The role of time and size in ontogenetic allometry: II. An empirical study of human growth.

The changes in shape that occur as a consequence of size changes during the third trimester (22 to 36 weeks post conception) of human growth are examined for muscular, tendinous and skeletal measurements of the lower limb. Gestational age and weight were highly correlated, but not linear. Several models appear to have equivalent fits to these data. The skeletal measurements were linear with body weight, but did not follow the predictions of any biomechanical model. Femur length grows more quickly than body weight, as does femur diameter. The ratio of length to diameter is nearly one, suggesting that the relative shape of the femur does not change with weight. The weights of the two muscles examined were nonlinear as functions of age or weight; Gompertz models with different coefficients fits these data best. The lengths of the two tendons were linear with age, but tendon weights were best described by nonlinear Gompertz models. One difficulty of the Gompertz models stems from the lack of an upper asymptote in these data. The data fit the lower half of the curve, however the model predicts an asymptote beyond the range of data available. The relative patterns of growth, i.e. muscles growing faster than tendons, suggest hypotheses that need to be tested with a larger data set. These include growth of muscles relative to bones or tendons in other regions, e.g., the upper limb or the cranium, and an extension of the data in time.