An unlearned principle for controlling natural movements

An unlearned principle for controlling natural movements. J. Neurophysiol. 82. 255-259, 1999. Recently, Gottlieb and colleagues discovered a linear re lation between elbow and shoulder dynamic torque in natural pointing moveme nts in the sagittal plane. The present study investigates if the process of learning to reach involves discovering this linearity principle. We inspec ted torque data from four infants who were learning to reach and grab a toy in front of them. In a longitudinal study, we collected data both in the p eriod before and after they performed their first successful reaches. Torqu e profiles at the shoulder and elbow were typically multipeaked and became more and more biphasic toward the end of the first year of life. Torques at the shoulder and elbow were correlated tightly for movements in the prerea ching period as well as for reaches later in the year. Furthermore, slopes of a regression of shoulder dynamic torque on elbow dynamic torque were rem arkably constant at a value similar to 2.5-3.0. If linear synergy is used b y the nervous system to reduce the controlled degrees of freedom, it will a ct as a strong constraint on the complex of possible coordination patterns for arm movement early in life. Natural reaching movements can capitalize o n this constraint because it simplifies the process of learning to reach.