Tendon Biomechanics

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Tendon Structure & Composition

Tendons have a hierarchy of fibrillar arrangement that is sequentially composed of collagen molecules, fibrils, fibers, fascicles (or fiber bundles), and the tendon unit. Tendon units are surrounded by epitenon, which functions to reduce friction with adjacent tissues.

The key to the tendons’ tensile strength is collagen.11 Type I collagen accounts for about 70–80% of the dry weight of normal tendons. In addition to type I collagen, many other types of collagen are also present, including type III (form rapid cross-links in stabilizing repair sites in the case of tears), type V (regulates collagen fibril diameter), and type XII (provides lubrication between collagen fibers).

In addition to collagen, many proteoglycans (e.g. aggrecan and decorin) and glycoproteins (e.g. tenascin-C, fibronectin, and elastin) also have important functions in tendons. Aggrecan holds water and resists compression, and decorin facilitates fibrillar slippage. Tenascin-C, fibronectin, and elastin function to enhance mechanical stability, facilitate tendon healing, and allow tendons to return to their prestretched lengths after physiological loading, respectively.

Tendon Mechanical Properties

The structure and composition of tendons allow for their unique mechanical behavior, reflected by a stress–strain curve consisting of four regions:

Tendon Stress-Strain Curve

1. Toe region (tendon strain <2%): this is where “stretching out” of crimped tendon fibrils occurs due to mechanical loading on the tendon. Depending on the type and location of the tendon, this wavy fibril pattern results in different initial mechanical properties due to the varying angle and length of “crimping.”
2. Linear region (tendon strain <4%): this is the physiological upper limit of tendon strain whereby the collagen fibrils orient themselves in the direction of tensile mechanical load. The slope of this region is the Young’s modulus of the tendon, which represents tendon stiffness.
3. Microscopic failure (tendon strain >4%): microscopic tearing of tendon fibers occurs, resulting in microtear failure of the tendon.
4. Macroscopic failure (tendon strain >8-10%): macroscopic tearing of tendon fibers ensues, eventually leading to tendon rupture.

Since there are many muscles in the body, each tendon differs in its function and therefore its mechanical properties. For example, the Young’s modulus of the human patellar tendon is 660 ± 266 MPa (mean ± standard deviation), whereas the tibialis anterior tendon is about 1200 MPa. Aging also significantly affects the mechanical properties of tendons: Young’s modulus of human patellar tendons aged 29–50 years is about 660 ± 266 MPa, but is about 504 ± 222 MPa in those aged 64–93 years.

Tendons also have viscoelastic properties (likely the result of collagenous proteins, water, and the interactions between collagens and proteoglycans), meaning their mechanical behaviour is dependent on the rate of mechanical strain A viscoeleastic material is more deformable at low strain rates but less deformable at high strain rates. Therefore, tendons at low strain rates tend to absorb more mechanical energy but are less effective in carrying mechanical loads. However, tendons become stiffer and more effective in transmitting large muscular loads to bone at high strain rates.

Factors Affecting Mechanical Forces

Wang 2006

Several factors affect the mechanical forces on tendons during normal locomotion. First, different tendons in the body are subjected to different levels of mechanical loads. For example, the Achilles tendon withstands higher tensile forces than those of the tibialis anterior. In humans, it has been estimated that the peak force transmitted through the Achilles tendon during running is 9 kN, which is equivalent to 12.5 times body weight. In human hand flexor tendons, it has been shown that the intratendinous force of the tendon depends on whether the force was generated passively or actively, and on whether the position of the joint was in flexion or extension. During passive mobilization of the wrist, the flexor tendon force was found to range between 1-6 N and up to 9 N during similar mobilization of the fingers. During a 35 N tip-pinch, the tendon force measured up to 12 N whereas during active, unresisted finger motion, the tendon force reached about 35 N. Second, both the level of muscle contraction and the tendon’s relative size influence mechanical forces on a tendon. In general, the greater the cross-sectional area of a muscle, the higher force it produces and the larger stress a tendon undergoes (e.g. patellar tendon vs. hamstring). Similarly, varying the rate and frequency of mechanical loading result in different levels of tendon forces.

The Differential Effects of Mechanical Loading on Tendons

Mechanical Load Level	Effects on Tendon
Low	• ↓ Tensile strength • ↓ Size • ↓ Collagen production • ↓ Anabolic activities • ↑ Catabolic activities
Moderate	• ↑ Tensile strength • ↑ Collagen synthesis • ↓ Collagen degradation • ↓ Adhesions • ↓ Inflammatory mediator (e.g. prostaglandin E2, PGE2) • ↑ Tendon stem cells (TSCs) differentiating into tenocytes
Excessive	• ↓ Tensile strength • ↓ Collagen organization • ↑ Myofibroblasts • ↑ Inflammatory mediators • ↑ TSCs differentiating into nontenocytes (adipocytes, osteocytes, chondrocytes) • ↑ Leukotrienes (↑ edema)

Recent Related Research (from Pubmed)

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