The Achilles tendon is the strongest, yet most commonly injured tendon in the human body. Repeated or sustained loading of the Achilles tendon during exercise could induce transient changes in gross mechanical and morphological properties that impact on muscle-tendon performance and tissue homeostasis, and may therefore be important in the context of tendon injury and adaptation. Characterising the immediate effect of commonly performed exercise routines on these properties would broaden our understanding of exercise-induced tendon adaptation and lead to more targeted exercise interventions. The aim of this thesis was to use freehand three-dimensional ultrasound to investigate the mechanical and morphological properties of the human Achilles tendon complex in healthy young adults and characterise the change in these properties immediately after eccentric exercise.