Editing Tendon (section)

===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]].