Analysis of “Invariant Characteristics” in the Motor Control of Down’s Syndrome and Normal Subjects

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Abstract
Following Asatryan and Fel’dman (1965), two experiments were conducted to describe the so-called invariant mechanical properties underlying movement control in Down’s syndrome and normal subjects. The invariant characteristic is a curve on a graph of joint torque versus joint angle. The task required subjects to maintain a steady joint angle against an external load (torque). Torque was systematically changed via partial unloading in order to obtain torque by length (joint angle) functions at three separate initial joint angles. Instructions required subjects “not to intervene” when unloading occurred in Experiment 1 and to “tense” their muscles prior to unloading in Experiment 2. Both normal and Down’s syndrome groups revealed systematic torque by length functions that might be expected according to a simple mass-spring system model. Although the gross organization of movement in Down’s syndrome subjects was nearly the same as normals, important differences between the two groups were found. Down’s syndrome subjects revealed underdamped motions relative to normals (as shown by differences in the degree of oscillation about the final equilibrium position) and were less able to regulate stiffness (as shown by differences in slope of the torque by angle functions in Experiment 2). We promote the notion that damping and stiffness may be sensitive indices of hypotonia—the most common description of neuromuscular deficiency in Down’s syndrome.