Like the bones, the skeletal muscles range in size and shape to suit the particular functions they perform. As you are reading this, raise your eyebrows. Now smile! Now frown! Now wiggle your tongue (not at anybody, please)! Now close your eyelids (but don't go to sleep, we still have work to do)! As you are performing these facial movements, you are using many different muscles, some of which are less than an inch long. Even with their shortness, the muscles of the eyelid are the fastest contracting muscles in the body. And in order to smile, you are using 14 different face muscles!
Of course, we can't cover all 700 or so muscles in the body, but we will review some of the more familiar ones. Let's look at some of the larger muscles in our bodies. Take a deep breath. You are experiencing the effect of movement of the diaphragm, which is the dome-shaped muscle located at the floor of the chest. It is the main muscle involved in breathing, and is also involved in coughing, sneezing, laughing and sighing. The diaphragm must constantly work whether we are on Earth or in space since breathing is essential to survival. That is, the function of the diaphragm does not change if gravity is removed.
Now let's look at some of the larger muscles that change their function when gravity is removed (Figure 5).
These are the muscles that we use for locomotion - the physical movement of our body. Although these muscles are not considered essential for survival in the same way that the muscles of the heart and some other organs are, they are extremely important for enabling us to carry out our day-to-day activities. All of these muscles have "grown up" and have been trained to work in the presence of gravity. For instance, the bulging triangular deltoids of the shoulders raise the arms. The biceps and triceps of the upper arms bend and unbend the elbows. The broad pectoralis major muscles, those rippling signs of the he-man, move the arms across the chest. Without gravity, their jobs would become easier.
There are other muscles, however, that function almost entirely because of gravity. That is, their function is to create movement that opposes the gravitational pull of the Earth. These are broadly referred to as anti-gravity muscles but are also known as postural muscles. They are located primarily from the lower lumbar spinal area down to the feet. For instance, the massive gluteal muscles of the buttocks (the largest combined muscle group in the body) help us maintain posture (to stand up) and to stabilize our hips (for walking and running). The longest muscle in the body is in the thigh; it is known as the sartorius muscle. The sartorius muscle and the four bundles of muscles on each side of it called the quadriceps not only move the legs but also help us maintain our balance. The soleus and the gastrocnemius muscles in the calf work together with the tendocalcaneus (or Achilles tendon) in the ankle to lift the body onto the heel and feet. And, of course, the feet have a multitude of muscles which help us to mobilize ourselves while in an upright position. Although we've mentioned only a few of the anti-gravity muscles, the main point to make here is that these muscles have been trained to do their work only in the presence of gravity. To a certain degree, these muscles owe their importance and strength to gravity!
Whether large or small, the skeletal muscles can perform with extraordinary speed and power. Such qualities can be literally of life-or-death importance here on Earth, enabling the body to move in response to sudden and drastic changes in the external environment. Skeletal muscle can get into action within a few hundredths of a second (not a few hundred seconds, a few hundredths of a second), exert an enormous concentrated pull on the bone to which it is attached and, when necessary, support 1000 times its own weight. But, as you are about to learn, different muscle types are equipped to handle different levels of activity.
As mentioned previously, all muscle (including skeletal, visceral, and cardiac) moves by contracting itself. This unique characteristic distinguishes it from any other body tissue. In the case of skeletal muscle, the individual cells (which are also called fibers), ordinarily long and thin, become shorter and fatter under stimulus and take on their tremendous pulling power. Once the stimulus has passed, the muscle relaxes, settling back into its original shape. There are two primary kinds of fibers. Marathon runners typically develop a type of slow-moving but high-stamina fiber, which is named Type I or, "slow twitch." For instance, the soleus muscle in the calf has a high percentage of slow twitch muscle fibers and therefore is said to be the muscle that is used to a greater extent for prolonged lower leg muscle activity.
On the other hand, sprinters and power lifters typically develop a type of
high-speed, high-output fiber called Type
II or "fast twitch." For instance, the
gastrocnemius muscle in the calf has a higher percentage of fast twitch
fibers, giving it the capability of very forceful and rapid contraction of
the type used in jumping or for quick, powerful "bursts" of movement. The
average person has about half of one type and half of the other throughout
the body. We will discuss this later. The important point here is that all
muscle cells, fast twitch or slow twitch, operate by contracting
(shortening) the microscopic filaments that each muscle fiber contains.
Let's examine how this process works.