Gastrocnemius medialis muscle-tendon behavior during eccentric exercise in young and active older adults.

Eccentric exercise is widely used in rehabilitation protocols for older adults. However, the age-related behavior of the muscle-tendon unit (MTU) during eccentric actions remains incompletely understood, especially regarding muscle-tendon decoupling and its influence on fascicle dynamics and force production. Better characterizing these mechanisms would help optimize the design of rehabilitation protocols in older individuals. This study aimed to investigate age-related differences, while ensuring comparable physical activity levels between groups, in gastrocnemius medialis (GM) MTU behavior during eccentric exercises performed at maximal and submaximal intensities, during controlled conditions on an isokinetic dynamometer and during a typical eccentric training exercise which can be done at home, i.e., heel-drop exercise. GM muscle fascicle length and tendon strain were simultaneously measured with ultrasonography during the eccentric exercises, while ankle angle, ankle torque and GM muscle activation were monitored. The results did not show any statistically significant between-group differences in GM MTU behavior during eccentric exercise between age groups, regardless of intensities (i.e., maximal versus submaximal) and conditions of the exercise (i.e., dynamometer versus heel-drop). Specifically, no statistically significant differences were detected in GM tendon strain, muscle fascicle length and velocity between active young adults and active non- or pre-frail older adults, in all exercise conditions and intensities (all p > 0.05). No significant differences were reported in Achilles tendon stiffness between groups (p > 0.05). These results suggest that the effects of aging on the MTU behavior and muscle-tendon interactions may be mitigated by maintaining important physical activity with increasing age.