Biomechanics in Achilles Tendinopathy

By Andrew M.Horwood, MSK Podiatrist M.Pod.A, D.Pod.M.

Achilles Anatomy

The Achilles tendon is the strongest and thickest tendon in the body (averaging 15cm in length) and is formed by the confluence of the gastrocnemius and soleus muscles; also known as the triceps surae (Schon & Anderson 2002: p.136).

The gastrocnemius lies superficially to the soleus and arises from two heads, with attachments to the posterior joint capsule of the knee. Its two muscular heads, sometimes known as gastrocnemius medialis and lateralis, pass distal to the knee joint and unite in a tendinous raphe that subsequently becomes an aponeurosis. It is this aponeurosis that unites with the soleus.

The gastrocnemius muscle. From: Draves, D.J. (1986) Anatomy of the Lower Extremity. Baltimore, Williams and Wilkins, p.262. and the soleus muscle clearly showing the twist of the tendon fibres prior to insertion. From : Travell, J.G., Simons, D.G., (1992) Myofascial Pain and Dysfunction: The Trigger Point Manual :The lower extremities. Baltimore, Williams & Wilkins. p.434.

The soleus lies deep and anterior to gastrocnemius, arising from the proximal posterior surface of the tibia and fibula, and the interosseous membrane. Its muscle fibres converge into its own aponeurosis, which blends with the gastrocnemius aponeurosis to form the Achilles tendon.

The aponeurosis has an important mechanical role and can strain almost 3 times as much as the tendon, although it is not homogeneous along its entire length (Maranaris & Narici, 2005 p.19) Although commonly believed to be least vascular in mid portion, Astrom et al (1994) showed even blood flow throughout the Achilles. The fibres of the Achilles tendon rotate externally approximately 90° before inserting on the posterior tuberosity of the calcaneus. The result of this is that the soleus fibres primarily insert medially and the gastrocnemius fibres insert laterally.

The insertion is approximately 1.5 cm distal to the superior calcaneal tuberosity. The tendon itself is enclosed by a paratenon that is not lined with synovial membrane. The paratenon allows for approximately 1.5cm of tendon glide.

Non-uniform stresses have frequently been described as an aetiological factor in Achilles tendon injury. Such non-uniform stresses in the Achilles tendon can occur through modifications of individual muscle contributions (Arndt et al 1998).

Tissue Stress Terms Related To Achilles Tendon

Tendons are a viscoelastic material. This means that like a viscous material, they deform away from load and dissipate absorbed energy; but like an elastic material, they return the energy loaded into it on release of that loaded stress. Tendons need to be more elastic than viscous to perform their normal function in gait. When discussing the properties of tendons, certain terms must be clearly understood:

  • Stress: the applied force per unit area. Its units are Pascals: N/mm².
  • Strain: the relative change in size in a prescribed direction. When acting along the axis of the material, it can be compressive or tensile.
  • When perpendicular or tangential to the cross-section, it is known as shear strain.
  • Materials that demonstrate little strain under high stress are brittle.
  • Materials that demonstrate lots of strain under little stress are ductile.
  • Toughness is a measure of the total energy needed to produce failure.
  • Ultimate tensile strength is the highest stress that a material is able to stand before it starts to fail.
  • Fail point is the breaking stress.

Stress/strain curves are used to assess a material’s resistance to deformation in a prescribed direction. Young’s modulus typically refers to a material’s range of strain for which the deformation is recovered immediately upon release of force. The unit of Young’s modulus is N/mm² or MPa (or often GPa) as the numbers involved are usually large.

Callister (1997: pp.774-786) lists the modulus of elasticity for many materials; some of interest to the podiatrist. Carbon fibre has a modulus of 145 GPa and polypropylene has a modulus of 1.14-1.55 GPa. Both of these materials are commonly used in the manufacture of foot orthoses. If a practitioner is looking for a material to return energy to the foot so as to help terminal stance phase gait, the carbon fibre may be a more effective choice. Added to this choice are the considerations of strength and ductility. Carbon fibre with a tensile strength of 3300 MPa and a ductility of 1.4% is a brittle material. A carbon fibre shell will hold its shape well, but will not easily yield to an arch that must flatten greatly during gait. However, if yield is exceeded then fracture will follow.
Young’s modulus for various materials as listed by Low and Reed (1996p30):








21 000


210 000

This diagram is from Anderson, D.D., Adams, D.J., Hale. J.E. (2000) Mechanical effects of forces acting on bone, cartilage, ligaments, and tendons. In: Nigg, B.M., MacIntosh, B.R., Mester, J., Biomechanics and Biology of Movement. p.285, Champaign, Human Kinetics.

It shows all the properties of a material undergoing force deformation. Up to the yield point, the material displays elastic properties with stress related to strain by a proportional constant (Young’s modulus, which is equal to the slope of the stress-strain curve over this linear range). Unloading the material in this range follows the same curve as loading, resulting in the complete recovery of any deformation.

From the yield point we go into area of plastic deformation where permanent deformation of the material remains following removal of the stress. The point just before the yield point is called the elastic limit.

All tendons demonstrate viscoelastic properties such as creep, hysteresis and strain rate sensitivity, with energy being stored in the stretch-shortening cycle (Alexander 2000: pp19-29). Tendons are composed of collagen. Type I collagen, the most common in tendons, gives it strength, and type III collagen gives it its elastic properties (Józsa and Kannus 1997: p69). The Young’s modulus of the Achilles tendon has been reported by Maganaris (2002) as being 1.16 GPa. Tendons are viscoelastic materials that undergo tensile stress in life, showing fatigue properties in repeated loading. Tendons are perfectly elastic as long as strain is equal or less than 4%.

This diagram is from Józsa, L., Kannus, P. (1997) Human Tendons: Anatomy,

Physiology and Pathology. Champaign, Human Kinetics (p101). It shows the stress- strain curve for a tendon. The toe area is caused by straightening of the crimp arrangement in the collagen fibres; the linear period is the elastic phase when removal of this stress will not cause any permanent deformation. The yield point occurs where collagen fibres begin to rupture beginning the plastic deformation phase. Yield begins at approximately 3%, while ultimate tensile strength occurs at about 8%.

Biological materials also demonstrate Time Dependent Properties in that their mechanical behaviour to stress is effected by the speed at which that stress is applied. Both Young’s modulus and strength increases with increased rates of loading stress. For tendons: the faster the tendon is loaded the stiffer it behaves.

Creep is deformation under constant load. The effect is slow and continuous with time and is dependent on temperature; which means the strain is dependent on stress, time, and temperature (Ashby and Jones 1996: p170). Creep is characteristic of plastic deformation (Moorcroft et al 2001).
Fatigue is the process by which a material subjected to repeated stress cycles may fail at stresses well below the tensile strength, and often below the yield strength. Material fatigue is studied using graphs that study stress over time (known as S-N curves), where S represents the stress (σ) and N represents the number of cycles. Materials usually fatigue and fail under a low stress over a long period of time, or at a high stress over a shorter period. The mechanical response seen in initial loading cycles may differ from the responses seen in later loading cycles, but may find a steady state eventually after a number of stress cycles.

The final material property that helps in the understanding of tendon injuries is Hysteresis. Tendons do not demonstrate linear storage of energy like a spring. They dissipate some energy stored in the material when force is applied. The nonlinearity on stress-strain diagrams represents the amount of energy dissipated; the curves created form the hysteresis.

This stress-strain diagram demonstrates nonlinearity and energy dissipation. The curves for increasing and decreasing forces is the hysteresis of the material. The area under the increasing curve (A&B) represents the energy that is put into the material or the work done on the material. The area under the decreasing curve (B) represents the energy that is returned or the work done by the material. The difference in the force is the area (A) between the two curves (known as the hysteresis loop). Therefore, the hysteresis loop represents the energy

dissipated within the material. A material that returns some energy but also dissipates some, is viscoelastic. A material that returns none is described as being viscous.

The Achilles tendon produces a hysteresis that clearly demonstrates that although it is viscoelastic, it’s behaviour is far more elastic than viscous. The Achilles tendon has good capacity to return energy, reported as being around 
80% (Kubo et al 2002; Maganaris 2002; Maganaris and Paul 2002).

From: Kubo, K., Kawakami, Y., Kanehisa, H., Fukunada, T. (2002) Measurement of viscoelastic properties of tendon structures in vivo. Scandinavian Journal of Medicine and Science in Sports. 12(1): 3-8. The diagrame demonstrates the hysteresis loop formed on testing the Achilles tendon’s viscoelastic properties. Because they have recorded changes in length of the tendon, the hysteresis loop is inverse to the previous diagram.

It is important to note age alters all the mechanical properties of tendons, not least by reducing tensile strength and increasing its stiffness (Kannus et al, 2005 p.27), this is probably why Achilles are commonly injured in older athletes (Paavola et al, 2005 p.34).

Biomechanical Properties of the Achilles

All tendons transfer force between muscle and bone. However, the peak stresses to which the tendon is subject is dependent on its anatomical site. Among adult mammalian limb tendons, this peak ranges between 10-70 mega-pascals (MPa), with the most common stress lying around 13MPa (Ker et al, 2000).

The human Achilles tendon has a high stress-in-life of 67 MPa because it acts as a spring to save energy during locomotion (Ker et al, 2000). The high incidence of Achilles tendon injuries is related to this mechanical loading imposed upon the tendon during physical activity, which produces higher in vivo stresses than most other tendons (Wren et al 2001). These forces have been reported to be as high as 110 MPa (Komi et al, 1992) which explains why the Achilles tendon is the largest human tendon, as adjustments
to structural stiffness is achieved in tendons by changing thickness rather than material stiffness (Ker et al, 2000).

Many studies have looked at the metabolic effects on tendons, with little cross over between the metabolic processes and the mechanical. Histology has revealed a lack of inflammatory cells (Khan et al, 1999), from which the conclusion of a degenerative process is the cause of tendon pathology. However Flick et al (2006) may have established a link between mechanics and tendon metabolism. They found that aggressive tendon loading resulted in prostaglandin E2 production, a powerful inflammatory substance that can cause tissue breakdown. When tendons were loaded moderately Nitric Oxide was produced, a substance used for tissue repair. Therefore they may well be an intimate relationship begin mechanics and metabolism.

Zifchock and Piazza (2004) assessed the propriety of modelling the Achilles tendon insertion on the calcaneus as a single point, when musculoskeletal models are used to predict subtalar joint moment arms. They found that the medial-lateral location of the effective insertion site varied linearly with subtalar joint angle in cadaver specimens; shifting medially when the foot was everted and laterally when inverted. This of course fits nicely with the anatomical features of the Achilles tendon arising from two separate muscles.

Schematic diagram. From: Zifchock & Piazza (2004) Investigation of the validity of modelling the Achilles tendon as having a single insertion site. Clinical Biomechanics. 19(3): 303-307.

(A) represents a single-point insertion of the Achilles tendon, where the insertion inverts more with inversion and everts more with eversion. However, the authors found that a two-point insertion model (B) was a more accurate representation.

The two-point model effectively shifts laterally with inversion and medially with eversion. Redirection of the tendon by soft tissues (right, B) may have resulted in the effective insertion site moving beyond the margins of the anatomical insertion site.


Achilles tendonitis has been reported as one of the most common overuse injury seen in sports medicine. In runners, its incidence has ranged from 6.5 to 18%. Although common in many athletic activities, middle-aged men lacking gastrocnemius-soleus flexibility, and having seronegative spondyloarthropathies (Reiter & Vad 1999: p423) are far more likely to develop symptoms.

Among 987 male and 663 female runners, Mechelen (1992) reported 7.9% of males and 3.2% of females reported Achilles tendon symptoms. Among males in this study, Achilles tendon symptoms represented the third most common pathology, whereas it was the fifth most common pathology in females.


Clinically clerk the patient first. Include details on activities, miles per outing, shoe conditions, running surface, and the character and quality of pain. Review of the patient’s health status and previous treatments are also helpful. The physical exam should involve examination of both extremities, looking for soft tissue swelling on the tendon, local tenderness, crepitus of the Achilles tendon, and noting the duration of the symptoms (Reiter & Vad 1999: p424). Achilles tendonitis is defined as being acute at less than 2 weeks duration, sub-acute at 3-6 weeks duration, and chronic over six weeks duration (Clancy Jr., et al, 1976). Any gaps or nodularities in the tendon should be noted, and foot and leg posture should be recorded. Ankle range of motion (with the knee flexed and extended) should be assessed with the foot supinated (Reiter & Vad 1999: p.424. A tendon rupture is suspected, a Thompson test should be performed wherein which the patient lies prone and the calf is squeezed to check for the integrity of the Achilles tendon. The foot should plantarflexed when the calf is squeezed for a negative result, which will still occur even if a partial rupture is present (Reiter & Vad 1999: p.424).

It should be first noted that in Puddu et al, (1976) classic classification of Achilles tendon disease that Achilles ‘tendinitis’ as inflammation of the tendon proper was not recognised as a true pathological entity. However, The term Achilles tendonitisis still commonly used non-specifically to refer to all symptomatic inflammatory conditions of the tendon and adjacent tissues (Wright, Crockett & Weiland 1999: p432). Achilles tendinopathy has been separated into insertional and non-insertional forms (Clain and Baxter 1992). The insertional form involves the adjacent bursa along with additional tendon changes, often in conjunction with a Haglund’s deformity; whereas the non- insertional type affects central zone. Pain localised to 2-6 cm above the tendon’s insertion is the predominant symptom of Achilles tendonitis. When pain is present only after long running, it is suggestive of a tendinopathy in its early stages, while pain at the start of a run suggests advanced disease (Reiter & Vad 1999: p424). Achilles tendinopathy encompasses two principal pathologic conditions: paratenonitis and paratenonitis with coexistent tendinosis (Wright et al 1999: p432); although Puddu et al (1976) also lists pure tendinosis as a separate form. However, the conditions present with identical symptoms, but those with longer standing symptoms are more likely to have tendinosis in addition to paratenonitis (Wright, Crockett & Weiland 1999: p432).

The painful arc sign has been described as a means to clinically identify inflammation of the paratenon from involvement of the tendon. With paratenonitis, the region of tenderness and swelling does not move as the ankle is plantarflexed and dorsiflexed slightly. With tendon involvement, the tender region and swelling moves in association with the tendon excursion (Wright et al, 1999: p432). Distinguishing one entity from the other affects the prognosis, but the methods of conservative treatment are identical at present, with isolated paratenonitis likely to respond more favourably because the tendon has less healing potential (Wright et al, 1999: p432).

Although an Achilles tendon injury can be diagnosed by physical examination and clinical history, plain radiographs are useful to exclude other osseous injuries such as avulsion fractures of the calcaneus. On lateral views of the ankle, the Achilles tendon should measure 4-9 mm across with a defined anterior interface. With a rupture, which is more likely with pre-exsisting degenerative changes, a focal fusiform thickening 2.5 cm proximal to the insertion is present, if there is complete absence of the tendon (Pavlov et al1999: p42). A loss of sharp interface between the tendon and the pre-Achilles fat pad suggests tendonitis, whereas linear-orientated calcifications in the distal portion of the Achilles tendon, indicates calcific tendinitis (Pavlov et al 1999: p42).

Sonography of the Achilles tendon and bursa has been reported to better define the internal anatomy of the tendon (Pavlov et al 1999: p42). MRI can distinguish an acute injury from a chronic injury, as well as being able to grade the injury (Pavlov et al 1999: p42).
Further to the above examinations, I believe it is also of use to locate on which side of the tendon the pain is located; medial or lateral.

Differential Diagnosis

If diagnosis of an Achilles tendinopathy has not been possible, then one should consider retrocalcaneal or superficial bursitis, a calcaneus or posterior talar process stress fracture, plantaris muscle rupture, os trigonum fracture, and an enthesopathy associated with Reiter’s disease (Reiter & Vad 1999: p424). Flexor hallucis longus tendonitis, tibialis posterior tendonitis, deep posterior exertion compartment syndrome, stress fracture of the calcaneus or distal tibial and posterior shin splints should also be kept in mind (Wright et al, 1999: p432).

Calcium-deficient diets have been shown to decrease the tensile strength of myotendinous junctions which may lead to junctional injuries, while iron-deficient diets can also affect healing. Certain antibiotics (the fluroquinolones), are associated with tendon disorders, while renal failure and prior corticosteroid treatment also increases the risk of rupture (Reiter & Vad
1999: p424).


Although the Achilles tendon is the strongest tendon in the leg, it is the most commonly injured tendon of the ankle, between 2-6 cm proximal to its calcaneal attachment, which was thought to correlate to the area of least vascularity (Applbaum 2002: p86). Achilles tendinopathy is the result of accumulative loading and repetitive microtrauma, but there are intrinsic and extrinsic factors that increase the risk (Reiter & Vad 1999: p423). Intrinsic factors are thought to include decreased vascularity within the tendon, tendon degeneration, poor gastrocnemius-soleus flexibility, and anatomic factors such as ‘overpronation’ (Reiter & Vad 1999: p423). Extrinsic factors include a sudden increase in training intensity, running surface change, and worn out or inappropriate footwear (Reiter & Vad 1999: p423).

Tendons are subject to compressive, tensile of shear forces, but resist tensile best. In the case of the Achilles compression can occur not least from footwear, but also over the bodies own anatomical structures, such as the posterior tubercle of the calcaneus. This is more likely in people with a high calcaneal inclination angle; that is cavoid feet.

However, as discussed in the section on biomechanics and anatomy, the area injured within the Achilles tendon may be dependent on foot motion during gait. I suggest that if pain is located medially, then the clinician should look for excessive or abnormal eversion moments occurring at the rearfoot. However, if the pain on palpation is more laterally located, then the clinician should look for abnormal or excessive inversion moments occurring at the rearfoot. Also, it is likely that medial sided symptoms are more likely occurring earlier in the gait cycle, with lateral sided symptoms occurring later; as the soleus fires earlier than gastrocnemius (Perry 1992: p59).


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