Age-related differences in gastrocnemii muscles and Achilles tendon mechanical properties in vivo.

Ageing is associated with alterations in the structure and function of the contractile and elastic tissues that enable movement, posture, and balance. Alterations in structure and mechanical properties of the ankle plantarflexors and Achilles tendon are of particular interest due to their important 'catapult-like' function during efficient and healthy human locomotion. In this study, we examined age-related differences in the in vivo mechanical properties of both muscle and tendon in the human ankle plantarflexors in healthy younger (21 ± 3.25 years) and older (69 ± 2.86 years) adults. All participants were physically active, to represent healthy ageing. B-mode ultrasound coupled with force measurements was used to determine in vivo Achilles tendon stiffness and shear-wave elastography was used to measure shear modulus, an index of muscle stiffness, in the medial and lateral gastrocnemii. We found that older adults displayed 43% lower (p = 0.004) Achilles tendon stiffness, 59% lower (p < 0.001) Achilles tendon Young's modulus, and 34% greater (p = 0.002) Achilles tendon cross-sectional area compared to younger participants. We found no difference in the shear modulus of the medial or lateral gastrocnemii between the younger and older individuals. The reduction in Achilles tendon stiffness coupled with similar gastrocnemii muscle shear modulus likely influences the integrated neuromechanical function of the ankle plantarflexor muscle-tendon units during locomotor tasks. Further investigations into the relationship between altered mechanical properties and in vivo muscle-tendon dynamics will provide greater insights into the age-related declines in mobility and locomotor function.