Altered movement strategy increases lower extremity stiffness during stepping down in the aged

One way the human neuromuscular system negotiates locomotory challenges is by stiffening the muscles and joints of a limb. Because aging reduces joint range of motion and muscle strength, the intrinsic elements of stiffness, we hypothesized that lower extremity stiffness would be greater in elderly than in young subjects during downward stepping. Fourteen elderly (mean age 70.1) and 16 young women (mean age 20.8) stepped down from a platform adjusted to 10% and 20% of body height. Subjects stepped down onto a force plate and were videotaped from the side. The lower extremity was modeled as a simple spring and stiffness was computed as the quotient of stress under the foot and the shortening of the limb. Ankle and knee joint angular position and velocity were also determined during stepping. Compared with young women, Elders had 50% greater lower extremity stiffness and 28% less linear shortening of the limb. Elders also performed downward stepping with 92% less dorsiflexion and 28% less knee flexion and 42% less ankle and 57% less knee joint range of motion. Elders executed downward stepping with a more erect lower extremity alignment, resulting in a stiffer leg and an apparently safer movement strategy.