Isometric and Isotonic Contraction

In muscle physiology, "contraction" does not always mean the shortening of a muscle—it may mean only that the muscle is producing internal tension while an external resistance causes it to stay the same length or even to become longer. Thus, physiologists speak of different kinds of muscle contraction as isometric versus isotonic and concentric versus eccentric.

Saladin: Anatomy & I 11. Muscular Tissue I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

426 Part Two Support and Movement

426 Part Two Support and Movement

Isotonic Contraction

Figure 11.16 Isometric and Isotonic Contraction. (a) Isometric contraction, in which a muscle develops tension but does not shorten. This occurs at the beginning of any muscle contraction but is prolonged in actions such as lifting heavy weights. (b) Isotonic concentric contraction, in which the muscle shortens while maintaining a constant degree of tension. In this phase, the muscle moves a load. (c) Isotonic eccentric contraction, in which the muscle maintains tension while it lengthens, allowing a muscle to relax without going suddenly limp. Name a muscle that undergoes eccentric contraction as you sit down in a chair.

Figure 11.16 Isometric and Isotonic Contraction. (a) Isometric contraction, in which a muscle develops tension but does not shorten. This occurs at the beginning of any muscle contraction but is prolonged in actions such as lifting heavy weights. (b) Isotonic concentric contraction, in which the muscle shortens while maintaining a constant degree of tension. In this phase, the muscle moves a load. (c) Isotonic eccentric contraction, in which the muscle maintains tension while it lengthens, allowing a muscle to relax without going suddenly limp. Name a muscle that undergoes eccentric contraction as you sit down in a chair.

Suppose you lift a heavy box of books from a table. When you first contract the muscles of your arms, you can feel the tension building in them even though the box is not yet moving. At this point, your muscles are contracting at a cellular level, but their tension is being absorbed by the series-elastic components and is resisted by the weight of the load; the muscle as a whole is not producing any external movement. This phase is called isometric10 contraction—contraction without a change in length (fig. 11.16a). Isotonic11 contraction—contraction with a change in length but no change in tension—begins when internal tension builds to the point that it overcomes the resistance. The muscle now shortens, moves the load, and maintains essentially the same tension from then on (fig. 11.16b). Isometric and isotonic contraction are both phases of normal muscular action (fig. 11.17).

There are two forms of isotonic contraction— concentric and eccentric. In concentric contraction, a muscle shortens as it maintains tension—for example, when the biceps brachii contracts and flexes the elbow. In an eccentric contraction, a muscle lengthens as it maintains tension. If you set that box of books down again (fig. 11.16c), your biceps brachii lengthens as you extend your elbow, but it maintains tension to act as a brake and keep you from simply dropping the box. A weight lifter

Isotonic And Isometric Contraction

Figure 11.17 Isometric and Isotonic Phases of Contraction.

At the beginning of a contraction (isometric phase), muscle tension rises but the length remains constant (the muscle does not shorten). When tension overcomes the resistance of the load, the tension levels off and the muscle begins to shorten and move the load (isotonic phase).

How would you extend this graph in order to show eccentric contraction?

Figure 11.17 Isometric and Isotonic Phases of Contraction.

At the beginning of a contraction (isometric phase), muscle tension rises but the length remains constant (the muscle does not shorten). When tension overcomes the resistance of the load, the tension levels off and the muscle begins to shorten and move the load (isotonic phase).

How would you extend this graph in order to show eccentric contraction?

I0iso = same, uniform + metr = length

Saladin: Anatomy & I 11. Muscular Tissue I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 11 Muscular Tissue 427

uses concentric contraction when lifting a barbell and eccentric contraction when lowering it to the floor.

In summary, during isometric contraction, a muscle develops tension without changing length, and in isotonic contraction, it changes length while maintaining constant tension. In concentric contraction, a muscle maintains tension as it shortens, and in eccentric contraction, it maintains tension while it is lengthening.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. Explain how warm-up is related to treppe and why it improves athletic performance.
  2. Explain the role of tetanus in normal muscle action.
  3. Describe an everyday activity not involving the arms in which your muscles would switch from isometric to isotonic contraction.
  4. Describe an everyday activity not involving the arms that would involve concentric contraction and one that would involve eccentric contraction.

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Responses

  • MHRET
    Is lifting a box from floor isotonic?
    7 years ago
  • Stefanie
    What is isometric tension physiology?
    1 month ago

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