Head-trunk coordination during hops using one or two feet in children and adults.

The main purpose of this study was to investigate the development of head-trunk coordination during single hops using one foot or two feet in children of two ages (5.5 to 6 and 7 to 7.5) and adults (n = 6/group). The kinematics of the subjects' hops were analyzed by means of an automatic optical TV-image processor called the ELITE system. The absolute angular dispersion of the head, trunk, and leg about the pitch and the roll axis were measured. Head and trunk pitch and roll anchoring indexes were calculated in order to compare the stabilization of a given segment with respect both to external space and to the underlying anatomical segment. Results were analyzed separately for 3 phases: take-off, flight, and landing. Only the last two phases, flight and landing, are presented in this paper, and the following was found: 1) During flight, under both unipedal and bipedal conditions, head and trunk stabilization in space about the pitch axis occurred in children as well as in adults, suggesting an articulated operation of the head-trunk unit. In contrast, during landing, in children and adults, head stabilization in space tended to disappear while trunk stabilization in space was still present, suggesting an en bloc operation of the head-trunk unit. Similarly, pelvis stabilization in space about the roll axis occurred in all subjects during both flight and landing under unipedal conditions, where lateral balance control is of primary importance. Taken together, these results suggest that head stabilization in space is phase dependent, while trunk stabilization is phase independent. The trunk, including the pelvis, may thus constitute a stable reference frame from which anteroposterior and lateral balance control is organized during hops. 2) For head-trunk coordination, whatever the component of rotation, the two groups of children differed from adults, but did not differ from each other, suggesting that, while jumping, the transition between 6 and 7 years of age in the organization of balance control takes place in the coordination of the lower limbs during the preparatory phase of the take-off.