On the human body, the limbs can be divided into segments, such as the arm and the forearm of the upper limb, and the thigh and the leg of the lower limb. If these segments are cut transversely, it is apparent that they are divided into multiple sections. These are called fascial compartments, and are formed by tough connective tissue septa. Compartments usually have separate nerve and blood supplies from their neighbours. All the muscles within a compartment will generally be supplied by the same nerve.
Skeletal muscle is separated into compartments which contain connective tissue, nerves, and blood vessels. Fascia is a type of strong connective tissue which separates neighboring muscles or muscle groups from each other into these compartments. The fascia is also responsible for separating skeletal muscles from the subcutaneous tissue. 
The fascia of the leg is extremely thick due to the effects of hydrostatic pressure: there are large amounts of pressure placed on the leg because of the weight of the column of blood from the heart to the feet, and the fascia needs to be thick enough to support the leg muscles.  Since the fascia is so thick, this can also be problematic because any inflammation in the leg has little area into which it can expand. Vascular structures and nerves have the most problem under this hydrostatic pressure, and any swelling in the leg can put pressure on these structures as well as to the muscle. If the pressure is great enough, blood flow to the muscle can be blocked, leading to a condition known as compartment syndrome. If there is great enough damage to the nerve and vascular structures around a muscle, the muscle can die, and serious complications, such as leg amputation, could occur.