Age-related declines in ballistic plantarflexion: Contributions of muscle activity, intermuscular coordination, and Achilles tendon stiffness.

Age-related impairments in balance and gait are strongly linked to deficits in plantarflexion, yet the underlying changes in force control and their neural and muscle-tendon determinants remain poorly understood. The purpose of this study was to determine whether central and/or peripheral mechanisms contribute to age-related declines in rapid, goal-directed plantar flexions. Twenty-two young (22 ± 2 years) and twenty-two older adults (72 ± 6 years) performed 3 sets of 10 plantar flexions to match force targets of 30% and 80% MVC as accurately and quickly as possible. Rapid force control was assessed using force error, time to peak force (TPF), force variability, and TPF variability. Surface EMG from plantar flexor and dorsiflexor muscles quantified neuromuscular activation and coordination, while Achilles tendon stiffness was measured to evaluate peripheral contributions. Older adults exhibited longer TPF, greater force error, and greater force and temporal variability compared with young adults, indicating impaired rapid force control relevant to mobility. They also demonstrated altered activation patterns and impaired coordination between agonist and antagonist muscles, whereas Achilles tendon stiffness did not differ between groups. Age-related slowing and increased variability were largely predicted by slower and more variable activation of the lateral gastrocnemius. These deficits were also associated with greater delays between antagonistic muscles and increased coactivation among synergistic plantar flexors. Together, these findings suggest that neural activation and coordination deficits may contribute more substantially than Achilles tendon stiffness to age-related declines in ballistic plantarflexion and identify candidate neural markers of mobility decline and fall risk.