This paper presents a brief overview of a control systems framework for understanding normal and abnormal posture, with a special emphasis on postural control during arm movements. The need for meaningful and valid definitions of postural output is discussed. A distinction is drawn between the joint posture and balance outputs of the system. The roles of commands and perturbing inputs in controlling and disturbing joint posture and balance are described. The effects of initial conditions on command inputs and postural outputs are considered. Mechanisms for correcting postural errors are described, with a major focus on active closed-loop and open-loop control processes. The method of objectively describing system behavior by the gains, time delays, and thresholds of input–output functions is presented. All concepts are developed in relation to their clinical implications.

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