Anatomy Review: Slow and Fast Twitch Muscle Fibers

There is no getting around it; all body functions that involve movement require muscle activity. It may be as obvious as when we walk, rise from sitting or toss a ball. It may be movement we take for granted such as our heartbeat or in the iris of the eye. Muscles control the movement of food through the digestive system and enable us to breath. Some muscles are used occasionally; some are in constant use, remaining contracted to help the body maintain posture in defiance of gravity. By their very movement and resulting friction, muscle fibers provide the heat that maintains a consistent body temperature as well as assist in the pumping of blood and lymph in and out of cells.

Types of Muscle

The human body has three types of muscle – skeletal, cardiac and smooth. Skeletal muscle is responsible for posture and movement of bones. It also guards the entrances and exits of the digestive, respiratory and urinary tracts. Cardiac muscle is what the heart is made of. Smooth muscle is found in the gut, around the bronchi, within the urinary tract, the reproductive organs and in the walls of the blood vessels. Skeletal muscle moves in response to nerve impulses. Cardiac and smooth muscle fibers respond to changes in local environment, hormone fluctuation, pH balance, ions and temperature among other factors.

Skeletal Muscles

Skeletal muscles connect bone to bone with tendonous attachments. When they contract, the bones generally pull closer together or flex. When they are at rest, the bones are further apart or extended. More complicated movements, such as circumduction, supination, pronation or rotation are a combination of flexion and extension, a blending of muscular contraction and relaxation.

Normally the muscles act together in a coordinated manner, producing smooth, efficient movements. Some movements are under conscious control, especially when first learning specific tasks such as writing or riding a bicycle, other less so like the blinking of eyes or scratching an itch. With disorders such as Parkinson’s, the signals from the nervous system to the muscles are confused, producing antagonist and agonistic movements at the same time, resulting in either oscillatory movement (tremors) or persistent spasm (rigidity).

Fast Twitch, Slow Twitch

There are two basic categories of skeletal muscle – fast twitch (also referred to as fast glycolic or Type IIB) and slow twitch (slow oxidative or Type I). Within the fast twitch there is a second category sometimes referred to as intermediate, Type IIA, or fast oxidative fibers. Each has their own set of characteristics and purpose. The percentage of fast, slow and intermediate twitch muscle fibers varies from person to person. The proportion is determined by genetics but can change with physical conditioning. Certain hormones, such as human growth hormone (HGH), testosterone and thyroid hormones can also stimulate the metabolism and size of muscle fibers.

Characteristics of Fast, Slow and Intermediate Twitch

Fast Twitch (Type IIB):

most skeletal muscles are of this type
large in diameter
use enormous reserves of glycogen rather than oxygen-rich blood for quick energy
densely packed myofibrils
few mitochondria
generate a lot of tension
rely largely on anaerobic metabolism
fatigue rapidly
appear pale to the naked eye because of low number of capillaries per unit
fast twitch fibers appear in muscles needed for fine movements, such as the small muscles of the hand and the eye

Slow Twitch (Type I):

are smaller than fast twitch muscles
take about three times longer to contract after receiving stimulus
many mitochondria
contain a large amount of myoglobin, which carry oxygen to muscle fibers (similar to hemoglobin, which helps carry oxygen to blood)
slow twitch fibers are needed for posture and movement, and in back muscles and muscles of the legs
because it needs high levels of blood and oxygen these fibers may receive the most benefit from massage

Intermediate (Type IIA):

have properties of both fast and slow twitch fibers
similar in appearance to fast twitch fibers
similar in endurance to slow twitch fibers

The physiology of fast or slow twitch muscles is most often utilized in sports. Athletes who run provide a good example. Those who spring short distances generally will have a higher percentage of fast twitch muscle fibers, while those who run in marathons have more slow twitch fibers.

Another example often used is the dark and light meat of poultry. The breast muscles or “white meat” of a free-range chicken consist of fast twitch muscle fibers needed for brief burst of flight, while the “red” or dark meat in their thighs and legs are used for walking and standing.

The Role of Massage

The type of skeletal muscle fiber is largely determined by heredity. Massage can’t change that. What massage can do is help to bring oxygen and nutrients to muscle cells, which in turn can help them to work more efficiently and heal quicker.

Individuals who are highly athletic or participate in competitive sports have a tendency to test their bodies to maximum endurance and, at the very least, experience sore muscles. They often get injured in their pursuit. Some injuries are serious – others are minor, involving microscopic tears in muscle tissue. When administering massage, care needs to be taken to customize the session to the athlete as well as the sport. Sports massage techniques, most of which are derived from traditional Swedish massage, have been found to be the most effective in promoting healing and maintaining optimal fitness.

To maximize the effectiveness of your massage, make sure to ask your clients about any physical training they participate in, whether it be recreationally or professionally. Knowing this may help you determine just which massage techniques will be best for them.