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 profile) are a reflection of the control process. In this study, we investigated 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 infants just begin to reach, a toy rattle was presented in three conditions, with varying external forces acting on the arm: sitting, sitting with masses on the forearms, and supine.The tangential velocity profile of the wrist during successful reaching attempts was decomposed to a gravity-dependent (φ_v), and a gravity-independent component (φ_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 time, number of movement units and duration ofphi;_hcomponent 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 moment of inertia of the arm, and the stiffness of the shoulder joint. Stiffness 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 reflection of the mechanical context.