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.