Aging substantially reduces movement capabilities. By age 70 muscle strength and mass decline 40%, range of motion decreases 50%, rate of tension development slows, and balance is impaired. Because such motor deficits can cause accidents while descending stairs, as an initial effort, a reduced model of stair descent was developed in the form of downward stepping to answer the question: What are the compensatory movement strategies used by elders to perform downward stepping? While stepping down the impact is countered by a stiffening of the limb. Limb stiffness reflects the dynamic interaction between the external force applied to the limb and the shortening of the limb as the joints are flexed. It is hypothesized that in compensation for the impaired movement capabilities, old compared with young people perform downward stepping with greater limb stiffness. One proposed compensatory movement strategy by elders during downward stepping leading to increased limb stiffness is the use of a straighter limb as elders have difficulty in flexing the joints and lowering the center of mass due to their motor deficits. Because downward stepping in many elders is associated with a fear of losing balance, it is also conceivable that elders would tend to activate muscles that are not specific to the task. Hence, a second compensatory movement strategy used by elders during stepping could be the increased voluntary coactivation of the antagonist muscles. As the stiffness contributions of opposing sets of muscles are additive at each joint, an increased antagonist muscle coactivation would also increase total limb stiffness. The benefits for elders performing downward stepping with a greater limb stiffness are lower force production by muscles, smaller accelerations, a simplified processing of the sensory array, and less time spent in the precarious one leg support making elders anticipate a safer landing. Whereas increased limb stiffness during downward stepping is an expected compensation for motor deficits by elders, excessive limb stiffness can cause elders to lose their balance. Thus the long-term objectives are: 1. To determine a threshold value of stiffness that predicts fallers as in this case the center of mass assumes too high a position and even a small error in foot positioning at impact could cause a fall and 2. To advance the current single stepping paradigm into a multiple stepping model to extensively examine elders' stair descent behavior.
