INFLUENCE OF MECHANICAL FACTORS ON MOVEMENT UNITS IN INFANT REACHING

In a number of studies it is assumed that movement units (combinations of an acceleration and a deceleration in the tangential velocity prof ile) are a reflection of the control process. In this study, we invest igated to what extent movement units reflect the oscillatory mechanics of the arm. At the ages of 16 and 20 weeks, the age range in which in fants just begin to reach, a toy rattle was presented in three conditi ons, with varying external forces acting on the arm: sitting, sitting with masses on the forearms, and supine. The tangential velocity profi le of the wrist during successful reaching attempts was decomposed to a gravity-dependent (phi(v)), and a gravity-independent component (phi (h)), and movement units were determined in the total velocity profile and in its components. Significant effects of experimental condition, but not age, were found for movement the, number of movement units an d duration of phi(h) component units. It was shown that these effects can be accounted for by a simple mechanical model, in which the arm is approximated as a harmonic oscillator governed by, gravity, the momen t of inertia of the arm, end the stiffness of the shoulder joint. Stif fness values, derived from fitting the model to the data, were in the order of 1 Nm/rad. From these results we conclude that movement units in an infant's initial attempts at reaching are to a large extent a re flection of the mechanical context. (C) 1997 Elsevier Science B.V.