Isometric And Isotonic Contractions
Isometric contraction (A) sustains the position of a joint without producing movement. Concentric (B) and eccentric (C) contractions are isotonic contractions in which the muscle changes length: concentric contractions by shortening and eccentric contractions by actively controlled lengthening (relaxation).
CONTRACTION OF MUSCLES
Skeletal muscles function by contracting; they pull and never push. However, certain phenomena—such as “popping of the ears” to equalize air pressure and the musculovenous pump—take advantage of the expansion of muscle bellies during contraction. When a muscle contracts and shortens, one of its attachments usually remains fixed while the other (more mobile) attachment is pulled toward it, often resulting in movement. Attachments of muscles are commonly described as the origin and insertion; the origin is usually the proximal end of the muscle, which remains fixed during muscular contraction, and the insertion is usually the distal end of the muscle, which is movable. However, this is not always the case. Some muscles can act in both directions under different circumstances. For example, when doing push-ups, the distal end of the upper limb (the hand) is fixed (on the floor), and the proximal end of the limb and the trunk (of the body) are being moved. Therefore, this book usually uses the terms proximal and distal or medial and lateral when describing most muscle attachments. Note that if the attachments of a muscle are known, the action of the muscle can usually be deduced (rather than memorized). When studying muscle attachments, act out the action; you are more likely to learn things you have experienced.
Although skeletal muscles are also referred to as voluntary muscles, certain aspects of their activity are automatic ( reflexive ) and therefore not voluntarily controlled. Examples are the respiratory movements of the diaphragm, controlled most of the time by reflexes stimulated by the levels of oxygen and carbon dioxide in the blood (although we can willfully control it within limits), and the myotatic reflex , which results in movement after a muscle stretch produced by tapping a tendon with a reflex hammer.
Even when “relaxed,” the muscles of a conscious individual are almost always slightly contracted. This slight contraction, called tonic contraction or muscle tone (tonus), does not produce movement or active resistance (as phasic contraction does) but gives the muscle a certain firmness, assisting the stability of joints and the maintenance of posture, while keeping the muscle ready to respond to appropriate stimuli. Muscle tone is usually absent only when unconscious (as during deep sleep or under general anesthesia) or after a nerve lesion resulting in paralysis.
There are two main types of phasic (active) muscle contractions : (1) isotonic contractions , in which the muscle changes length in relationship to the production of movement, and (2) isometric contractions , in which muscle length remains the same—no movement occurs, but the force (muscle tension) is increased above tonic levels to resist gravity or other antagonistic force. The latter type of contraction is important in maintaining upright posture and when muscles act as fixators or shunt muscles as described below.
There are two types of isotonic contractions. The type we most commonly think of is concentric contraction , in which movement occurs as a result of the muscle shortening—for example, when lifting a cup, pushing a door, or striking a blow. The ability to apply exceptional force by means of concentric contraction often is what distinguishes an athlete from an amateur. The other type of isotonic contraction is eccentric contraction , in which a contracting muscle lengthens—that is, it undergoes a controlled and gradual relaxation while continually exerting a (diminishing) force, like playing out a rope. Although people are generally not as aware of them, eccentric contractions are as important as concentric contractions for coordinated, functional movements such as walking, running, and setting objects (or one’s self) down.
Often, when the main muscle of a particular movement (the prime mover ) is undergoing a concentric contraction, its antagonist is undergoing a coordinated eccentric contraction. In walking, we contract concentrically to pull our center of gravity forward, and then as it passes ahead of the limb, we contract eccentrically to prevent a lurching during the transfer of weight to the other limb. Eccentric contractions require less metabolic energy at the same load but, with a maximal contraction, are capable of generating much higher tension levels than concentric contractions—as much as 50% higher (Marieb, 2016).
Source: Clinically Oriented Anatomy
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