[Measuring short-term periodicity of pediatric growth]

The study of human growth is traditionally based on investigations of body height. Differences of height which significantly surpass the technical error of the measuring device then by definition represent growth. However, this definition proved to be unsatisfying if applied to changes of body stature within very short time intervals. We have measured 73 healthy children, aged between 2.9 and 15.9 years, for periods of 180 to 306 days once or twice weekly, and 23 healthy children of similar ages for periods of 3 months, 4 to 5 times per week. The individual series of lower leg length measurements (technical error 160 microns) were analyzed by a smoothing procedure yielding mean daily lower leg growth rates. The series of almost daily measurements were additionally analyzed in order to find periodic elements. In the case of weekly measurements, marked periodic changes of mean daily lower leg growth rates were found with sharp growth spurts once every 30 to 55 days. Additional investigations of the series of almost daily lower leg length measurements showed evidence that the above periodic changes of growth rate were caused by aliasing between shorter periods overlapping each other and the exact 7-day intervals of measurements. We found a number of dominant periods particularly at 7 to 9-day intervals.