High force versus Low force
The folowing is direct from one of my physiology text books called Sports Injury Assessment and Rehabilitation, David C. Reid, b.p.t.,M.D.
Chapter 4 - Connective tissue healing and classification of ligament and tendon pathology
Stetching refers to the process of elongation and may be achieved by either elastic or plastic deformation. Elastic deformation is spring-like, the stretched material recovering its pretensile dimensions after the applied force is removed. Plastic deformation refers to a putty-like behavior. where the linear deformation produced by the tensile stress remains even after the stress is removed. Plastic deformation is nonrecoverable, or permanent, elongation. Materials that have viscous properties are characterized by plastic deformation. Viscoelastic materials exhibit both viscous and elastic behavior, a point that is important to appreciate as connective tissue has viscoelastic properties when submitted to stretch or tensile stress.
When connective tissue is stretched, some of the deformation occurs in the elastic elements and some occurs as plastic deformation in the viscous elements. With withdrawl of the tensile stress, the elastic deformation recovers, but the plastic deformation remains. The deformation of connective tissue varies widely depending on the amount, duration, and speed of application of stress, as well as the tissue temperature. Attempts at gaining a permanent increase of range of motion should make use of the conditions that are conducive to plastic deformation: (1) increased tissue temperature; (2) slow, prolonged stretching; and (3) long duration. These concepts and data on which they are based are examined further.
High Force versus Low Force
The amount of stretching achieved by tensile forces is proportional to the amount of force. Also, the corollary that a low force stretching technique requires more time to produce an equal amount of stretching is also true. However, the proportion of tissue lengthening that remains after tensile stress is removed is greater for the low force, long duration method, evidencing its influence on the plastic or viscous elements. High force, short duration stretching favors the recoverable, elastic-type deformation. This principle does not necessarily prohibit the use of high force, prolonged duration stertching, but obviously high force application may generate pain, trigger spasm, and produce tissue rupture.
Furthermore, elongation of connectiove tissue is accompanied by some structural weakening, and highforce stretching appears to produce more structural weakening for a given amount of stretch. Hence low force, prolonged duration stretching is usually a more comfortable, safer, and effective method.
Temperature has a significant effect on the behavior of connective tissue. Therapeutic heat is usually within the range of 102 to 110 degrees F. Using selected modalities to raise connective tissue temperature to 103F increases the amount of permanent elongation resulting from a given amount of stretcing. At 104 F and above there is a thermal transition in the microstructure of collagen that significantly enhances the viscous stress relaxation of collagen tissue, allowing greater plastic deformation. The mechanism by which this occurs is probably partial destabilization of the intermolecular bonding, allowing molecules to ” creep ” thereby enhancing the viscous flow properties of the tissue.
Evidently there are also events during the cooling phase that eventually influence the permanent deformation. Tissues that are stretched under heating conditions and the allowed to cool under tensile force maintain a greater proportion of therir plastic deformation than do structures allowed to cool in the unloaded state. Cooling under tension may allow the collagenous microstructure to stabilize at the new stretched length.
A further point worthy of consideration is th fact that at temperatures within the normal therapeutic range the amount of structural weakening produced by a given amount of connective tissue elongation varies inversely with the temperature. This fact is probably related to the thermal destabilization of the molecular bonds, which allows creeping of the tissue with less structural damage.
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