PLASTICITY OF HUMAN TENDON’S MECHANICAL PROPERTIES: EFFECTS ON SPORT PERFORMANCE
AbstractINTRODUCTION: In the literature it is often mentioned, that the tendon is very relevant for the work producing capability of the muscle fibers and for the motion and the performance of the human body. During a given movement, strain energy can be stored in the tendon and this way the whole energy delivery of the muscle can be enhanced. Further, the higher elongation capability of the tendon with respect to the muscle fiber, allows a bigger change in length of the muscle-tendon unit. Therefore, the muscle fibers may work on a lower shortening velocity and as a consequence of the force-velocity relationship their force producing potential will be higher. Generally, the main functions of the tendon during locomotion are: (a) to transfer muscle forces to the skeleton (b) to store mechanical energy coming from the human body or/and from muscular work as strain energy and (c) to create favorable conditions for the muscle fibers to produce force as a result of the force-length-velocity relationship. A higher force potential of the muscle fibers due to the force-length-velocity relationship during submaximal contractions would decrease the volume of active muscle at a given force or a given rate of force generation and consequently would decrease the cost of force production. In the same manner during maximal muscle contractions (maximal activation level) the higher force potential of the muscle fibers will allow the muscles to exert higher forces. The reports about the influence of the non rigidity of the tendon on the effectivity of muscle force production reveal the expectation that sport performance during submaximal as well as maximal running intensities may be affected by the mechanical and morphological properties of the tendon. In a series of experiments we examined the mechanical properties of the lower extremities muscle-tendon units (MTU) from athletes displaying different running economy and sprint performance. The most economical runners showed a higher contractile strength and a higher tendon stiffness in the triceps surae MTU and a higher compliance of the quadriceps tendon and aponeurosis at low level tendon forces (Arampatzis et al., 2006). The faster sprinters exhibited a higher elongation of the vastus lateralis (VL) tendon and aponeurosis at a given tendon force and a higher maximal elongation of the VL tendon and aponeurosis during the MVC (Stafilidis and Arampatzis, 2007). Furthermore, the maximal elongation of the VL tendon and aponeurosis showed a significant correlation with the 100 m sprint times (r = -0.567, P = 0.003). It has been supposed that, the more compliant quadriceps tendon and aponeurosis will increase the energy storage and return as well as the force potential of the muscle due to the force-velocity relationship. These studies provide evidence that the mechanical properties of the tendons at the lower extremity at least partially explain the performance of the human musculoskeletal system during running activities. However, until now no study exist in reference to the potential for improving running performance by manipulating the tendon mechanical properties. Mechanical load induced as cyclic strain on connective soft tissues such as tendons is an important regulator of fibroblast metabolic activity as well as for the maintenance of tendon matrix (Chiquet et al., 2003). An increased loading typically stimulates cells for remodelling and, therefore, for increasing the mechanical properties of the tissue (Arnoczky et al., 2002). Whereas, a decreased loading leads to tissue destruction and weak mechanical properties of the tissue (Arnoczky et al., 2004). These reports demonstrate the highly plastic nature of tendons within the muscle-tendon unit of mammals and give evidence that tendon strain is an important mechanical factor regulating tendon properties. Generally, from a mechanobiological point of view strain magnitude, strain frequency, strain rate and strain duration of cells influence the cellular biochemical responses and the mechanical properties of collagen fascicles. Although it is known that mechanical loading induced as cyclic strain affects the mechanical properties of human tendons in vivo, the effect of a controlled modulation in cyclic strain magnitude, frequency, rate or duration applied to the tendon on the plasticity of human tendons in vivo is not well established. Understanding the details of tendon plasticity in response to mechanical loading applied to the tendon in vivo may help to improve tendon adaptation, reduce tendon injury risks and increases the performance potential of the human system. This paper aimed (a) to present the effects of a controlled modulation of strain magnitude and strain frequency applied to the Achilles tendon on the plasticity of tendon mechanical and morphological properties and (b) to investigate whether an exercise induced increase in tendon-aponeurosis stiffness and contractile strength at the triceps surae muscle-tendon unit affect running economy.
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