Agonists are also referred to as prime movers since they are the muscles that are primarily responsible for generating the movement. These muscles act in opposition to the movement generated by the agonists and are responsible for returning a limb to its initial position. These muscles perform, or assist in performing, the same set of joint motion as the agonists. Synergists are sometimes referred to as neutralizers because they help cancel out, or neutralize, extra motion from the agonists to make sure that the force generated works within the desired plane of motion.
These muscles provide the necessary support to assist in holding the rest of the body in place while the movement occurs. Fixators are also sometimes called stabilizers. This is a contraction in which no movement takes place, because the load on the muscle exceeds the tension generated by the contracting muscle.
This occurs when a muscle attempts to push or pull an immovable object. This is a contraction in which movement does take place, because the tension generated by the contracting muscle exceeds the load on the muscle.
This occurs when you use your muscles to successfully push or pull an object. Isotonic contractions are further divided into two types:. This is a contraction in which the muscle decreases in length shortens against an opposing load, such as lifting a weight up. This is a contraction in which the muscle increases in length lengthens as it resists a load, such as lowering a weight down in a slow, controlled fashion.
Although a number of muscles may be involved in an action, the principal muscle involved is called the prime mover , or agonist. To lift a cup, a muscle called the biceps brachii is actually the prime mover; however, because it can be assisted by the brachialis, the brachialis is called a synergist in this action Figure 1.
A muscle with the opposite action of the prime mover is called an antagonist. Antagonists play two important roles in muscle function:.
For example, to extend the knee, a group of four muscles called the quadriceps femoris in the anterior compartment of the thigh are activated and would be called the agonists of knee extension. However, to flex the knee joint, an opposite or antagonistic set of muscles called the hamstrings is activated. As you can see, these terms would also be reversed for the opposing action.
If you consider the first action as the knee bending, the hamstrings would be called the agonists and the quadriceps femoris would then be called the antagonists. See Table 1 for a list of some agonists and antagonists. There are also skeletal muscles that do not pull against the skeleton for movements. For example, there are the muscles that produce facial expressions. The insertions and origins of facial muscles are in the skin, so that certain individual muscles contract to form a smile or frown, form sounds or words, and raise the eyebrows.
There also are skeletal muscles in the tongue, and the external urinary and anal sphincters that allow for voluntary regulation of urination and defecation, respectively. In addition, the diaphragm contracts and relaxes to change the volume of the pleural cavities but it does not move the skeleton to do this. When exercising, it is important to first warm up the muscles. Stretching pulls on the muscle fibers and it also results in an increased blood flow to the muscles being worked.
Without a proper warm-up, it is possible that you may either damage some of the muscle fibers or pull a tendon. A pulled tendon, regardless of location, results in pain, swelling, and diminished function; if it is moderate to severe, the injury could immobilize you for an extended period. Recall the discussion about muscles crossing joints to create movement. Most of the joints you use during exercise are synovial joints, which have synovial fluid in the joint space between two bones.
Exercise and stretching may also have a beneficial effect on synovial joints. Synovial fluid is a thin, but viscous film with the consistency of egg whites. When you first get up and start moving, your joints feel stiff for a number of reasons. After proper stretching and warm-up, the synovial fluid may become less viscous, allowing for better joint function. Skeletal muscle is enclosed in connective tissue scaffolding at three levels. Each muscle fiber cell is covered by endomysium and the entire muscle is covered by epimysium.
Fascicle arrangement by perimysia is correlated to the force generated by a muscle; it also affects the range of motion of the muscle. Based on the patterns of fascicle arrangement, skeletal muscles can be classified in several ways. What follows are the most common fascicle arrangements.
Parallel muscles have fascicles that are arranged in the same direction as the long axis of the muscle Figure 2. The majority of skeletal muscles in the body have this type of organization. Some parallel muscles are flat sheets that expand at the ends to make broad attachments. The amounts of certain vitamins and minerals that you eat, especially vitamin D and calcium , directly affect how much calcium is stored in the bones.
In this soft bone is where most of the body's blood cells are made. The bone marrow contains stem cells, which produce the body's red blood cells and platelets, and some types of white blood cells. Red blood cells carry oxygen to the body's tissues, and platelets help with blood clotting when someone has a cut or wound.
White blood cells help the body fight infection. Bones are fastened to other bones by long, fibrous straps called ligaments pronounced: LIG-uh-mentz. Cartilage pronounced: KAR-tul-ij , a flexible, rubbery substance in our joints, supports bones and protects them where they rub against each other.
The bones of kids and young teens are smaller than those of adults and contain "growing zones" called growth plates. These plates consist of multiplying cartilage cells that grow in length, and then change into hard, mineralized bone. These growth plates are easy to spot on an X-ray. Because girls mature at an earlier age than boys, their growth plates change into hard bone at an earlier age.
Bone-building continues throughout life, as a body constantly renews and reshapes the bones' living tissue. Bone contains three types of cells:.
Muscles pull on the joints, allowing us to move. They also help the body do such things as chewing food and then moving it through the digestive system. Even when we sit perfectly still, muscles throughout the body are constantly moving. Muscles help the heart beat, the chest rise and fall during breathing, and blood vessels regulate the pressure and flow of blood. When we smile and talk, muscles help us communicate, and when we exercise, they help us stay physically fit and healthy.
The movements your muscles make are coordinated and controlled by the brain and nervous system. The involuntary muscles are controlled by structures deep within the brain and the upper part of the spinal cord called the brain stem.
The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum pronounced: ser-uh-BEL-um. When you decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, causing them to contract. The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa.
The cerebellum coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints send messages back through peripheral nerves to tell the cerebellum and other parts of the brain where and how the arm or leg is moving and what position it's in. This feedback results in smooth, coordinated motion. If you want to lift your arm, your brain sends a message to the muscles in your arm and you move it. When you run, the messages to the brain are more involved, because many muscles have to work in rhythm.
0コメント