Fig. 5

Strain patterns of the Achilles tendon during a voluntary contraction. The two cylinders represent a close-up of the free tendon at rest (panel 1) and during contraction (panel 2), assuming no change in knee and ankle joint angles. In panel 2, the red arrows indicate the shortening and bulging of the gastrocnemius muscle during contraction, while the black arrows illustrate the positive transversal strain (widening) around the gastrocnemius aponeurosis, as well as the negative transverse strain (thinning) at the free tendon. Additionally, the circular black arrow around the free tendon represents the rotation undergone during contraction. The grey scale scheme represents the magnitude of the longitudinal strain undergone by the tissue: the darker the colour, the greater the longitudinal strain. The white region around the soleus muscle tendon junction indicates no strain or even a negative longitudinal strain [86]. (S-I: superior–inferior, A-P: anterior–posterior, M-L: medio–lateral)