yogabook / movement physiology / tendon

Tendon

A tendon (lat. tendinum) is a connective tissue, non-contractile part of a muscle that connects the contractile part (muscle belly) to a bone. In some cases, muscles also attach to other tendons, fascia or directly to bones. It is not uncommon for tendons of several muscles or muscle bellies to confluence into one. Near joints, tendons often run in a connective tissue sheath, the tendon sheath. They are often padded with bursae to protect them from bone pressure. Tendons consist of parallel, firmly connected fibers and are round or flat, depending on the shape of the muscle. Tendons are generally weakly vascularized and therefore belong to the bradytrophic tissues, which increases their regeneration time or turn over. They therefore belong to the bradytrophic tissues. They are supplied approximately one third each by their muscle, by the periosteum of the bony insertion site and by the lymph flow. The sensitive innervation is also rather low. The Golgi tendon organ, which measures tendon strength, is one of the most important proprioceptive receptors in the transition from the contractile to the tendinous area of the muscle. The elasticity of tendons is up to about 15%.

Cellularly, tendons consist of 90-59% tenocytes and tenoblasts, with the remainder consisting of vascular cells, chondrocytes, synovial cells and smooth muscle cells. The extracellular matrix consists mainly of collagen (95% type 1) and elastin. Tendons are categorised into gliding tendons, which change direction along their course, and traction tendons, which connect the insertion and origin in a largely straight line. Sliding tendons are most strongly compressed in the area of contact with their redirection, while the tension is greatest diametrically opposite. They are therefore exposed to shear forces. In the case of the Achilles tendon, which is biomechanically a tensile tendon, the fibres of the soleus twist from anterior and medial with those of the gastrocnemius from posterior and lateral. This effect is most pronounced in the area around 2-7 cm proximal to the insertion on the calcaneus (the Achilles waist) and leads to hypovascularisation there, resulting in reduced metabolism and increased susceptibility. Damage to tendons can occur from 4% elongation, at more than 8-12% it is likely (tears). Healing takes place (except in the case of a total tear) in three overlapping phases, the first of which is the 3-7 day inflammatory phase. The prodelivery phase lasts from the 5th to the 21st day, followed by the maturation and restructuring phase lasting up to one year.

Tendon elasticity

The elasticity of tendons is about 15%. According to recent research, an ion channel protein in the tendons acts as a force sensor that detects the longitudinal displacement of collagen fibres against each other. When larger shear forces are measured, the force sensor emits calcium ions into the interior of the tendon cells, which promotes the production of enzymes that bind the fibres together, increasing stiffness and resilience at the expense of elasticity. There are several genetic variants of this ion channel protein. The E756del variant, which originated in West Africa and is the result of a counter-reaction to malaria plasmodia, causes an excessive release of calcium, which results in firmer tendons. Carriers of this variant have an advantage in sports where the condition of the tendons is important (high-speed sports such as sprinting, fast jumping disciplines).

Tendon force

The force with which a muscle pulls on its attachment and origin. In the simplified model, a muscle attaches to a bone with a tendon at both the origin and the attachment. If you were to cut a tendon and insert an element to measure the traction force, you would obtain the tendon force of the muscle. The actual tendon force depends on the resting tone, the positions of the covered joints and, of course, the innervation to a high degree. Even without voluntary innervation, a muscle has a certain resting tone, so the tendon force is different from zero even in the most favorable joint positions. If the tendon were to be severed on one side, the head of the muscle would contract.

Tendon healing

Tendon healing takes place in three phases, of which phase 1 in particular gradually merges into phase 2 towards the end:

1. Inflammatory phase, up to approx. 7 days after trigger: platelet accumulation; fibrinous cross-linking of collagen fibres; increase in permeability due to mediators such as bradykinin and histamine
2. Prodelivery phase, 2-3 weeks: the fibrin construct is replaced by granulation tissue, prodelivery of myofibroblasts and fibroblasts, the latter form uncrosslinked collagen III
3. Maturation and remodelling phase, from 3 weeks to approx. 6 months: successive replacement of collagen III by collagen I. After 6 months, the tendon is fully resilient, but the healing is not a restitutio ad integrum, the resilience of the repaired tendon is reduced.

Tendon sheath / vagina synovialis tendinis

The tendon sheath is the double-walled connective tissue covering of the tendon in which it glides. It is directly enveloped by a two-leaf stratum (inner and outer leaflet). The inner leaflet is fused with the tendon, the outer leaflet with the stratum fibrosum, the outer part of the tendon sheath. Between these two strata lies a buffer of synovia, which produces the synovial layer of the joint capsule, to which the inside of the tendon sheath is connected.

Tendinitis

Inflammation of a tendon. This can occur together with or without inflammation of its tendon sheath (tendovaginitis) and is then also referred to as tendosynovitis. The cause is usually overuse.

Tendopathy / Tendinopathy

Tendon disorders with no specified etiology, but usually of an inflammatory or degenerative nature. Overuse is a common cause, with ergonomic or material deficiencies being a risk factor, as is a lack of regeneration.