Altered plantarflexor muscle-tendon mechanics and gearing during stretch-shortening contractions in older adults

Ageing is associated with impairments in muscle force-generating capacity and tendon mechanical properties, yet its effects on stretch-shortening cycle (SSC) function remain unclear. We combined in vivo ultrasonography with dynamometric and kinematic measurements to investigate the muscle-tendon mechanisms underpinning SSC performance in older adults. Fourteen young (28 ± 5 years) and fifteen older adults (67 ± 3 years) performed electrically evoked plantarflexion contractions on an isokinetic dynamometer. Pure shortening and stretch-shortening (SSC) contractions were performed at angular velocities of 60 and 150°·s^-1. Ankle torque, muscle-tendon unit (MTU) force, fascicle force, fascicle and Achilles tendon (AT) length changes, muscle belly gearing, tendon gearing, and mechanical work were quantified. Older adults exhibited lower maximal plantarflexor force and AT stiffness than young participants (both P < 0.001). SSCs increased ankle torque, MTU force, and fascicle force in both groups, with a greater enhancement at the higher shortening velocity. The magnitude of SSC-induced force enhancement was similar between groups, with no significant age×SSC interactions for force-related variables. Older adults exhibited greater AT elongation and tendon gearing, but less fascicle shortening, smaller changes in pennation angle, and lower muscle belly gearing. AT mechanical work increased during SSCs in both groups, whereas fascicle work was reduced only in the young participants. These findings demonstrate no significant age-related differences in SSC-effect between populations despite marked reductions in maximal force-generating capacity. The comparable SSC-effect could be achieved through altered muscle-tendon interaction strategies characterised by greater AT compliance and modified fascicle behaviour. Ageing, therefore, seems to preserve SSC function while altering the distribution of mechanical work within the MTU.