Achilles Tendon Mechanical Behavior and Ankle Joint Function at the Walk-to-Run Transition

SIMPLE SUMMARY: In this study we investigated the ankle joint functional indexes and the Achilles tendon mechanical behaviour (changes in AT force and power as a function of speed and gait) during walking and running at speeds close to transition speed (about 7.2–7.5 km/h in healthy adults), to better elucidate the mechanical determinants of the walk-to-run transition. Our results indicate that, when walking at speeds faster than the typical transition speed (7.5–8.5 km/h), the Achilles tendon mechanical behavior is impaired: the force acting along its line of action is reduced, as well as its contribution in determining the total mechanical power of the muscle–tendon unit. Moreover, our data suggest that the walk-to-run transition could be partially explained by the need to preserve the spring-like function of the ankle joint (which is indeed lower in walking than in running at speeds > 7.5 km/h). ABSTRACT: Walking at speeds higher than transition speed is associated with a decrease in the plantar-flexor muscle fibres’ ability to produce force and, potentially, to an impaired behaviour of the muscle–tendon unit (MTU) elastic components. This study aimed to investigate the ankle joint functional indexes and the Achilles tendon mechanical behaviour (changes in AT force and power) to better elucidate the mechanical determinants of the walk-to-run transition. Kinematics, kinetic and ultrasound data of the gastrocnemius medialis (GM) were investigated during overground walking and running at speeds ranging from 5–9 km·h(−1). AT and GM MTU force and power were calculated during the propulsive phase; the ankle joint function indexes (damper, strut, spring and motor) were obtained using a combination of kinetic and kinematic data. AT force was larger in running at speeds > 6.5 km/h. The contribution of AT to the total power provided by the GM MTU was significantly larger in running at speeds > 7.5 km/h. The spring and strut indexes of the ankle were significantly larger in running at speeds > 7.5 km/h. These data suggest that the walk-to-run transition could (at least partially) be explained by the need to preserve AT mechanical behaviour and the ankle spring function.