![]() INSERT FIGURE LIKE FOCUS FIGURE 10.1c IN MARIEB-11E For example, the deltoid muscle on the lateral side of the upper arm causes abduction of the shoulder. A muscle that crosses the lateral side of a joint results in abduction, which results in the body part moving away from the midline of the body.For example, the muscles in the posterior arm cause elbow extension. A muscle that crosses the posterior side of a joint results in extension, which results in an increase in joint angle with movement.For example, the anterior arm muscles cause elbow flexion. A muscle that crosses the anterior side of a joint results in flexion, which results in a decrease in joint angle with movement.For example, in the case of the knee, muscles of the posterior thigh cause knee flexion and anterior thigh muscles cause knee extension, which is opposite of the rules stated below for most other joints. ![]() There are four helpful rules that can be applied to all major joints except the ankle and knee because the lower extremity is rotated during development. 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. 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 are also muscles that do not pull against the skeleton for movements such as the muscles of facial expressions. In this case the hamstrings would be called the agonists and the quadriceps femoris would be called the antagonists. These terms are reversed for the opposite action, flexion of the leg at the knee. ![]() A set of antagonists called the hamstrings in the posterior compartment of the thigh are activated to slow or stop the movement. Antagonists play two important roles in muscle function: (1) they maintain body or limb position, such as holding the arm out or standing erect and (2) they control rapid movement, as in shadow boxing without landing a punch or the ability to check the motion of a limb.įor example, to extend the leg at 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 leg extension at the knee). The brachoradialis, in the forearm, and brachialis, located deep to the biceps in the upper arm, are both synergists that aid in this motion.Ī muscle with the opposite action of the prime mover is called an antagonist. ![]() Figure 11.1.1 – Prime Movers and Synergists: The biceps brachii flex the lower arm. A synergist can also be a fixator that stabilizes the muscle’s origin. Because it can be assisted by the brachialis, the brachialis is called a synergist in this action ( Figure 11.1.1). During forearm flexion, for example lifting a cup, a muscle called the biceps brachii is the prime mover. Upon activation, the muscle pulls the insertion toward the origin.Īlthough a number of muscles may be involved in an action, the principal muscle involved is called the prime mover, or agonist. The moveable end of the muscle that attaches to the bone being pulled is called the muscle’s insertion, and the end of the muscle attached to a fixed (stabilized) bone is called the origin. Interactions of Skeletal Muscles in the Body ![]()
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