Influence of calcaneus angle and muscle forces on strain distribution in the human Achilles tendon

BACKGROUND: Heterogeneous distribution of tendon strain is considered to contribute to the development of the Achilles tendon overuse injuries. Force distribution between the three portions of the triceps surae muscle and position of the calcaneus might affect the extent of strain differences within the Achilles tendon. Purpose of this study was to determine the effect of changes in force distribution within the triceps muscle and changes in calcaneus position on intratendinous strain distribution of the Achilles tendon.

METHODS: Five cadaveric Achilles tendons including complete triceps surae and calcaneus were dissected. Specimens were mounted in a loading simulator allowing independent force application for the three parts of triceps muscle and changes calcaneus eversion and inversion position. Strain was determined in different aspects of the Achilles tendon.

FINDINGS: Changes of calcaneus position resulted in intratendinous strain differences up to 15%, changes in force distribution within the triceps muscle resulted in strain differences up to 2.5%. Calcaneal eversion was connected to a higher degree of strain in medial tendon portions, while inversion increased strain in lateral tendon portions.

INTERPRETATION: Medio-lateral, proximo-distal and dorsal-ventral distribution of tendon strain is rather influenced by kinematics of the subtalar joint than by muscular imbalances within the triceps muscle. Clinical movement analyses should focus on motion pattern combining rearfoot eversion with high Achilles tendon load. The results indicate that twist of the Achilles tendon fascicles seems of paramount importance in balancing tendon strain. To get more insight into the Achilles tendon injuries pathogenesis future research should focus on methods monitoring heterogeneous distribution of strain in vivo.