The structure of the tendon is complex and important to understand due to the effect of torsion has on the effectiveness of the Achilles tendon in its important role in the efficiency of gait. The Gastrocnemius and Soleus complex acts on three joints of the lower extremity: the knee (Gastrocnemius only) and the Ankle and Subtalar joint (both muscles). Even though they have the same insertion point they may at certain times have the same or different modes of action on these joints (28)
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The Achilles tendon changes shape over a time, not just in cross sectional area and length but most importantly in torsion. In adults the Achilles tendon demonstrates various amounts of torsion. This torsion is seen to be important in elongation and elastic energy return. The complex makeup of the tendon allows it to play a greater role in creating moments at the Ankle and Subtalar Joint (26) All tendons have evolved primarily to transmit tensile load and have a fibrous tissue structure to accommodate this(29) .
The post-natal progressive changes in cellular and fibrous components are closely related to the intensity and duration of mechanical stress. As loads on the tendon increase the collagen fibers grow in diameter (30). It appears that this growth and change in the torsion of the Achilles is a result of physical activity (26)
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This suggests that greater activity, results in greater torsion of the Achilles tendon and the ability elastic energy return increases.
Another important feature when looking at the Achilles tendon is the stiffness, various Achilles tendon stiffness will mean differing amounts of elongation under load and therefore differing amounts of returned stored elastic energy .The greater the amount of stored and returned elastic energy the greater efficiency of gait. There however needs to be a balance with the fact that excessive elongation of the Achilles may lead to plastic deformation and injury. Therefore there needs to be a balance between torsion and stiffness. The torsion of the tendon will allow it to function more like a spring, with the stiffness determining how much compression and elongation the tendon can tolerate without becoming injured. Muraoka et al note that the cross-sectional area of the Achilles does not relate to stiffness, ie the greater the cross-sectional area the stiffer the tendon (31) no correlation was noted. But they did discover that the greater the muscle strength of the Gastrocnemius and Soleus complex the stiffer the tendon (32). So the two important features of the tendon which give it the ability to tolerate and produce forces, store and release elastic energy are effected by one the torsion of the tendon itself and the stiffness, which is controlled by the strength of the
Gastrocnemius and Soleus complex.
26- Feehery Jr RV : Surgery of the Achilles Tendon and Posterior Muscle group. Clinical Podiatric medical Surgery: 9: 781-811,1992
27- Alexander RM, Bennet-Clarke HC, Storage of elastic strain energy in muscle and other tissues. Nature : 265:114-117, 1994.
28- Neptune RR, Kautz SA, Zajac FF : Contributions of the individual ankle plantarflexors to support, forward progression and Swing initiation during walking. Journal of Biomechanics. 34: 1387-1398, 2001
29- Cook J, Purdam C : Is compression load a factor in the development of Tendionopathy ? Br J Sports Med doi:10.1136/bjsports-2011-090414
30- Strocchi R, DePasquale V, Guizzardi S et al : Human Achilles tendon: morphological and morpometric variation as a function of age. Foot and Ankle. 12: 100-104, 1991
31- Murako T, Muramatsc T, Fukunaga T et al : Geometric and Elastic Properties of in vivo Human Achilles Tendon in Young adults. Cells Tissues Organs. 178: 197-203,2004
32- Murako T, Muramatsc T, Fukunaga T et al: Elastic properties of human Achilles tendon are correlated to muscle strength. J Appl Physiol. 99: 665-669, 2005
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