<P> Tendon length is determined by genetic predisposition, and has not been shown to either increase or decrease in response to environment, unlike muscles, which can be shortened by trauma, use imbalances and a lack of recovery and stretching . </P> <P> Traditionally, tendons have been considered to be a mechanism by which muscles connect to bone, functioning to transmit forces . This connection allows tendons to passively modulate forces during locomotion, providing additional stability with no active work . However, over the past two decades, much research focused on the elastic properties of some tendons and their ability to function as springs . Not all tendons are required to perform the same functional role, with some predominantly positioning limbs, such as the fingers when writing (positional tendons) and others acting as springs to make locomotion more efficient (energy storing tendons). Energy storing tendons can store and recover energy at high efficiency . For example, during a human stride, the Achilles tendon stretches as the ankle joint dorsiflexes . During the last portion of the stride, as the foot plantar - flexes (pointing the toes down), the stored elastic energy is released . Furthermore, because the tendon stretches, the muscle is able to function with less or even no change in length, allowing the muscle to generate greater force . </P> <P> The mechanical properties of the tendon are dependent on the collagen fiber diameter and orientation . The collagen fibrils are parallel to each other and closely packed, but show a wave - like appearance due to planar undulations, or crimps, on a scale of several micrometers . In tendons, the collagen fibres have some flexibility due to the absence of hydroxyproline and proline residues at specific locations in the amino acid sequence, which allows the formation of other conformations such as bends or internal loops in the triple helix and results in the development of crimps . The crimps in the collagen fibrils allow the tendons to have some flexibility as well as a low compressive stiffness . In addition, because the tendon is a multi-stranded structure made up of many partially independent fibrils and fascicles, it does not behave as a single rod, and this property also contributes to its flexibility . </P> <P> The proteoglycan components of tendons also are important to the mechanical properties . While the collagen fibrils allow tendons to resist tensile stress, the proteoglycans allow them to resist compressive stress . These molecules are very hydrophilic, meaning that they can absorb a large amount of water and therefore have a high swelling ratio . Since they are noncovalently bound to the fibrils, they may reversibly associate and disassociate so that the bridges between fibrils can be broken and reformed . This process may be involved in allowing the fibril to elongate and decrease in diameter under tension . However, the proteoglycans may also have a role in the tensile properties of tendon . The structure of tendon is effectively a fibre composite material, built as a series of hierarchical levels . At each level of the hierarchy, the collagen units are bound together by either collagen crosslinks, or the proteoglycans, to create a structure highly resistant to tensile load . The elongation and the strain of the collagen fibrils alone have been shown to be much lower than the total elongation and strain of the entire tendon under the same amount of stress, demonstrating that the proteoglycan - rich matrix must also undergo deformation, and stiffening of the matrix occurs at high strain rates . This deformation of the non-collagenous matrix occurs at all levels of the tendon hierarchy, and by modulating the organisation and structure of this matrix, the different mechanical properties required by different tendons can be achieved . Energy storing tendons have been shown to utilise significant amounts of sliding between fascicles to enable the high strain characteristics they require, whilst positional tendons rely more heavily on sliding between collagen fibres and fibrils . However, recent data suggests that energy storing tendons may also contain fascicles which are twisted, or helical, in nature - an arrangement that would be highly beneficial for providing the spring - like behaviour required in these tendons . </P>

Where are tendons located in the human body