On variability and stability in human movement

Current research in biology and physiology has unequivocally demonstrated the significance of variability for the optimal functioning of healthy and adaptable systems. Different pathologies are characterized by reductions in complexity of organization, often signified by loss of variability and adaptability. It is argued that the traditional perspective on biology in general and movement science in particular that tended to associate noise and variability with performance decrements and pathology is no longer tenable. Tools and methodologies that have emerged from the dynamical systems perspective to coordination and control are discussed in the context of postural control and transitions in interlimb coordination and locomotion. First, it is shown that variability can play a functional role in the detection and exploration of stability boundaries during balance control. Second, pattern transitions are characterized by increased variability in movement coordination dynamics. Under conditions of movement pathologies, such as in Parkinson's disease, reductions in variability in coordination dynamics clearly identify movement coordination and transition problems so characteristic for these patients. It is concluded that the relation between variability and stability is complex and that variability cannot be equated with instability without knowledge of the underlying movement dynamics