The objectives of this work were (i) to investigate whether chronic endurance running is a sufficient stimulus to counteract the age-related changes in the mechanical and morphological properties of human triceps surae(TS) and quadriceps femoris (QF) muscle–tendon units (MTUs) by comparing runners and non-active subjects at different ages (young and old), (ii) to identify adaptational phenomena in running mechanics due to age-related changes in the mechanical and morphological properties of the TS and QF MTUs,and finally (iii) to examine whether chronic endurance-running exercise is associated with adaptational effects on running characteristics in old and young adults.

The investigation was conducted on 30 old and 19 young adult males divided into two subgroups according to their running activity: endurance-runners vs non-active. To analyse the properties of the MTUs, all subjects performed isometric maximal voluntary (MVC) ankle plantarflexion and knee extension contractions at 11 different MTU lengths on a dynamometer. The activation of the TS and QF during MVC was estimated by surface electromyography. The gastrocnemius medialis and the vastus lateralis and their distal aponeuroses were visualized by ultrasonography at rest and during MVC, respectively. Ground reaction forces and kinematic data were recorded during running trials at 2.7 m s^–1.

The TS and QF MTU capacities were reduced with aging (lower muscle strength and lower tendon stiffness). Runners and non-active subjects had similar MTU properties, suggesting that chronic endurance-running exercise does not counteract the age-related degeneration of the MTUs. Runners showed a higher mechanical advantage for the QF MTU while running (lower gear ratio) compared to non-active subjects, indicating a task-specific adaptation even at old age. Older adults reacted to the reduced capacities of their MTUs by increasing running safety (higher duty factor, lower flight time) and benefitting from a mechanical advantage for the TS MTU, lower rate of force generation and force generation per meter distance. We suggest that the improvement in running mechanics in the older adults happens due to a perceptual motor recalibration and a feed-forward adaptation of the motor task aimed at decreasing the disparity between the reduced capacity of the MTUs and the running effort.