A demi plié is one of the most important movements in classical ballet. Its the start and end of every jump, pirouette and is used between steps to look more fluid and protect the dancer from injury. Since its so common, its important to have a really solid plié, and part of that is keeping your heels down. Lifting your heels in a demi plié, by definition, no longer makes it a demi plié! Plus, you’ve lost most of its usefulness and strength for your jumps and pirouettes.
Another important aspect of a demi plié is that it allows you to properly stretch the muscles in your calf. When you keep your heel on the floor in a plié, it allows the calf muscles to stretch better and for your body to learn where the “end” of your plie is. Having this sense is vital to stronger jumps and for endurance. When you allow your muscles to contract and then fully relax and stretch in a plié, you’re using them more efficiently, allowing you to dance more difficult movement and for longer periods of time. If you shorten your plié too much by lifting your heels, you’ll find your calf muscles will begin to shorten overtime and your power when pushing from the floor begins to decrease.
Keep your heels on the floor when you demi plie and you’ll be on your way to being an overall stronger, healthier and more capable dancer.