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Tendon Injuries

Tendon injuries constitute a frequent diagnostic and therapeutic problem in sports medicine. If these injuries are not treated well, they result in chronic and long-lasting problems. The basis for successful management is a correct diagnosis.

A muscle is usually attached to bone by a tendon through which the effects of muscle contraction are conveyed. The main function of a tendon is to transfer forces from the muscle to the bone. The muscle produces force only when contracting, and this has a stretching effect on the tendon. Tendons display great anatomic diversity, with variations in shape, length, vascularity, and the extent of synovial lining. There is also variability in biochemical and biomechanical characteristics.
Tendons regularly used in sport are usually very strong. Peak force in the Achilles tendon has been estimated in running to be nearly 9000 N (1 ton-force) or 12.5 times bodyweight. The rate of force application may, however, be more important than the magnitude of the force as a cause of tissue damage. Tendons withstand tensile forces well, but resist shearing forces less effectively, and provide little resistance to compressive forces. Tendons are composed of collagen, which provides great mechanical strength, and elastin, which provides elasticity.
In the normal resting state, a tendon has a wavy configuration, but if it is strained (elongated) by more than 2%, the wavy pattern disappears and the collagen fibers are subjected to stress. At 4–8% strain, the crosslinks joining the collagen molecules together will start to break as the fibers slide past one another. At 8–10% strain, the tendon will begin to fail and the weakest fibers will rupture.

Tendons are most vulnerable to injury when:
1. Tension is applied quickly and sustained without adequate warm-up.
2. Tension is applied obliquely.
3. The tendon is tensed before the trauma.
4. The attached muscle is maximally innervated and contacted.
5. The muscle group is stretched by external forces.
6. The tendon is weak in comparison to the muscle.

All these factors can apply to athletes of all ages.
Tendon injuries are common in sports because force is focused on the tendon as a part of the muscle unit, thereby increasing the risk of injury. Tendon tissue readily undergoes adaptation to the conditions imposed upon it. A tendon can be subjected to overload (which includes a rapid increase in resistance) or overuse (which is a repetitive motion without increase in resistance). An injury to a tendon represents a failure of the cell matrix (surrounding supportive tissue) to adapt to the load exposure, whether it is a sudden overload or a cumulative overload secondary to cyclic overuse.

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