SEASONALITY IN LOWER LEG LENGTH VELOCITY IN PREPUBERTAL CHILDREN

Despite low measuring error, annual growth is poorly predicted by short-term measurements of the lower leg. In order to study if this low correlation can be explained by seasonal changes in lower leg length (LLL) velocity, we followed short-term growth longitudinally in 50 prepubertal children with normal height and growth velocities. Height measurements were performed at 4-week intervals and LLL measurements at 1-2-week intervals. Analysis of annual growth showed seasonality in the monthly mean height velocity values: 5.06 (SD 5.02) cm/year during the autumn and 8.15 (SD 5.22) cm/year in the spring. Similarly, the monthly mean LLL velocity values changed from 1.80 (SD 1.07) cm/year in the autumn to 2.63 (SD 0.92) cm/year in the spring. The correlation between monthly LLL and annual height velocity was low (r = 0.27). The technical error of the LLL measurement was 7-8% of the mean monthly LLL velocity, while the mean values changed by 31% over the seasons. The annual height velocity was virtually independent of the variation in growth rate over the seasons. It is concluded that there is significant seasonality both in height and LLL velocity and that it takes place at the same time for both measures. Seasonality in LLL has not been reported previously and must be considered when studying short-term growth, for example when LLL is used for prediction of annual height velocity or when a short-term treatment effect is examined using LLL.