Muscles make up the major part of fleshy portions of the body and account for one half of body weight. Muscles vary in proportion to body size, and the shape of the body is determined by muscles covering bones.
Muscles account for 40 percent of our total body weight; the skeleton accounts for only about 18 percent.
Muscular expanding and contracting doesn’t just happen in your biceps. It happens all over the body. Muscles support and maintain posture and produce body heat. They help form many internal organs and regulate the work those organs do behind the scenes (such as the heart, uterus, lungs, and intestines) even when the body is not moving. The muscles of arteries, intestines, heart, and stomach, for example, are always at work even when we aren’t thinking of them. However, the silent work muscles do inside your body is wholly different from more obvious muscular work done by your arms and legs, for example.Internal movement involves the contraction and relaxation of involuntary muscles, muscles that we cannot consciously control. For example, heartbeats are performed by cardiac muscles. Breathing and digestion are facilitated by muscles called visceral (involuntary) muscles, whereas external movement is accomplished by contraction and relaxation of muscles that are attached to bones. The muscles that provide this external movement are known as voluntary muscles, as they perform movements on command.
All bodily movement, whether lifting of an arm, or the beating of the heart, involves the contraction and relaxation of voluntary or involuntary muscles.
Classes of muscles
The class system is alive and well, at least as far as your muscles are concerned. There are three classes of muscles: skeletal, visceral, and cardiac.- Cardiac (involuntary striated) muscle has branching fibers and forms most of the wall of the heart. Its contraction produces the heartbeat.
- Skeletal (voluntary striated, meaning striped) muscles, are attached to the skeleton. They are called voluntary, of course, because they are controlled by your will. This type of muscle can be easily seen by flexing the forearm, which makes the biceps muscle become hard and thick.
- Visceral (involuntary smooth) muscle is found in the stomach, intestines, and blood vessels, and cannot be controlled at will.
Unlike other muscle, cardiac (heart) muscle keeps beating even when removed from the body, as in a heart transplant. And even if it stops beating, it can be jump-started with an external electrical charge. Not so with the other muscles.
Types of muscles
Oh, if it were only that easy. But there are also types of muscles. At first glance, the types of muscles are the same as the classes of muscle. But pay close attention and you’ll see there are subtle differences. There are three types of muscles in the body.Striated muscle
Striated muscles are also called skeletal or voluntary muscles. These are the muscles that move all the bones, as well as the face and the eyes. The body is able to consciously control the activity of a striated muscle.Smooth muscle
The second type of muscle is smooth muscle, also known as visceral, involuntary or unstriated muscle. The body has no conscious control over smooth muscles, which move the internal organs such as the digestive tract. The smooth muscles are also found in blood vessels and secretory ducts leading from glands.Skeletal muscle fibers are arranged in bundles, but smooth muscles form sheets of fibers that wrap around tubes and vessels.
Cardiac muscles
The third type of muscle is cardiac muscle. It is striated in appearance but is like smooth muscle in its actions. Movement of cardiac muscle cannot be consciously controlled. Cardiac muscle has branching fibers forming most of the wall of the heart and controlling the contractions producing the heartbeat.Muscles and tendons
Now that you know the classes and types of muscles, let’s take a more in-depth look at how they work. You already know that skeletal muscles, or striated muscles, are the muscles that move the bones of the body. Now get ready for the scoop on what makes it possible.When a muscle contracts, one of the attached bones remains stationary, as a result of other muscles holding it in place. The point of attachment of the muscle to the stationary bone is called the origin or beginning of that muscle. When the muscle contracts, another bone to which it is attached, does move. The junction of the muscle to the bone that moves is called the insertion of the muscle. Near the point of insertion, the muscle narrows and is connected to the bone by a tendon. One type of tendon that helps attach bone to muscles is called an aponeurosis.
It takes 200 muscles working together to take a step.
Roundup of the superficial muscles
Sometimes, being superficial isn’t a bad thing. Take, for example, your superficial muscles, so named because these are the muscles you’re most likely to see with the naked eye. These workhorses of the muscular system help make you unique. Though they all have complicated-sounding names, they help your body perform everyday functions like picking up objects and smiling.The following figures illustrate four major muscle groups.
Illustration by Kathryn BornPosterior view of the neck and shoulder muscles.
- Arm muscles consist of the upper arm muscles, biceps brachii and triceps brachii. In the forearm (lower arm) are the flexor and extensor muscles of the hands and fingers.
- Head and face muscles include the frontalis, temporalis, orbicularis oculi, orbicularis oris, occipitalis, mentalis, buccinator, zygomatic major and minor, and the masseter
- Shoulder and neck muscles include the sternocleidomastoid, pectoralis major, latissimus dorsi, and trapezius muscle, leading to the deltoid muscle of the shoulder.
- The major chest and abdominal muscles consist of the diaphragm, pectoralis major, the rectus abdominis, and the external oblique. Also associated with this region is the linea alba. The linea alba (meaning “white line”) is a vertical band of connective muscular tissue that begins at the xiphoid process (sternum) and ends at the symphysis pubis (where the iliac bones join at the front of the pelvis).
- The major muscles of the back include the seventh cervical vertebral muscle, infraspinatus, supraspinatus, latissimus dorsi, and the rhomboid major muscle.
Illustration by Kathryn BornAnterior muscles of the chest and abdomen.
Illustration by Kathryn BornThe muscles of the upper limb, anterior (A) and posterior (B).
The seventh cervical vertebra muscle is a muscle, whereas the seventh vertebra is a bone. Many muscles, tendons, and ligaments have the same name but don’t have the same function. In this case, the seventh cervical muscle is a point of attachment aiding in support and movement of head and neck.
- The pelvis and anterior thigh muscles include the tensor fascia lata, the adductors of the thigh, the vastus lateralis and vastus medialis, the rectus femoris, and the
- The lower leg muscles from the knee to the ankle includes the gastrocnemius, which makes up a large portion of the calf muscles, the tibialis anterior, soleus, peroneus longus, and peroneus brevis. By the way, things aren’t always what they seem: The Achilles tendon is technically classified as a muscle.
- From the back, the buttocks are composed of the gluteus maximus and gluteus medius. In the thigh are the adductor magnus, vastus lateralis, and gracilis, whereas the biceps femoris, semitendinosus, and semimembranosus combined comprise the hamstrings.
Illustration by Kathryn BornThe muscles of the lower limb.
So, the moral of our muscle story is teamwork. Bones cannot move alone without being attached to muscle. Muscles cannot move alone, without being attached to stationary bones to allow support for that movement. Neither bones nor muscles could function without the attachments provided by the tendons and ligaments. Body movement then, is made possible by the bones and skeletal muscles working together in addition to the visceral and cardiac muscles that function to maintain the muscular rhythm of our vital organs. It’s all one big, happy family of muscles.