While the structure of mammalian tendons is well known do the mechanical properties of the tendons vary depending upon their anatomical position in mammals. Large amounts of research has looked at individual mechanical properties of tendons but very little has looked at a wide range all at once.
Tendons are mainly composed of water and collagen. 55% of the fluid in tendons is water (Vogel, 2003) while 65-85% of the dry mass of tendons is type I collagen. Collagen is a fibrous protein, each collagen fibre is made of polymerised molecules in a jelly like matrix. Fibroblasts between parallel bundles of collagen are responsible for the production and maintenance of collagen. (Vogel, 2003) 1-2% of the tendon is elastic fibres although (Kannus, 2000) suggests they aren’t present in all tendons. Elastic fibres have lower strength but greater extensibility than collagen fibres.
Collagen fibres consist of soluble tropocollagen joined by cross links to form insoluble collagen molecules which gradually form collagen fibrils, the greater the number of cross links the stronger the fibre and tendon (Dressler, Butler, Wenstrup, Awad, Smith and Boivin, 2002; Magnusson, Beyer, Abrahamsen, Aagaard, Neegaard and Kjaer, 2003). A small group of collagen fibrils forms a collagen fibre, the base structure of tendons, these are roughly 300nm in length and 1.5nm in diameter (Franchi, Trirẻ, Quaranta, Orsini, Ottani, 2007). Collagen fibres in a group form a sub fascicle (fibre bundle), which in turn group to form a secondary fibre bundle (fascicle). A large number of fascicles form a tertiary bundle. The fascicle, sub fascicle and tertiary bundle are encompassed by a thin sheet of connective tissue called the endotenon composed of crisscrossed collagen fibres. Three to four tertiary bundles encompassed by an epitenon form a tendon. (Franchi, Trirẻ, Quaranta, Orsini, Ottani, 2007) The epitenon is a fine layer of connective tissue containing a dense network of collagen fibres, each strand of collagen being 8-10nm in diameter. (Kannus, 2000) Most tendons are then covered sheet by a comprising of type I and III collagen fibres along with elastic firbres. This acts as an elastic sleeve for the tendon. Collagen fibres are aligned parallel to the length of the tendon so force is transmitted along the tendon.
Collagen fibres have a crimped structure when relaxed, this ranges from 0° to 60°. (Kannus, 2000; Kubo, Kanehisa, Kawakami and Fukunaga, 2001) The crimp reduces the stress required to initially stretch the tendon. As force is applied the fibres straighten, the stress required to stretch the tendon increases to protect the muscle by taking up strain. Some research suggests the crimp angle decreases with age (Franchi, Trirẻ, Quaranta, Orsini, Ottani, 2007; O’Brien, Reeves, Baltzopoulos, Jones and Maganaris, 2010) increasing tendon stiffness. Tendons usually operate in the elastic region (linear region) of a stress strain curve, below 4%. A stretch greater than this prevents the crimped structure reforming on cessation of the stretch. Fracture occurs at values of 8-13%., however the human Patellar tendon reaches 23-30% (Reeves, Maganaris and Narici, 2003; Ker, Alexander and Bennett, 1988).
Tendon cross sectional area depends on the diameter of the fascicles and collagen fibres. Tertiary fibre bundles range from 1000 to 3000µm, secondary fibre bundles 150 to 1000µm and sub fascicles 15 to 400µm in diameter and they are usually triangular. Individual collagen fibres range in diameter from 5µm in rats tails to 300µm in the human Achilles tendon. (Kannus, 2000; Franchi, Trirẻ, Quaranta, Orsini, Ottani, 2007) Individual collagen fibrils are 20nm to 150nm in diameter. Thicker tendons are required for muscles that provide large resistive forces while muscles that perform subtle delicate movements require longer thinner tendons (Ker, Wang, Pike, 2000)
Large volumes of research state energy is required to stretch mammalian tendons, the stress required increases exponentially as strain increases due to the elastic properties of tendons. However energy can also be absorbed by the tendon from movement and contact with the environment. As the tendon shortens they return energy to the muscles and boned acting as power amplifiers (Bryant, Clark, Bartold, Murphy, Bennell, Hohmann, Marshall-Gradisnik, Payne and Crossley, 2008; Maganaris, 2002). Energy return isn’t 100% efficient, some is lost as heat shown by the area inside the two curves that form a work loop (hysteresis) on a stress strain graph, a fact theory that has been well proven in current literature. Energy lost as heat in human walking is 22-25% (Kubo, Kanehisa and Fukunaga, 2003; Kubo, Kawakami, Kanehisa and Fukunaga, 2002) Some research suggests hysteresis is lower in women, however this is a theory which at the moment has little evidence to support it. (Kubo, Kanehisa and Fukunaga, 2003)
Tendons allow muscles to be situated proximal to the body reducing the force required to move the limb through reductions in inertia. As a result the tendons on the whole have high tensile strength but it varies depending upon the mammal and the anatomical position (Ker, Wang, Pike, 2000). A human Achilles tendon ruptures at roughly 100MPa, (Kongsgaard, Aagaard, Kjaer and Magnusson, 2005) but a wallaby tail tendon ruptures at 144MPa. (Wang and Ker, 1995) Loads on the wallaby tail tendon are lower than the Achilles tendon which suggests the magnitude of loading isn’t the determinant of tendon strength. Other research contradicts this showing tensile strength increases with age as the load placed upon them increases through the increase in body mass with stresses in the human Achilles tendon being 67MPa (Ker, Wang, Pike, 2000) and 28-74 in a deer’s ankle exyensor but only 18MPa in a camel, 15-41MPa in a wallaby ankle extensor. (Ker, Alexander and Bennett, 1988; O’Brien, Reeves, Baltzopoulos, Jones and Maganaris, 2010; Reeves, Maganaris and Narici, 2003; Muraoka, Muramatsu, Fukunaga and Kanehisa, 2005) Peak tensile stress in human tendons occurs at roughly 25-35 years old although physical activity can affect this, however this might not be a property that is present in all mammalian tendons as research on this is only present in human tendons. Normal stresses on mammalian tendons range from 10-70MPa, the average being 13MPa. Tendons which act like a spring have to cope with higher stresses. (Ker, Wang, Pike, 2000)
The Patellar tendon however experiences much lower stresses than the Achilles with stress values of 22-23MPa supporting the idea that the anatomical position plays a key role in determining tendon strength (Haraldsson, Aagaard, Krogsgaard, Alkjaer, Kjaer and Magnusson, 2005).
Tendon strain can be affected by the forces applied through locomotion or through movement. Achilles tendon strain in humans while walking is roughly 2.5% but is 5.5% in running, this is supported with the stress value of up to 84MPa in a dog when jumping. (Bryant, Clark, Bartold, Murphy, Bennell, Hohmann, Marshall-Gradisnik, Payne and Crossley, 2008) Limited research suggests tendons in women have lower stiffness and greater comliance. (Kubo, Kanehisa and Fukunaga, 2003) While no research suggests a reason for this in terms of cross sectional area Bryant, Clarke, Bartold, Murphy, Bennell, Hohmann, Marshall-Gradisnik, Payne, and Crossley, 2008 suggest an interesting argument with their findings that the higher estrogen levels could play a role in reducing the amount of collagen in the tendons.
Force transfer from muscle to bone is a key role the tendon plays (Maganaris, 2002; Reeves, Maganaris and Narici, 2003; Ker, Alexander and Bennett, 1988; Kubo, Kanehisa, Kawakami and Fukunaga, 2001; Kubo, Kawakami, Kanehisa and Fukunaga, 2002; Ker, Wang, Pike, 2000) and the length is important, longer tendons have lower stiffness without compromising strength. A 2% stretch of a longer tendon is greater than a 2% stretch of a short tendon in terms of relative length meaning greater power amplification (Muraoka, Muramatsu, Fukunaga and Kanehisa, 2005).
Increasing the cross sectional area (C.S.A.) on the other hand increases the stiffness and force required to achieve rupture for the same strain value. This has a cost in muscle mass required to stretch the tendon if energy isn’t absorbed from locomotion. (Ker, Alexander and Bennett, 1988) Stiffer tendons act more like rods than strings so are better for power athletes and the quick transfer of force. (Kubo, Kanehisa, Kawakami and Fukunaga, 2001; Kubo, Kanehisa, Ito and Fukunage, 2001) A higher tendon to muscle cross sectional area ratio would be advantageous for quick force transfer as stresses upon the tendon will be reduced. Lower tendon to muscle CSA ratio causes tendons to act more like a spring beneficial for someone like a high jumper or a mammal such as a cat but they have higher stresses such as the human Achilles tendon. The tendon:muscle cross sectional area ratio can vary, Cutts, Alexander and Ker, 1991 suggest an optimum ratio of 34. Their research shows little variation in the cross sectional area between anatomical positions in the human body. Both the Achilles tendon, Abductor Pollicics Longus (APL) and Flexi carpi ulnaris all show ratios of 34 and all undergo very different tensile forces but they suggest that strain values would be about 1.3% for this ratio but don’t state if this is the case in other mammals.
Strength training increases collagen levels and cross links in the tendon causing greater stiffness similar to changes that occur from an increase in body mass but with no change in strength, similar to the changes with aging. (Dressler, Butler, Wenstrup, Awad, Smith and Boivin, 2002; Magnusson, Beyer, Abrahamsen, Aagaard, Neegaard and Kjaer, 2003; O’Brien, Reeves, Baltzopoulos, Jones and Maganaris, 2010) Endurance training also increases collagen content, strength and stiffness in some tendons through the repetitive loading. Repetitive loading is the stimulus for the increase in tendon strength not the force of loading (Kjaer, Langberg, Heinemeier, Bayer, Hansen, Holm, Doessing, Kongsgaard and Magnusson, 2009). However repeated lengthening of the tendon causes micro damage at sub-maximal stresses. This causes the tendons to become more elastic similar to stretching. If the rate of damage is higher than the rate of repair it can cause rupture. This rupture can occur at stresses well below the maximum tensile strength of the muscle. (Ker, Wang, Pike, 2000) Stretching increases tendon elasticity and reduces energy lost through hysteresis but at the cost of stiffness reducing the rate of force transfer. (Kubo, Kanehisa, Kawakami and Fukunaga, 2001; O’Brien, Reeves, Baltzopoulos, Jones and Maganaris, 2010; Reeves, Maganaris and Narici, 2003)
Research on aging appears to be relatively conclusive with common themes occurring in a variety of animals showing increased stiffness of the tendon with age. (O’Brien, Reeves, Baltzopoulos and Jones, 2010) reported tendons will grow in length and cross sectional area, up to 22% in some studies. (Magnusson, Beyer, Abrahamsen, Aagaard, Neergaae-rd and Kjaer, 2003) Stiffness was reported to increase between 84-94% between adults and children in one study, a trait also shown in rabbit tendons (Nakagawa, Hayashi, Yamamoto and Nagashima, 1996). Resistance to fatigue in the tendon increases with age (Bryant, Clark, Bartold, Murphy, Bennell, Hohmann, Marshall-Gradisnik, Payne and Crossley, 2008; Reeves, Maganaris and Narici, 2003)
Overall the current literature presents sufficient evidence to suggest that the strength, elasticity and energy return properties of tendons vary greatly dependent upon their anatomical position within the mammals. The current literature also supports the theory of tendons in different mammals displaying different mechanical properties and on the whole can be explained by the different loadings that they have to regularly undergo. However I feel that more research on this could be help to solidify the theory, for example directly comparing tendons from the same relative anatomical position in different mammals. Along side this the effects of aging on tendons has been conclusively proven, the increases in stiffness have been well documented in a wide variety of mammalian tendons.