Development of passive (viscoelastic) force

	HUMAN TISSUES, INCLUDING MUSCLES, BEHAVE LIKE SPRINGS; THEY ARE ELASTIC. At lengths greater than their resting length (l0), they develop tension or force. This force is passive, since it exists whether or not the muscle is active. Passive force, like active force, acts in a direction from the muscle’s points of attachment toward its center.

ELASTICITY OR STIFFNESS

	The figure (from Soderberg, 1986, p. 32) illustrates that muscles differ in their passive properties. Some are stiffer than others; they develop more force during a given amount of elongation. In Soderberg's figure, the stiffer the muscle, the steeper the slope of its passive stress-strain curve.

The distinctive stiffness properties of any soft tissue are illustrated by its length-tension curve. The knee's anterior cruciate ligament has a distinct length-tension curve.

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VISCOSITY OR DAMPING PROPERTIES

In addition to displaying the property of elasticity, many tissues display viscosity, a resistance to flow. Viscous materials resist elongation more at higher rates of elongation. Highly viscous fluids are "thick" and flow more slowly than "thinner," less viscous fluids.

	The passive length-tension curve of a viscous material has a slope that changes, increasing as the velocity of elongation increases. The faster the rate of elongation, the stiffer the material's behavior.

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THREE SOURCES OF STIFFNESS IN MUSCLE

1. Intrinsic muscle stiffness
   • connective tissue has its own length-tension properties

   • cross-bridges in inactive actin-myosin produce thixotropy, so that a muscle's stiffness depends on its recent movement history.

2. Stiffness due to voluntary muscle activity

3. Reflex stiffness

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References:

• Carey, J.R., & Burghardt, T.P. (1993). Movement dysfunction following central nervous system lesions: A problem of neurologic or muscular impairment. Physical Therapy, 73 , 538-547.

• Hagbarth, K-E. (1994). Evaluation of and methods to change muscle tone. Scandinavian Journal of Rehabilitation Medicine, 30 (Sup), 19-32.

• Nordin, M., & Frankel, V.H. (2001). Basic biomechanics of the musculoskeletal system. Chapter 6, pp. 148-171. Baltimore: Lippincott, Williams & Wilkins.

• Soderberg, G.L. (1986). Kinesiology: Application to pathological motion. Baltimore: Williams & Wilkins.

• Young, R.R. (1994). Spasticity: A review. Neurology,44,9,S12-20.

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Last updated 2-25-08 Dave Thompson PT
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