Chapter 9: Gross Anatomy and Functions of Skeletal Muscles

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.

Classes of Levers

Levers are probably the most common simple machine because just about anything that has a handle on it has a lever attached. The point on which the lever moves is called the fulcrum. By changing the position of the fulcrum, you can gain extra power with less effort.

In a class I lever system, the fulcrum is located between the pull and the weight. A child’s seasaw is an example of this type of lever. The children on the seasaw alternate between being the weight and the pull across a fulcrum in the center of the board. The head is an example of this type of lever in the body. The atlanto-occipital joint is the fulcrum, the posterior neck muscles provide the pull depressing the back of the head, and the face, which is elevated, is the weight. With the weight balanced over the fulcrum, only a small amount of pull is required to lift a weight. For example, only a very small shift in weight is needed for one child to lift the other on a seasaw. This system is quite limited, however, as to how much weight can be lifted and how high it can be lifted. For example, consider what happens when the child on one end of the seasaw is much larger than the child on the other end.

In a class II lever system, the weight is located between the fulcrum and the pull. An example is a wheelbarrow; the wheel is the fulcrum and the person lifting on the handles provides the pull. The weight, or load, carried in the wheelbarrow is placed between the wheel and the operator. In the body, an example of a class II lever is depression of the mandible.

In a class III lever system, the most common type in the body, the pull is located between the fulcrum and the weight. An example is a person using a shovel. The hand placed on the part of the handle closest to the blade provides the pull to lift the weight, such as a shovelful of dirt, and the hand placed near the end of the handle acts as the fulcrum. In the body, the action of the biceps brrachii muscle pulling on the radius to flex the elbow and elevate the hand is an example of a class III lever. This type of lever system does not allow as great a weight to be lifted, but the weight can be lifted a greater distance.

Here is a video about levers:

MUSCLES FUNCTIONS

Muscles which move the shoulder and their action

• Levator scapulae - Raises shoulder blade
• Pectoralis minor - Lowers shoulder blade
• Trapezius - Lifts clavicle. Adducts, elevates and rotates scapular outwards
• Rhomboideus major - Adducts scapular and rotates it inwards
• Serratus anterior - Stabilises scapula when hand exerts pressure on an object

Muscles which move the arm and their action

• Pectoralis major - Flexes, adducts and rotates arm medially
• Latissimus dorsi - Extends, adducts and rotates arm medially. Moves arm downward and backwards
• Deltoid - Abducts, flexes, extends and medially and laterally rotates arm
• Teres major - Extends arm, assists in adduction and medial rotation of arm

Muscles which move the forearm and wrist and their action

• Biceps brachii - Flexes and supinates forearm. Flexes arm
• Brachialis - Flexes the forearm
• Brachoradialis - Flexes, semi-supinates and semi-pronates the forearm
• Triceps brachii - Extends forearm. Extends arm
• Pronator teres - Pronates and flexes forearm
• Pronator quadratus - Pronates the forearm and hand
• Supinator - Supinates forearm and hand

Muscles which move the abdominal wall and their action

• Rectus abdominis - Compresses abdomen and flexes vertebral column
• External obliques - Bends vertebral column laterally and rotates vertebral column
• Transversus abdominis - Compresses abdomen
• Quadratus lumborum - Side flexion

Muscles which move the vertebral column and their action

• Iliocostalis lumborum - Extends lumbar region
• Iliocostalis thoracis - Maintains the spine's erect position
• Iliocostalis cervicis - Extends cervical region
• Longissimus thoracis - Extends thoracis region
• Longissimus cervicis - Extends cervical region
• Longissimus capitis - Extends the head and rotates it to opposite side
• Spinalis thoracis - Extends vertebral column
• Spinalis cervicis - Extends vertebral column
• Spinalis capitis - Extends vertebral column

Muscles which move the thigh and their action

• Psoas major - Flexes and rotates thigh medially and flexes vertebral column
• Iliacus - Flexes and rotates thigh medially and flexes vertebral column
• Gluteus maximus - Extends and rotates thigh laterally.
• Adductor longus - Adducts, medially rotates and flexes the thigh
• Adductor brevis - Adducts, laterally rotates and flexes the thigh
• Adductor magnus - Adducts, flexes, laterally rotates and extends the thigh.

Muscles which act on the leg and their action

• Rectus femoris - Extends knee and flexes hip
• Vastus lateralis - Extends knee
• Vastus medialis - Extends knee
• Vastus intermedius - Extends knee
• Sartorius - Flexes knee. Flexes hip and rotates femur laterally
• Biceps femoris - Flexes leg and extends thigh
• Semitendinosus - Flexes leg and extends thigh
• Semimembranosus - Flexes leg and extends thigh

Muscles which move the foot and their action

• Tibialis anterior - Dorsiflexes and inverts foot
• Peroneus tertius - Dorsiflexes and everts foot
• Gastrocnemius - Plantar flexes foot and flexes knee
• Soleus - Plantar flexes foot
• Plantaris - Plantar flexes foot
• Tibialis posterior - Plantar flexes and inverts the foot
• Peroneus longus - Plantar flexes and everts the foot
• Peroneus brevis - Plantar flexes and everts the foot

Muscles in the human body

Muscles of the back

Muscle of upper arm

Muscles in the arm

Muscle in the upper leg

Muscles in the lower leg

Muscles in the lower calf

Here’s a video to understand better the muscles of our body

I know it’s very hard to memorize these muscles because of it’s complicated names, numbers, different functions and it’s very hard to distinguish what is what because it all looks the same. I had a hard time also memorizing these things, but through the help of diligence and brainstorming with friends, I memorized most of the muscles of the body.

Here’s a link to tutorials and quizzes about muscles that would help you sharpen your mund about muscles:

FUN QUIZZES

End of Chapter 9