There are two types of physical strength, Tai Chikara and Kokyu Chikara. Tai Chikara, usually referred to as external power, this is normal physical strength. It is developed by such exercises as weight lifting. Kokyu Chikara is commonly referred to as internal power; it seems quite different to a beginner. This difference has given rise to many tales of invincibility caused by Kokyu Chikara. It is said to derive from some mysterious power usually called Ki or Chi. In reality, the only difference between Tai Chkara and Kokyu Chikara is the muscles involved. When you use Tai Chikara it is with the feeling of compression even when lifting a weight over your head it is with a feeling of compressing your body. The use of Kokyu Chikara, however is with the feeling of expanding. One widely used analogy is that of a balloon, when you apply internal strength you pretend that your entire body is expanding. This is the main difference between internal and external strength. When you use Tai Chikara, you can only use one muscle group and relax the rest of your body. However when you use Kokyu Chikara it must be done with every part of your body working equally.