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9.3E: Types of Muscle Contractions: Isotonic and Isometric

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    Muscle contractions are defined by changes in the length of the muscle during contraction.

     

    LEARNING OBJECTIVES

     

    Differentiate among the types of muscle contractions

     

    KEY TAKEAWAYS

    Key Points

     

    • Isotonic contractions generate force by changing the length of the muscle and can be concentric contractions or eccentric contractions.
    • A concentric contraction causes muscles to shorten, thereby generating force.
    • Eccentric contractions cause muscles to elongate in response to a greater opposing force.
    • Isometric contractions generate force without changing the length of the muscle.

     

    Key Terms

     

    • Isometric: A muscular contraction in which the length of the muscle does not change.
    • isotonic: A muscular contraction in which the length of the muscle changes.
    • eccentric: An isotonic contraction where the muscle lengthens.
    • concentric: An isotonic contraction where the muscle shortens.

    A muscle fiber generates tension through actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same. Although the term contraction implies shortening, when referring to the muscular system, it means the generation of tension within a muscle fiber. Several types of muscle contractions occur and are defined by the changes in the length of the muscle during contraction.

     

    Isotonic Contractions

     

    Isotonic contractions maintain constant tension in the muscle as the muscle changes length. Isotonic muscle contractions can be either concentric or eccentric.

     

    Concentric Contractions

     

    A concentric contraction is a type of muscle contraction in which the muscles shorten while generating force, overcoming resistance. For example, when lifting a heavy weight, a concentric contraction of the biceps would cause the arm to bend at the elbow, lifting the weight towards the shoulder. Cross-bridge cycling occurs, shortening the sarcomere, muscle fiber, and muscle.

     

    Eccentric Contractions

     

    An eccentric contraction results in the elongation of a muscle while the muscle is still generating force; in effect, resistance is greater than force generated. Eccentric contractions can be both voluntary and involuntary. For example, a voluntary eccentric contraction would be the controlled lowering of the heavy weight raised during the above concentric contraction. An involuntary eccentric contraction may occur when a weight is too great for a muscle to bear and so it is slowly lowered while under tension. Cross-bridge cycling occurs even though the sarcomere, muscle fiber, and muscle are lengthening, controlling the extension of the muscle.

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    Types of Muscle Contraction: An isotonic concentric contraction results in the muscle shortening, an isotonic eccentric contraction results in the muscle lengthening. During an isometric contraction the muscle is under tension but neither shortens nor lengthens.

     

    Isometric Contractions

     

    In contrast to isotonic contractions, isometric contractions generate force without changing the length of the muscle, common in the muscles of the hand and forearm responsible for grip. Using the above example, the muscle contraction required to grip but not move a heavy object prior to lifting would be isometric. Isometric contractions are frequently used to maintain posture.

    Isometric contractions are sometimes described as yielding or overcoming.

     

    Yielding

     

    A yielding contraction occurs when a muscle contraction is opposed by resistance. For example, when holding a heavy weight steady, neither raising nor lowering it.

     

    Overcoming

     

    An overcoming contraction occurs when a muscle contraction is opposed by an immovable object, such as the contraction generated in the muscles when pushing against a wall.

    In both instances, cross-bridge cycling is maintaining tension in the muscle; the sarcomere, muscle fibers, and muscle are not changing length.

     

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