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

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

1. Human gastrocnemius medialis architecture was analysed in vivo, by ultrasonography, as a function of joint angle at rest and during voluntary isometric contractions up to the maximum force (MCV). maximum force (MVC). 2. At rest, as ankle joint angle increased from 90 to 150 deg, pennation increased from 15.8 to 27.7 deg, fibre length decreased from 57.0 to 34.0 mm and the physiological cross-sectional area (PCSA) increased from 42.1 to 63.5 cm2. 3. From rest to MVC, at a fixed ankle joint angle of 110 deg, pennation angle increased from 15.5 to 33.6 deg and fibre length decreased from 50.8 to 32.9 mm, with no significant change in the distance between the aponeuroses. As a result of these changes the PCSA increased by 34.8%. 4. Measurements of pennation angle, fibre length and distance between the aponeuroses of the gastrocnemius medialis were also performed by ultrasound on a cadaver leg and found to be in good agreement with direct anatomical measurements. 5. It is concluded that human gastrocnemius medialis architecture is significantly affected both by changes of joint angle at rest and by isometric contraction intensity. The remarkable shortening observed during isometric contraction suggests that, at rest, the gastrocnemius muscle and tendon are considerably slack. The extrapolation of muscle architectural data obtained from cadavers to in vivo conditions should be made only for matching muscle lengths.

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In vivo human gastrocnemius architecture with changing joint angle at rest and during graded isometric contraction

Architectural properties of the triceps surae muscles were determined in vivo for six men. The ankle was positioned at 15° dorsiflexion (−15°) and 0, 15, and 30° plantar flexion, with the knee set ...

Architectural and functional features of human triceps surae muscles during contraction

Contractile force is transmitted to the skeleton through tendons and aponeuroses, and, although it is appreciated that the mechanocharacteristics of these tissues play an important role for movement performance with respect to energy storage, the association between tendon mechanical properties and the contractile muscle output during high-force movement tasks remains elusive. The purpose of the study was to investigate the relation between the mechanical properties of the connective tissue and muscle performance in maximal isometric and dynamic muscle actions. Sixteen trained men participated in the study. The mechanical properties of the vastus lateralis tendon-aponeurosis complex were assessed by ultrasonography. Maximal isometric knee extensor force and rate of torque development (RTD) were determined. Dynamic performance was assessed by maximal squat jumps and countermovement jumps on a force plate. From the vertical ground reaction force, maximal jump height, jump power, and force-/velocity-related determinants of jump performance were obtained. RTD was positively related to the stiffness of the tendinous structures (r = 0.55, P < 0.05), indicating that tendon mechanical properties may account for up to 30% of the variance in RTD. A correlation was observed between stiffness and maximal jump height in squat jumps and countermovement jumps (r = 0.64, P < 0.05 and r = 0.55, P < 0.05). Power, force, and velocity parameters obtained during the jumps were significantly correlated to tendon stiffness. These data indicate that muscle output in high-force isometric and dynamic muscle actions is positively related to the stiffness of the tendinous structures, possibly by means of a more effective force transmission from the contractile elements to the bone.

Muscle performance during maximal isometric and dynamic contractions is influenced by the stiffness of the tendinous structures

To obtain an insight into tendon elasticity during human movement, a real-time ultrasonography was applied to the contracting tibialis anterior muscle. The insertion point of fascicles onto the aponeurosis was clearly visualized, and its position relative to a fixed marker on the skin moved proximally (Δ1) according to the increasing dorsiflexion force (ΔF) with a fixed ankle joint. Notably, the Δ1 − ΔF relationship in the tendon was found to be quadratic in nature (ΔF = cΔ12; c=1.48 ∼ 2.24, r=0.985 ∼ 0.992, n=9) as has been reported in the isolated tendon, although the ΔF − Δ1 curves were slightly underestimated in comparison with the stiffness constant estimated from tendon architecture. This underestimation might be caused by changes in the height of the foot arch with the application of force.

M Itoh

Papers

Ultrasonography gives directly but noninvasively elastic characteristic of human tendon in vivo.