Perspective on variability in the development of human action.

Humans are designed not only with variability but for variability. This article explores the important contribution of variability to successful human action. Human systems for action have abundant variability of tissues and processes. This plasticity provides for the necessary flexibility when humans encounter the metric and dynamic changes of growth, development, and adaptation of action across the life span. However, variability must have definable limits. The reduction of possible solutions to probable solutions and the reduction of variability appear to be common assumptions of most theories of human action. The lack of variability of action is a hindrance to the development of skilled, functional action, and excessive variability interferes with the production of automatic, dependable, and typical functional action. The lack of variability and excessive variability are hallmarks of the movement patterns produced by people across the life span following neurological insult. Active problem solving as therapy, with its inherent error as a part of the therapeutic process, is critical to the successful learning of functional actions. The role of the physical therapist is to create movement environments and provide personal and environmental constraints that elicit and support self-produced functional actions.