The extracellular matrix (ECM), and especially the connective tissue with its collagen, links tissues of the body together and plays an important role in the force transmission and tissue structure maintenance especially in tendons, ligaments, bone, and muscle. The ECM turnover is influenced by physical activity, and both collagen synthesis and degrading metalloprotease enzymes increase with mechanical loading. Both transcription and posttranslational modifications, as well as local and systemic release of growth factors, are enhanced following exercise. For tendons, metabolic activity, circulatory responses, and collagen turnover are demonstrated to be more pronounced in humans than hitherto thought. Conversely, inactivity markedly decreases collagen turnover in both tendon and muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as, dependent on the type of collagen in question, some degree of net collagen synthesis. These changes will modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress, and likely make it more load resistant. Cross-linking in connective tissue involves an intimate, enzymatical interplay between collagen synthesis and ECM proteoglycan components during growth and maturation and influences the collagen-derived functional properties of the tissue. With aging, glycation contributes to additional cross-linking which modifies tissue stiffness. Physiological signaling pathways from mechanical loading to changes in ECM most likely involve feedback signaling that results in rapid alterations in the mechanical properties of the ECM. In developing skeletal muscle, an important interplay between muscle cells and the ECM is present, and some evidence from adult human muscle suggests common signaling pathways to stimulate contractile and ECM components. Unaccostumed overloading responses suggest an important role of ECM in the adaptation of myofibrillar structures in adult muscle. Development of overuse injury in tendons involve morphological and biochemical changes including altered collagen typing and fibril size, hypervascularization zones, accumulation of nociceptive substances, and impaired collagen degradation activity. Counteracting these phenomena requires adjusted loading rather than absence of loading in the form of immobilization. Full understanding of these physiological processes will provide the physiological basis for understanding of tissue overloading and injury seen in both tendons and muscle with repetitive work and leisure time physical activity.

Role of Extracellular Matrix in Adaptation of Tendon and Skeletal Muscle to Mechanical Loading

Large mammals, including humans, save much of the energy needed for running by means of elastic structures in their legs and feet. Kinetic and potential energy removed from the body in the first half of the stance phase is stored briefly as elastic strain energy and then returned in the second half by elastic recoil. Thus the animal runs in an analogous fashion to a rubber ball bouncing along. Among the elastic structures involved, the tendons of distal leg muscles have been shown to be important. Here we show that the elastic properties of the arch of the human foot are also important.

The spring in the arch of the human foot

Achilles, the warrior and hero of Homer's Iliad, lends his name to the Achilles tendon, the thickest and strongest tendon in the human body138. Thetis, Achilles's mother, made him invulnerable to physical harm by immersing him in the river Styx after learning of a prophecy that Achilles would die in battle. However, the heel by which he was held remained untouched by the water and thus Achilles had a vulnerable point. Achilles led the Greek military forces, which captured and destroyed Troy after killing the Trojan prince Hector. However, Hector's brother Paris killed Achilles by firing a poisoned arrow into his heel164.

Hippocrates, in the first recorded description of an injury to the Achilles tendon, stated that “this tendon, if bruised or cut, causes the most acute fevers, induces choking, deranges the mind and at length brings death.”26 Ambroise Pare, in 1575, recommended that a ruptured Achilles tendon be strapped with bandages dipped in wine and spices, but advised that the result was dubious27. Operative repair of a ruptured Achilles tendon was advocated in 1888 by another Frenchman, Gustave Polaillon27, although an Arabian physician performed such procedures as early as the tenth century A.D. In the twelfth century, an Italian surgeon, Guglielmo di Faliceto, believed that nature was unable to unite divided tendons and that operative treatment was necessary27.

Much research has been performed to elucidate the etiology of a rupture of the Achilles tendon, but its true nature still remains unclear190. Also, the best method of treatment is still fiercely debated. Some physicians advocate operative repair, whereas others insist that an operation is unnecessary and poses an unacceptable risk.

The tendinous portions of the gastrocnemius and soleus muscles merge to form the Achilles tendon. The plantaris …

Rupture of the Achilles Tendon

Two-thirds of Achilles tendon injuries in competitive athletes are paratenonitis and one-fifth are insertional complaints (bursitis and insertion tendinitis). The remaining afflictions consist of pain syndromes of the myotendineal junction and tendinopathies. The majority of Achilles tendon injuries from sport occur in males, mainly because of their higher rates of participation in sport, but also with tendinopathies a gender difference is probably indicated. Athletes in running sports have a high incidence of Achilles tendon overuse injuries. About 75% of total and the majority of partial tendon ruptures are related to sports activities usually involving abrupt repetitive jumping and sprinting movements. Mechanical factors and a sedentary lifestyle play a role in the pathology of these injuries. Achilles tendon overuse injuries occur at a higher rate in older athletes than most other typical overuse injuries. Recreational athletes with a complete Achilles tendon rupture are about 15 years younger than those with other spontaneous tendon ruptures. Following surgery, about 70 to 90% of athletes have a successful comeback after Achilles tendon injury. Surgery is required in about 25% of athletes with Achilles tendon overuse injuries and the frequency of surgery increases with patient age and duration of symptoms as well as occurrence of tendinopathic changes. However, about 20% of injured athletes require a re-operation for Achilles tendon overuse injuries, and about 3 to 5% are compelled to abandon their sports career because of these injuries. Myotendineal junction pain should be treated conservatively. Partial Achilles tendon ruptures are primarily treated conservatively, although the best treatment method of chronic partial rupture seems to be surgery. Complete Achilles tendon ruptures of athletes are treated surgically, because this increases the likelihood of athletes reaching preinjury activity levels and minimises the risk of re-ruptures. Marked forefoot varus is found in athletes with Achilles tendon overuse injuries, reflecting the predisposing role of ankle joint overpronation. Athletes with the major stress in lower extremities have often a limited range of motion in the passive dorsiflexion of the ankle joint and total subtalar joint mobility, which seems to be a predisposing factor for these injuries. Various predisposing transient factors are found in about one-third of athletes with Achilles tendon overuse injuries; of these, traumatic factors (mostly minor injuries) predominate. The typical histological features of chronically inflamed paratendineal tissue of the Achilles tendon are profound proliferation of loose, immature connective tissue and marked obliterative and degenerative alterations in the blood vessels. These changes cause continuing leakage of plasma proteins, which may have an important role in the pathophysiology of these injuries. The chronically inflamed paratendineal tissues of the Achilles tendon do not seem to have enough capacity to form mature connective tissue.

Achilles Tendon Injuries in Athletes

Achilles tendinopathy is prevalent and potentially incapacitating in athletes involved in running sports. It is a degenerative, not an inflammatory, condition. Most patients respond to conservative measures if the condition is recognised early. Surgery usually involves removal of adhesions and degenerated areas and decompression of the tendon by tenotomy or measures that influence the local circulation.

https://bjsm.bmj.com/content/bjsports/36/4/239.full.pdf

Patellar tendinopathy: some aspects of basic science and clinical management.

Storage of strain energy in elastic materials has important roles in mammal running, insect jumping and insect flight. The elastic materials involved include muscle in every case, but only in insect flight is the proportion of the energy stored in the muscle substantial.

Storage of elastic strain energy in muscle and other tissues

MANY vertebrate eyes possess a depression (fovea) in the retinal tissue that overlies an area of receptor surface specialised for high acuity vision. The shape of this depression varies but two extreme forms have been characterised1. Many birds and lizards2 and some fish1,3 possess deep ‘convexiclivate’ foveas, while shallow bowl-like ‘concaviclivate’ foveas are characteristic of the eyes of some primates and the binocular field of some birds (Fig. 1). Various suggestions have been made to explain the existence and shape of these different foveal surfaces. The functions proposed have been mainly concerned with visual acuity and include reduction of chromatic aberration4, clearing of the retinal tissue to minimise image degradation (concaviclivate foveas)5 and image magnification (convexiclivate foveas)6–8. It has also been suggested that the deep convexiclivate fovea acts as a fixation device and/or movement detector at the expense of loss of visual acuity over a small area of image9. We show here how the deep convexiclivate fovea can act as a sensitive, directional focus indicator.

The deep fovea as a focus indicator

THE songs of most insect species are stereotyped and show regular rhythms1,2 which can be readily defined and described. In such cases, closely related species produce songs which differ in rhythmic structure3–5, carrier frequency5,6 or combinations of these parameters3,7. We have examined the songs produced during courtship of the sibling drosophilid fly species Zaprionus tuberculatus, Z. sepsoides and Z. mascariensis which are known from morphometric8 and chromosomal evidence to be closely related. Drosophilid songs are produced by wing beating and normally consist of a regular series of similar mono- or polycyclic sound pulses with carrier frequencies of 150 to 500 Hz and pulse periods of 8 to 250 ms (for review, see ref. 9). Some species produce more than one type of song, and the pulsed song may be interrupted by protracted bursts of constant frequency tone10.

Regularity versus irregularity in specific songs of closely-related drosophilid flies

Animals. By R. McNeill Alexander. Cambridge University Press: 1990. Pp.509. pbk£50, $89.50; pbk£19.50, $34.50.

H. C. Bennet-Clark

Papers

Storage of elastic strain energy in muscle and other tissues

The deep fovea as a focus indicator

Regularity versus irregularity in specific songs of closely-related drosophilid flies

Formed for function

The lives of the spineless