Fascia is a type of connective tissue made up of a network of fibers. It is best thought of as being the packing material of the body. Fascia surrounds muscles, bones, joints and lies between the layers of skin. It functions to hold these structures together, protecting these structures and defining the shape of the body. When surrounding a muscle, fascia helps prevent a contracting muscle from catching or causing excessive friction on neighboring muscles.

Fascia is made up of two materials: collagen and elastin. Collagen can change from a fluid to a solid and visa versa, depending on the forces acting on it. Because of this property, collagen is a colloid. Elastin is a network of protein chains. When stressed, the network becomes organized in the direction of the stress, so as to provide some resistance. Under relaxed conditions, elastin assumes a more disorganized structure.

Fascia contains two layers. The so-called superficial layer is attached to the skin. A large proportion of this connective tissue contains fat. The superficial layer serves to anchor the skin to underlying tissues. This layer is especially plentiful in the scalp, back of the neck and the palms of the hands. Elsewhere in the body, the layer can be more loosely associated with the skin, and so can be moved more easily. The second layer of fascia is called the deep fascia. It is located under the superficial layer, and is joined to this layer by strands of fiber. The deep fascia helps to organize muscles into their various functional groups.

When fascia is under chronic tension, it tends to become shorter and harder. This reduces its ability to react to stresses. A well-known example in athletes is plantar fasciitis, which is damage to the plantar fascia located on the bottom of the foot. Swelling of the plantar fascia can be painful. The condition can be corrected by relieving the stress through the use of orthotics, or the tempering of the activity that causes the swelling (such as jogging). Stretching exercises can cause the fascia to regain some of it pliability.

More severe cases of fascitis may require the use of anti-inflammatory drugs or surgery.