Longitudinal principal component analysis of somatic and motor characteristics

Longitudinal principal component analysis was applied to the study of growth patterns of several somatic and motor characteristics. The data are from the "Leuven Growth Study of Belgian Boys," in which six annual observations were made. Peak growth velocity and age at peak growth velocity for each variable were estimated. The results for height show three components sufficient to provide an adequate representation of the original information. The first component can be interpreted as a growth distance component, which characterizes the general position of an individual growth curve relative to the average growth curve during the period analyzed. Components 2 and 3 reflect fluctuations in percentile level during the age period studied and can be conceived as indices of stability. Component 2 provides additional information about the age at peak height velocity; component 3 characterizes variation in peak height velocity. These interpretations can, in general, be assumed for the other characteristics analyzed.