Changes in the lower leg moment of inertia due to child's growth.

During growth the size and shape of the child's body changes. It is not clear whether the shape of a body segment changes proportionally in children between the age 5 and 18 years. The aim of this study is to describe these changes for the lower leg moment of inertia in a population of children. The segment moment of inertia describes the mass distribution along the body segment axis. The moment of inertia of the lower leg (including the foot) was measured by the free oscillation technique in 90 healthy children (61 boys and 29 girls) between and 5 and 18 years of age. The period of free oscillation was measured with and without external mass loading. The moment of inertia was calculated using a relation between the mass and the period of oscillation. A two-cylinder model of constant body density was used to predict the moment of inertia. Anthropometric measurements of length of the lower leg and foot, the circumference of the knee, ankle and foot were made. Experimental and model data of the lower leg of inertia were described by a fifth power function of body height. The experimental and model data showed high degree of convergence, confirming that the segment growth of the human body can be treated like the volume growth of a cylindrical object of constant body density. Thus it was experimentally confirmed that the lower leg segment growth between age 5 and 18 years may be considered as proportional.