Improving our understanding of the origins and functional consequences of achilles subtendon sliding in walking

Mechanical power generated via triceps surae muscle-tendon interaction is an important determinant of walking performance. This interaction is made complex by the architecture of the Achilles tendon (AT), with distinct bundles of tendon fascicles, known as "subtendons", arising from the biarticular gastrocnemius (GAS) and uniarticular soleus (SOL) muscles. In addition to its potential relevance to tendon injury, there is growing appreciation that inter-fascicular sliding between and/or within these subtendons may be functionally important during walking for at least two reasons. First, subtendon sliding in walking could facilitate independent actuation among the triceps surae muscles and thereby the appropriate transmission of muscle forces to vertical support and forward propulsion (Fig. 1A). Second, comparative data and our own in vivo evidence allude to a reduced capacity for sliding between adjacent subtendons in old age (Fig. 1B) - an outcome that correlates with reduced ankle moment and positive ankle work during push-off in