The Length Tension Relationship and Muscle Tone

The amount of tension generated by a muscle, and therefore the force of its contraction, depends on how stretched or contracted it was before it was stimulated, among other

Optimum resting length

Optimum resting length

Length Tension Relationship

Sarcomere length (|im) before stimulation

Figure 11.12 The Length-Tension Relationship. Center: In a resting muscle fiber, the sarcomeres are usually 2.0 to 2.25 ^m long, the optimum length for producing maximum tension when the muscle is stimulated to contract. Note how this relates to the degree of overlap between the thick and thin filaments. Left: If the muscle is overly contracted, the thick filaments butt against the Z discs and the fiber cannot contract very much more when it is stimulated. Right: If the muscle is overly stretched, there is so little overlap between the thick and thin filaments that few cross-bridges can form between myosin and actin.

Sarcomere length (|im) before stimulation

Figure 11.12 The Length-Tension Relationship. Center: In a resting muscle fiber, the sarcomeres are usually 2.0 to 2.25 ^m long, the optimum length for producing maximum tension when the muscle is stimulated to contract. Note how this relates to the degree of overlap between the thick and thin filaments. Left: If the muscle is overly contracted, the thick filaments butt against the Z discs and the fiber cannot contract very much more when it is stimulated. Right: If the muscle is overly stretched, there is so little overlap between the thick and thin filaments that few cross-bridges can form between myosin and actin.

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factors. This principle is called the length-tension relationship. The reasons for it can be seen in figure 11.12. If a fiber is overly contracted at rest, its thick filaments are rather close to the Z discs. The stimulated muscle may contract a little, but then the thick filaments butt up against the Z discs and can go no farther. The contraction is therefore a weak one. On the other hand, if a muscle fiber is too stretched before it is stimulated, there is relatively little overlap between its thick and thin filaments. When the muscle is stimulated, its myosin heads cannot "get a good grip" on the thin filaments, and again the contraction is weak. (As mentioned in chapter 10, this is one reason you should not bend at the waist to pick up a heavy object. Muscles of the back become overly stretched and cannot contract effectively to straighten your spine against a heavy resistance.)

Between these extremes, there is an optimum resting length at which a muscle produces the greatest force when it contracts. The central nervous system continually monitors and adjusts the length of a resting muscle, maintaining a state of partial contraction called muscle tone. This maintains optimum length and makes the muscles ideally ready for action. The elastic filaments of the sarcomere also help to maintain enough myofilament overlap to ensure an effective contraction when the muscle is called into action.

Before You Go On

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

  1. What change does ACh cause in an ACh receptor? How does this electrically affect the muscle fiber?
  2. How do troponin and tropomyosin regulate the interaction between myosin and actin?
  3. Describe the roles played by ATP in the power and recovery strokes of myosin.
  4. What steps are necessary for a contracted muscle to return to its resting length?

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Responses

  • Andrea
    What steps are necessary for a contracted muscle to return to its resting length?
    7 years ago
  • thomas
    What is muscle fiber stimulation for the butt?
    7 years ago
  • H
    What is length tension relationship and muscle tone?
    3 years ago
  • joel
    What Is The Lengthtension Relationship And Muscle Tone.?
    2 years ago

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