Editing Tendon (section)

==Functions==
[[File:Magnified view of a Tendon.jpg|thumb|261x261px|Magnified view of a tendon]]
Traditionally, tendons have been considered to be a mechanism by which muscles connect to bone as well as muscles itself, 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 has 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).<ref name="pmid23718692">{{cite journal | vauthors = Thorpe CT, Birch HL, Clegg PD, Screen HR  | author-link4 = Hazel Screen| title = The role of the non-collagenous matrix in tendon function | journal = International Journal of Experimental Pathology | volume = 94 | issue = 4 | pages = 248–259 | date = August 2013 | pmid = 23718692 | pmc = 3721456 | doi = 10.1111/iep.12027 }}</ref> 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 [[muscle contraction#Force-length and force-velocity relationships|no change in length]], allowing the muscle to generate more force.

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.<ref>{{cite journal | vauthors = Hulmes DJ | title = Building collagen molecules, fibrils, and suprafibrillar structures | journal = Journal of Structural Biology | volume = 137 | issue = 1–2 | pages = 2–10 | year = 2002 | pmid = 12064927 | doi = 10.1006/jsbi.2002.4450 }}</ref> 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.<ref>{{cite journal | vauthors = Silver FH, Freeman JW, Seehra GP | title = Collagen self-assembly and the development of tendon mechanical properties | journal = Journal of Biomechanics | volume = 36 | issue = 10 | pages = 1529–1553 | date = October 2003 | pmid = 14499302 | doi = 10.1016/S0021-9290(03)00135-0 }}</ref> 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.<ref>{{cite journal | vauthors = Ker RF | title = The implications of the adaptable fatigue quality of tendons for their construction, repair and function | journal = Comparative Biochemistry and Physiology. Part A, Molecular & Integrative Physiology | volume = 133 | issue = 4 | pages = 987–1000 | date = December 2002 | pmid = 12485688 | doi = 10.1016/S1095-6433(02)00171-X }}</ref>

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.<ref name="pmid7592005">{{cite journal | vauthors = Cribb AM, Scott JE | title = Tendon response to tensile stress: an ultrastructural investigation of collagen:proteoglycan interactions in stressed tendon | journal = Journal of Anatomy | volume = 187 | issue = Pt 2 | pages = 423–8 | date = October 1995 | pmid = 7592005 | pmc = 1167437 | doi = | url = }}</ref> 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.<ref>{{cite journal | vauthors = Screen HR, Lee DA, Bader DL, Shelton JC | title = An investigation into the effects of the hierarchical structure of tendon fascicles on micromechanical properties | journal = Proceedings of the Institution of Mechanical Engineers, Part H | volume = 218 | issue = 2 | pages = 109–119 | year = 2004 | pmid = 15116898 | doi = 10.1243/095441104322984004 | s2cid = 46256718 }}</ref> 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.<ref>{{cite journal | vauthors = Puxkandl R, Zizak I, Paris O, Keckes J, Tesch W, Bernstorff S, Purslow P, Fratzl P | title = Viscoelastic properties of collagen: synchrotron radiation investigations and structural model | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 357 | issue = 1418 | pages = 191–197 | date = February 2002 | pmid = 11911776 | pmc = 1692933 | doi = 10.1098/rstb.2001.1033 }}</ref> 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.<ref name="pmid19822213">{{cite journal | vauthors = Gupta HS, Seto J, Krauss S, Boesecke P, Screen HR | title = In situ multi-level analysis of viscoelastic deformation mechanisms in tendon collagen | journal = Journal of Structural Biology | volume = 169 | issue = 2 | pages = 183–91 | date = February 2010 | pmid = 19822213 | doi = 10.1016/j.jsb.2009.10.002 }}</ref> 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.<ref>{{cite journal | vauthors = Thorpe CT, Udeze CP, Birch HL, Clegg PD, Screen HR | title = Specialization of tendon mechanical properties results from interfascicular differences | journal = Journal of the Royal Society, Interface | volume = 9 | issue = 76 | pages = 3108–3117 | date = November 2012 | pmid = 22764132 | pmc = 3479922 | doi = 10.1098/rsif.2012.0362 }}</ref> 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.<ref>{{cite journal | vauthors = Thorpe CT, Klemt C, Riley GP, Birch HL, Clegg PD, Screen HR | title = Helical sub-structures in energy-storing tendons provide a possible mechanism for efficient energy storage and return | journal = Acta Biomaterialia | volume = 9 | issue = 8 | pages = 7948–7956 | date = August 2013 | pmid = 23669621 | doi = 10.1016/j.actbio.2013.05.004 }}</ref>

===Mechanics===
{{main|Soft tissue}}
Tendons are [[viscoelastic]] structures, which means they exhibit both elastic and viscous behaviour. When stretched, tendons exhibit typical "soft tissue" behavior. The force-extension, or stress-strain curve starts with a very low stiffness region, as the crimp structure straightens and the collagen fibres align suggesting negative Poisson's ratio in the fibres of the tendon. More recently, tests carried out in vivo (through [[Magnetic resonance imaging|MRI]]) and ex vivo (through mechanical testing of various cadaveric tendon tissue) have shown that healthy tendons are highly anisotropic and exhibit a negative Poisson's ratio ([[auxetics|auxetic]]) in some planes when stretched up to 2% along their length, i.e. within their normal range of motion.<ref>{{cite journal | vauthors = Gatt R, Vella Wood M, Gatt A, Zarb F, Formosa C, Azzopardi KM, Casha A, Agius TP, Schembri-Wismayer P, Attard L, Chockalingam N, Grima JN | title = Negative Poisson's ratios in tendons: An unexpected mechanical response | journal = Acta Biomaterialia | volume = 24 | pages = 201–208 | date = September 2015 | pmid = 26102335 | doi = 10.1016/j.actbio.2015.06.018 | url = https://eprints.staffs.ac.uk/3517/1/AA_Negative_Possionsratio_Paper_M.pdf }}</ref> After this 'toe' region, the structure becomes significantly stiffer, and has a linear stress-strain curve until it begins to fail. The mechanical properties of tendons vary widely, as they are matched to the functional requirements of the tendon. The energy storing tendons tend to be more elastic, or less stiff, so they can more easily store energy, whilst the stiffer positional tendons tend to be a little more viscoelastic, and less elastic, so they can provide finer control of movement. A typical energy storing tendon will fail at around 12–15% strain, and a stress in the region of 100–150 MPa, although some tendons are notably more extensible than this, for example the superficial digital flexor in the [[Muscular system of the horse|horse]], which stretches in excess of 20% when galloping.<ref>Batson EL, Paramour RJ, Smith TJ, Birch HL, Patterson-Kane JC, Goodship AE. (2003). ''Equine Vet J.'' |volume=35 |issue=3 |pages=314–8. Are the material properties and matrix composition of equine flexor and extensor tendons determined by their functions?</ref> Positional tendons can fail at strains as low as 6–8%, but can have moduli in the region of 700–1000 MPa.<ref>{{cite book | vauthors = Screen HR, Tanner KE | date = 2012 | chapter = Structure & Biomechanics of Biological Composites. | title = Encyclopaedia of Composites | edition = 2nd | publisher = Nicolais & Borzacchiello.Pub. John Wiley & Sons, Inc. | isbn = 978-0-470-12828-2 | pages = 2928–2939 }}</ref>

Several studies have demonstrated that tendons respond to changes in mechanical loading with growth and remodeling processes, much like [[bone]]s. In particular, a study showed that disuse of the [[Achilles tendon]] in rats resulted in a decrease in the average thickness of the collagen fiber bundles comprising the tendon.<ref name="Nakagawa, Y. 1989">{{cite journal | vauthors = Nakagawa Y, Totsuka M, Sato T, Fukuda Y, Hirota K | title = Effect of disuse on the ultrastructure of the achilles tendon in rats | journal = European Journal of Applied Physiology and Occupational Physiology | volume = 59 | issue = 3 | pages = 239–242 | year = 1989 | pmid = 2583169 | doi = 10.1007/bf02386194 | s2cid = 20626078 }}</ref> In humans, an experiment in which people were subjected to a simulated micro-gravity environment found that tendon stiffness decreased significantly, even when subjects were required to perform restiveness exercises.<ref name="Reeves, N. D. 2005">{{cite journal | vauthors = Reeves ND, Maganaris CN, Ferretti G, Narici MV | title = Influence of 90-day simulated microgravity on human tendon mechanical properties and the effect of resistive countermeasures | journal = Journal of Applied Physiology | volume = 98 | issue = 6 | pages = 2278–2286 | date = June 2005 | pmid = 15705722 | doi = 10.1152/japplphysiol.01266.2004 | s2cid-access = free | hdl-access = free | s2cid = 10508646 | doi-access = free | hdl = 11379/25397 }}</ref> These effects have implications in areas ranging from treatment of bedridden patients to the design of more effective exercises for [[astronauts]].