To train muscles at all without the need of prior kreng, and may select the particular muscles the only problem. Which muscles do not need to be both an enables less convenient and more.
To train muscles without straining them before. And may choose only the muscle is only optional. It is not necessary to loosen the muscles, both to help the less time and is more convenient.
To train the muscles it without the need of contraction. , and may choose to relax muscle specific part is the only problem. Which don't need to loosen the muscle body will help take less time and more convenient!