Abstract
Tendons transfer forces produced by muscle to the skeletal system and can therefore have a large influence on movement effectiveness and safety. Tendons are mechanosensitive, meaning that they adapt their material, morphological and hence their mechanical properties in response to mechanical loading. Therefore, unloading due to immobilization or inactivity could lead to changes in tendon mechanical properties. Additionally, ageing may influence tendon biomechanical properties directly, as a result of biological changes in the tendon, and indirectly, due to reduced muscle strength and physical activity. This review aimed to examine age-related differences in human leg extensor (triceps surae and quadriceps femoris) muscle-tendon unit biomechanical properties. Additionally, this review aimed to assess if, and to what extent mechanical loading interventions could counteract these changes in older adults. There appear to be consistent reductions in human triceps surae and quadriceps femoris muscle strength, accompanied by similar reductions in tendon stiffness and elastic modulus with ageing, whereas the effect on tendon cross sectional area is unclear. Therefore, the observed age-related changes in tendon stiffness are predominantly due to changes in tendon material rather than size with age. However, human tendons appear to retain their mechanosensitivity with age, as intervention studies report alterations in tendon biomechanical properties in older adults of similar magnitudes to younger adults over 12–14 weeks of training. Interventions should implement tendon strains corresponding to high mechanical loads (i.e., 80–90% MVC) with repetitive loading for up to 3–4 months to successfully counteract age-related changes in leg extensor muscle-tendon unit biomechanical properties.
Highlights
The leg extensor muscle-tendon units (MTUs) play important roles in locomotion, with the muscles opposing gravity and controlling and generating progression by decelerating and accelerating the center of mass and the tendons storing and returning elastic energy to the musculoskeletal system (Biewener and Roberts, 2000; Roberts, 2002; Pandy and Andriacchi, 2010)
This review aims to examine age-related differences in human leg extensor MTU biomechanical properties and if changes in these properties can be counteracted in older adults
We can conclude that the observed changes in tendon stiffness due to ageing are predominantly due to changes in tendon material properties rather than reduced CSA
Summary
The leg extensor muscle-tendon units (MTUs) play important roles in locomotion, with the muscles opposing gravity and controlling and generating progression by decelerating and accelerating the center of mass and the tendons storing and returning elastic energy to the musculoskeletal system (Biewener and Roberts, 2000; Roberts, 2002; Pandy and Andriacchi, 2010). In vitro animal studies have associated ageing with an increase in irreducible collagen cross-linking, a reduction in collagen fibril diameter and its crimp angle, an increase in more extensible elastin content, a reduction in extracellular water and glycosaminglycans content and an increase in collagen type V (Vogel, 1991; Nakagawa et al, 1994; Tuite et al, 1997; Dressler et al, 2002; Kjaer, 2004) These changes may lead to altered biomechanical properties of tendons in vivo, which in turn, could affect the overall function of the MTUs. In addition to biological changes, altered environmental mechanical stress may influence ageing tendon. Mechanical loading interventions to trigger alterations in these properties in older adults are reviewed, in order to determine if and to what extent age-related changes can be counteracted and if particular criteria for successful interventions exist
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