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At the completion of this chapter, the reader will be able to:


  1. Outline the various roles of human skeletal muscle.

  2. List the various roles of muscle in the human body.

  3. Differentiate among muscle strength, endurance, and power.

  4. Describe strategies to increase muscle strength.

  5. List the different types of resistance that can be used to strengthen muscles.

  6. List the different types of muscle contractions and the advantages and disadvantages of each.

  7. Outline the various types of exercise progression and the components of each.

  8. Describe strategies to increase muscle endurance.

  9. Describe strategies to increase muscle power.

  10. Explain the basic principles behind plyometrics.

  11. Define delayed onset muscle soreness (DOMS) and explain why it occurs.

  12. Define senescence sarcopenia.

  13. List the changes that can occur with muscles during aging.

  14. Describe the concept of specificity of training.


Movement of the body or any of its parts involves considerable activity from those muscles directly responsible. Muscle is the only biological tissue capable of actively generating tension. This characteristic enables human skeletal muscle to perform the important functions of maintaining upright body posture, moving body parts, and absorbing shock. For body motions to take place, the muscles producing movement must have a stable base from which to work from. Muscles serve a variety of roles depending on the required movement:


  • Prime agonist. A muscle that is directly responsible for producing movement.
  • Synergist (supporter). Performs a cooperative muscle function in relation to the agonist. Synergists can function as stabilizers or neutralizers.


  • Stabilizers (fixators). Muscles that contract statically to steady or support some part of the body against the pull of the contracting muscles, against the pull of gravity, or against the effect of momentum and recoil in certain vigorous movements.
  • Neutralizers. Muscles that act to prevent an undesired action of one of the movers.


  • Antagonist. A muscle that has an effect opposite to that of the agonist.


Types of Muscle Contraction


The basic function of a muscle is to contract. The word contraction, used to describe the generation of tension within muscle fibers, conjures up an image of shortening of muscle fibers. However, a contraction can produce shortening or lengthening of the muscle, or no change in the muscle length. Thus, three types of contraction are commonly recognized (see Chap. 1): isometric, concentric, and eccentric.


  • Isometric contraction. Isometric exercises provide a static contraction with a variable and accommodating resistance without producing any appreciable change in muscle length.1
  • Concentric contraction. A concentric contraction (Fig. 12-1) produces a shortening of the muscle. This occurs when the tension generated by the agonist muscle is sufficient to overcome an external resistance and to move the body segment of one attachment toward the segment of its other attachment.1
  • Eccentric contraction. An eccentric contraction (Fig. 12-2) occurs when a muscle slowly lengthens as it gives in to an external ...

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