Age increases the skeletal versus muscular component of lower extremity stiffness during stepping down

Elderly adults step down with greater lower extremity stiffness than young adults. The purpose of this study was to compare skeletal and muscular components of lower extremity stiffness between elderly and young adults during stepping down. Fourteen elderly (age, 70.1 years) and 16 young (age, 20.8 years) adults stepped down onto a force plate from 10% and 20% body heights while being videotaped. Lower extremity stiffness was defined as the ratio between the floor reaction force directed along the limb and limb compression. It was partitioned into skeletal and muscular components using the angular relationship (phi) between the direction of the force and the line of the leg. Our results showed that phi was 21% smaller (p < .03), the skeletal component was 48% larger (p < .025), and the ratio of skeletal to muscular components was 32% larger (p < .01) in elderly adults compared with young adults. Elderly adults rely more on their skeletal and less on their muscular systems when stepping down compared with young adults, producing a stiffer lower extremity.