<p>Proteins such as the collagen of tendons and other structures of the body tend to form cross-links with their neighbors as a result of aging. These new chemical bonds are known to increase the lateral force transfer between collagen molecules and, as a consequence, increase the shear stresses present in tissues with a high degree of fiber organization such as the Achilles tendon. A finite element model of the Achilles tendon is proposed to compare three levels of lesion severity on two different Achilles tendon models (young and old). The low and high levels of lesion severity show no difference between the old and the young cases, but the medium level of severity shows that the lower shear modulus of the young case protects the tendon by resulting in a more homogeneous stress field, whereas the old case shows an increase in the stress concentration near the lesion site.</p>