ADAPTIVE DYNAMICS OF THE LEG MOVEMENT PATTERNS OF HUMAN INFANTS .1. THE EFFECTS OF POSTURE ON SPONTANEOUS KICKING

This is the first of two articles in which we describe how infants ada pt their spontaneous leg movements to changes in posture or to elicita tion of behaviors by a mechanical treadmill. In this article, we compa re the kinematics of kicks produced by 3-month-old infants in three po stures, supine, angled (45-degrees), and vertical, and examine the cha nges in muscular and nonmuscular force contributions to limb trajector y. By manipulating posture we were able to assess the sensitivity of t he nascent motor system to changes in the gravitational context. The p ostural manipulation elicited a distinct behavioral and dynamic effect . In the more upright postures, gravitational resistance to motion at the hip was 4 to 10 times greater than resistance met in the supine po sture, necessitating larger muscle torques to drive hip flexion. Kicks produced in the vertical posture showed a reduction in hip joint rang e of motion and an increase in synchronous joint flexion and extension at the hip and knee. At the same time, hip and knee muscle torques we re also more highly correlated in kicks performed in the vertical than in the supine or angled posture. This increased correlation between m uscle torques at the hip and knee implicates anatomical and energetic constraints-the intrinsic limb dynamics-in creating coordinated limb b ehavior our of nonspecific muscle activations.