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Every ballet exercise can be broken down into tiny parts. A good dance technique is in store for you if you can get the correct technical information on every basic exercise. If you select one exercise, or one component of an exercise, to put into your daily routines, and rotate the selections that relate to your particular technical weaknesses, you'll have an ongoing regimen for building strength. Here is a little analysis that you might not find in a book at the ballet store.
Ronde de jambe a terre is the phrase of French ballet words that mean "rotation of the leg on the floor", as opposed to "en l'air" which means you raise the leg up off the floor. You can build strength with extra standing or floor exercises.
Ronde de jambe a terre develops the rotator muscles in the back of the pelvic area, specifically. It is a whole body workout as you are also holding the turnout in the supporting leg, keeping the weight balanced properly on the supporting foot, using the support of the barre sparingly, maintaining your postural plumb line, and working your core muscles well so that the neck and shoulders are not straining.
And, if you can take a barre ronde de jambe exercise into the center and execute it, you have achieved a considerable result in building strength!
Ronde de jambe en dehors, or circling outward from your tendu devant, seems simple. You draw a circle outward from tendu devant, towards a la seconde, maintaining your maximum turnout in both legs. Nothing else changes. You end the movement at your second position, as far to the side that YOU can go without losing turnout or balance. Lift the foot off the floor an inch, and take your hand off the barre. Be aware of any change of position, or ankle and foot movement to maintain balance on the supporting side. These are the details that tell you how to improve.
Floor exercises can be very helpful, especially if you are tired from previous classes, or your other daily routines. When you lie on the floor, place a cervical pillow or a rolled towel under your neck, to support the curve and help avoid straining those muscles. Place your arms a la seconde, and your feet in first.
Extend one leg forward to a tendu devant position. (If you roll to either side, your turnout has changed in one of your legs, or your abs and back muscles cannot hold the position). From your balanced position, slowly circle the tendu foot toward your second position. If you lose balance again, go back the point where you can hold your position with your turnout and core muscles.
You can also relax your arms, folding them across your stomach. Don't place them on the floor unless you are really off balance. If any muscles cramp, stop and stretch and relax them, and continue.
It doesn't matter if you do this on each side for a while, only moving partially to a la seconde. You are strengthening your turnout position exactly where you need to.
You can also tendu a la seconde, and circle toward the tendu devant position, working for a balanced position without strain. This is ronde de jambe en dedans, or inwards.
Always relax the legs, and turn IN to relax the rotator muscles, often during class and in your daily practice routines at home. Use a rubber ball, leaning into it around the hips, under the thighs and calves, under the tibial muscles and the foot muscles. It's a great massage. You will hit some very tender spots that you can work on daily. This will also tell you where you are straining, and where you need to strengthen.
No matter what your level is, whether you are pre-pointe, pre-pas-de-deux, or more advanced, your daily routines will be useful especially during holidays, so you don't get strained or injured when you return to class.
Although many ballet stores sell technical books on ballet, I am not aware of any right now that pick things apart the way technical points needs to be explained.
This is just one piece of one exercise, but very powerful.
Look here for a wonderful detailed analysis of daily routines with self-assessment charts and other organizational material, about ballet moves.