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

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.

The interplay between gastrocnemius medialis force–length and force–velocity potentials, cumulative EMG activity and energy cost at speeds above and below the walk to run transition speed

“…Thus, combining our results with those recently published by Monte, Tecchio, Nardello and Zamparo ( 2022 ), it seems plausible that the transition from walking to running (which typically occurs between 7 and 8 km h −1 ) could be explained by a combination of mechanical factors that involve, among the others, a decrease in the force potentials and a reduction of the ankle spring‐like function. These mechanical alterations ultimately lead to an increase in metabolic energy expenditure; to avoid it, people switch from walking to running (regaining high force potentials and improving the ankle spring‐like function).…”

“…A muscle needs to produce larger forces at faster shortening speeds to drive motion, and the mechanical disadvantage imposed by the decrease in F–L and F–V potential at sustained walking speeds possibly exacerbates this effect. As recently shown by Monte, Tecchio, Nardello & Zamparo ( 2022 ), the elastic energy provided by the Achilles tendon (estimated by means of an inverse dynamic approach) is reduced at high walking speed (>6 km h −1 ); thus, the contribution of the Achilles tendon in determining the mechanical power generated by the ankle decreases and this is associated with an increase in the contribution of the contractile components of the plantar flexors’ muscle–tendon unit (Monte, Tecchio, Nardello & Zamparo, 2022 ).…”

“…Last but not least, a recent study suggested that the transition speed could also be explained by the need to preserve the Achilles tendon mechanical behaviour (i.e., AT force and power) and the ankle spring function (Monte, Tecchio, Nardello & Zamparo, 2022 ).…”