CHAPTER 3 Structure and Function of Muscles


63


Iliac crest


Tensor fascia latae


Iliotibial band


Gluteus maximus


Inferior extensor retinaculum


Superior extensor retinaculum


Patella


AB


Figure 3.12 (A) The superior and inferior retinaculum on the dorsal surface of the foot hold the extensor tendons in a secure position. (B) The fascial iliotibial tract transmits muscle forces along the lateral surface of the thigh before inserting distal to the knee joint.


the muscle lengthens until it reaches its critical length. When a muscle lengthens, the noncontractile fascia is also stretched and is responsible for the passive tension generated. An analogy of this tension and stored energy in the connective tissue is stretching a rubber band. When the rubber band is released, the stored energy is released. As the muscle tissue is elongated past its critical length, there is a sharp increase in passive tension produced by the noncontractile connective tissue. If the muscle continues to be stretched, it will reach a level of passive tension that will result in tissue rupture or failure. Figure 3.13 illustrates the relationship between the passive tension generated in muscle tissue as it is lengthened. Taking up the slack is depicted before the muscle reaches its critical length. Once reaching this length, there is a sharp increase in tension as the muscle tissue is further stretched.


Lengthening the connective tissue can augment the function of the muscle. The release of the connective tissue force adds to any force produced by the muscle and, up to a point, can result in a greater force produced by the muscle. An example of how this passive tension assists with function is seen at the end of stance phase during walking just before the heel comes off the ground. When the foot is fl at on the ground and the center of mass is shifting over the foot, the calf muscles and Achil- les tendon are stretched. The stored energy of this stretch


Critical length


Slack


Muscle Length Tension


Figure 3.13 The passive length-tension curve. The graph illustrates a generalized concept concerning the relationship between the passive tension generated in muscle tissue as it is increasingly stretched. Although the steepness of the curve varies among muscles depending on the fiber length and muscle architecture, the relationship between the two factors remains the same.


is released as the heel comes off the ground and the calf muscles contract. Together, the passive tension of the stretched connective tissue and the active tension pro- duced by the muscles provide propulsion to advance the body forward. Because the connective tissue has viscoelas- tic properties, increasing the force and speed of stretch


Passive tension in muscle


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