Biology

A tendon (or sinew) is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae; all three are made of collagen. Ligaments join one bone to another bone; fasciae connect muscles to other muscles. Tendons and muscles work together to move bones.

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Function

Tendons have been traditionally considered to simply be a mechanism by which muscles connect to bone, functioning simply to transmit forces. This connection allows tendons to passively modulate forces during locomotion, providing additional stability with no active work. 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.

Mechanics

Tendons are viscoelastic structures, meaning 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, after which the structure becomes significantly stiffer, and behaves reasonably linearly 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 stretchy, 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

Healing

The tendons...