What Is a Tendon? The Rope Between Muscle and Bone
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Muscles do not pull directly on bones. They pull on tendons, dense cords of collagen that transmit the force, and the arrangement is what lets a hand have slender fingers while the muscles working them sit in the forearm. Tendons also store and return energy like springs, which is why a running human uses far less energy than the leg muscles alone could explain.
What it is made of
Tendon is among the strongest tissues in the body and its structure is essentially a rope. Collagen molecules, mostly type one, assemble into fibrils, fibrils bundle into fibres, fibres into fascicles, and fascicles into the tendon, with each level wrapped in a thin connective sheath. The fibres are aligned almost entirely along the direction of pull, which is why tendon is extremely strong in tension and weak in shear or compression. Scattered among them are tenocytes, the resident cells, which are sparse, and a ground substance of proteoglycans that binds water and handles the loads acting across the fibres. Some elastin is present, giving a little stretch. The crucial property for injury and healing is that tendon has a poor blood supply, far below that of muscle, which is why it heals slowly, why damage accumulates, and why a torn tendon frequently needs surgical repair while a torn muscle usually does not.
The spring
A tendon is not a rigid cable and its slight stretch does real mechanical work. When a runner lands, the Achilles tendon and the arch of the foot stretch under load, storing elastic energy, and they recoil during push-off, returning a large fraction of it. Estimates suggest this recycling supplies a substantial share of the energy needed for each stride, which is why human running is far more economical than a calculation based on muscle work alone would predict, and why kangaroos can hop faster with almost no increase in metabolic cost. The arrangement also allows muscles to work more efficiently, since the tendon absorbs the rapid length changes while the muscle fibres operate closer to a constant length where they generate force best. A related benefit is that placing the muscles proximally and running long tendons to the extremities keeps mass close to the body, reducing the energy needed to swing a limb, which is why hooved runners have thin lower legs containing almost no muscle at all.
How they get injured
Tendon problems are among the most common complaints in sport and in manual work, and the vocabulary has been revised as the pathology became clearer:
- •Tendinopathy is the general term, and it is usually a degenerative change rather than inflammation, with disorganised collagen, increased ground substance and abnormal blood vessel growth, which is why the older name tendinitis is now avoided and why anti-inflammatory drugs help pain more than they help the tissue
- •Overuse is the dominant cause, since tendon adapts more slowly than muscle, so a training load the muscles can handle may exceed what the tendon has adapted to, which is why rapid increases in volume cause problems
- •Rupture, a complete tear, typically affects a tendon already degenerated, commonly the Achilles, and frequently happens during a sudden acceleration
- •Tenosynovitis affects the sheath around tendons that run through tight channels, including the wrist and ankle, producing pain and sometimes an audible creak
- •Certain antibiotics in the fluoroquinolone class raise rupture risk enough to carry warnings, and corticosteroid injection relieves pain while weakening the tissue
- •Age reduces collagen turnover and cross-link quality, so tendon stiffness and injury risk rise even without any change in activity
How they heal and how they are trained
Repair is slow and imperfect: healed tendon contains more type three collagen, which is weaker and less organised, so a repaired tendon rarely regains full original strength and remains at elevated risk. What the evidence supports for treatment is loading rather than rest. Progressive resistance training, particularly programmes emphasising the lowering phase under control and heavy slow loading, stimulates the tenocytes to remodel the tissue and has the strongest evidence base of any conservative treatment for tendinopathy, with rest producing short-term relief and long-term deterioration. Adaptation is slow, taking months rather than weeks, which is why rehabilitation programmes are long and why returning to sport when pain resolves is premature. Preventive practice follows the same logic: increase load gradually, include the specific tissue in training rather than only the muscles, and treat pain during loading as information about capacity rather than as a signal to stop entirely, since mild pain that settles quickly is generally acceptable while pain that worsens the following day is not.
The takeaway
A tendon is a dense rope of aligned collagen transmitting muscle force to bone, extremely strong in tension and poorly supplied with blood, which is why it heals slowly. Its slight elasticity stores and returns energy at each stride, supplying much of the economy of human running, and long tendons let heavy muscles sit near the body. Most tendon injury is degenerative overuse rather than inflammation, which is why the name tendinopathy replaced tendinitis, and progressive loading rather than rest is the treatment with the best evidence.