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Biomechanics of Tendon

Tendon possesses unique linear and viscoelastic properties, allowing it to transmit high muscular forces to the skele- ton with minimal elongation during muscle contraction. Subjecting tendons to various tensile loads and examining tendon response can help to define tendon biomechanical properties.

A classic load–elongation curve results from lengthening a tendon at a constant rate under tensile stress. Initially, little force is required to elongate the tendon as the wavy collagen pattern becomes straight because of fiber reorientation in the direction of stress. The first concave portion of the curve, which is known as the “toe” region, represents this phenomenon.

The second portion of the curve is linear as the tendon becomes stiffer and elongates at the molecular and fibrillar levels. Progressive failure of collagen begins in the third region, until complete failure and tendon rupture occur in an unpredictable, sudden episode.

The vast majority of physiologic tensile loads are well below the maximal force that is required to rupture  tendon, but athletes occasionally sustain higher loads and may experience tendon rupture.

Tendon also is viscoelastic, and it exhibits different behaviors at different rates of tensile loading. Tendons exhibit properties known as load relaxation and creep . Load relaxation occurs when a submaximal load is held at a fixed length in the “toe” region of the load–elongation curve.

Tensile stress will diminish rapidly at first, then stabilizes with time. Tendon creep occurs when load is held constant and tendon length is variable. Tendon elongation will increase rapidly before eventually stabilizing over time. These concepts also are important clinically.

For example, a patient with Achilles tendinitis will develop a plantar flexion contracture requiring casting in a neutral position to regain range of motion through the creep model.

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