Yin tissues and Yang Tissues
Yin and yang are relative terms and need a context to be appropriately applied. They can be used as adjectives, although they are often used as nouns. Within our bodies, if we use the context of position or density, the yang tissues could be said to be our muscles, blood, and skin compared to the yin tissues of ligaments, bones, and joints. The contexts of flexibility or heat could also be used; muscles are elastic, bones are plastic.  Muscles love to get hot while ligaments generally remain cool. However, we are not making an absolute definition here. In the context of water content, the muscles are yin and the ligaments are yang. Muscles love to get juicy, thus, they have lots of water in them, which is a yin quality; ligaments have less water content, which means they are relatively yang.
Despite the context of water, there are more ways in which the muscles are yang-like relative to the ligaments than they are yin-like. This is one reason this particular style of yoga is called Yin Yoga. Yang styles of yoga generally target the muscles and employ rhythmic, repetitive movements to stress the fibers of the muscles. Being elastic and moist, the muscles appreciate this form of exercise and respond well to it. Yin tissues, however, being dryer and much less elastic, could be damaged if they were stressed in this way. Instead, the more plastic tissues appreciate and require gentler pressures, applied for longer periods of time, in order to be stimulated to grow stronger. This is why orthodontic braces must be worn for a long time, with a reasonable (and not always comfortable) amount of pressure, in order to reshape the bones of the jaw.