Developmental aspects of equilibrium control during stance: a kinematic and EMG study

Five groups of children from 2 to 6 years of age and a group of adults were tested during stance on a treadmill moving sinusoidally at different frequencies (0.25, 0.33, and 0.50 Hz) and a constant displacement of 12 cm in anterior-posterior direction. Biomechanical data together with electromyographic responses and videoanalysis of body movements were recorded. Differential effects of treadmill frequency on the kinematic data between children and adults were observed. For slow and for fast frequencies a reduction of the oscillations from foot to head was found, being greatest in the adult subjects and smallest in the youngest children. This damping of the oscillations was due to a forward inclination of the body around the posterior turning point and its erection around the anterior turning point. With respect to the timing this represents a feedforward mechanism to compensate for the inertial forces in an economical way for the older children and the adults. In the smaller children these body movements were delayed, indicating a more passive mode of reaction to the forward-backward sway. Correspondingly the amplitudes of the electromyographic activity were largest in the groups of the youngest children for all conditions tested, especially for the leg flexor muscles. It is suggested that the greater amplitude of leg muscle activation of the children is required to compensate for the inertial forces acting at the body less economically than for the adults. Extensor load receptors are supposed to be responsible to keep the centre of gravity over the feet. The immaturity of both the function of long-latency stretch reflexes and of the extensor load receptors may play a role in the difficulties of balance control observed in small children.