Increased Safety Margins with Motor Learning in Older Adults

Abstract

Aging is associated with changes in the sensorimotor system that result in slow and variable movements, which require adequate safety margins to prevent injury. When controlling objects with complex dynamics, older adults maintain smaller safety margins than young adults. However, the opposite is true when controlling simple objects, i.e. older adults use safety margins that are larger than younger adults. Thus, the size of safety margins in older adults may be related to their ability to control the dynamics of the task. We hypothesized that when learning to control a complex object, like a cup of coffee, older adults will increase their safety margins as they practice manipulating the object and become familiar with the objectÍs dynamics. Four healthy older subjects (65-80 years) practiced moving a virtual ball in a cup, representing a cup filled with coffee, from one point to another in two seconds without losing the ball a total of 240 times. Subjects grasped a robotic arm that simulated the ball and cup forces. The safety margin was defined as the ball energy relative to ball escape. With practice subjects became better at completing the movement in the two second target-time. Compared to young subjects, the older subjects began with safety margins that were about half the size of the young subjects. With practice three out of four subjects increased their safety margins to levels approximating the young subjects. This supports the hypothesis that older adults increase their safety margins as they learn the dynamics of an object.