1〜2歳児の歩行獲得過程における運動力学的変化

Abstract

The aim of this research is to examine the kinetic and kinematic changes of infant gait by longitudinal observation of infants at the onset of bipedal walking. We are especially interested in the trajectory of the foot force as a kinetic parameter which is important in determining the developmental stage of an infant. Six infants, ranging from 10 months to 2 years and 1 month of age, were examined longitudinally by means of a force plate system and a three-dimensional motion analysis system. Two characteristics of the trajectory of foot force distinguish an infant from an adult during a single stance phase. First, the sagittal track length of the trajectory of the infant, standardized by foot length, is shorter than that of the adult. The length is correlated with the duration of single stance phase. Second, the transverse sway of the trajectory is characteristic of the infant gait. The above results are related to displacement of the joints, that is, transverse displacement of the hip joint, vertical displacement of knee joint, and the pattern of transverse movements of shoulder, hip, and knee joints. The transverse displacement of the hip joint during stance phase is larger in the infant gait than in the adult gait. It decreases with development. Rapid decrease is observed between 10 months and 15 months. The vertical drop of the knee joint is conspicuous at the earlier month of independent walking. This action of the knee joint causes an impact to the force plate. The pattern of transverse movements of shoulder and hip joints differs between infants and adults. The shoulder and hip joints of infants sway together during stance phase. The shoulder and hip joints of adults first begin to move laterally following heel contact. When the load acts on one foot, the adult shoulder joint moves in the opposite direction. The knee joints of infants and adults both move in the opposite direction to the shoulder joints at that time. The sway of each joint causes instability of the center of gravity and changes the trajectory of foot force. The trajectory of foot force and the displacement of joints accordingly express the stability of the center of gravity. They can be added to the series of parameters useful in judging the developmental stage of the infant gait.