pelvis is neutral, you can use your muscles properly. You don't need to grip or clench.
You want your thighs pulled up long. You want a feeling of pushing down through the center of your leg bones into the floor. And imagine a space between your hip bones and the top of your thighs. You will achieve a real lengthening by doing this, your deep lower abs will pull up and in nicely, and now your core muscles are supporting the length you want.
You will be able to hold your turnout and have a strong supporting leg. When you get into pointe shoes, you will have your body aligned well, over that tiny point on the floor.
Always remember in your ballet stretching, to relax and lengthen all your muscles. Use a soft rubber ball to knead the muscles, leaning on a wall, or use it with the floor. Deborah Vogel demonstrates this on her DVD as to how to get more flexible and release tension.
Get a guide with expert ballet tips for working with hyper-extended knees.