THE EFFECT OF POSTURE ON EARLY REACHING MOVEMENTS

Infants of about 5 months of age who have just mastered the ability to reach succeed more frequently in contacting an object when they are s eated upright than when they are supine or reclined. That effect of po sture disappears in the subsequent months. Whether that effect can be attributed either to insufficient muscular strength or to insufficient control over the mechanically unstable arm was the subject of the pre sent investigation. Kinematics and electromyography (EMG) of reaching movements of 8 sitting and supine infants at 12, 16, and 20 weeks of a ge were recorded. Maximum levels of shoulder torque as well as kinemat ic stability measures were similar in both postures. Coactivation leve ls and the frequency of on-off switching of muscles turned out to be h igher in the sitting than in the supine posture. The authors suggest t hat the difference in reaching behavior resulted from the degree of er ror in the feedforward control signal that was allowed by the differen t postures rather than either insufficient muscular strength or insuff icient control over the mechanically unstable arm.