HEAD AND TRUNK MOVEMENTS IN THE FRONTAL PLANE DURING COMPLEX DYNAMIC EQUILIBRIUM TASKS IN HUMANS

Eight normal human subjects were asked to maintain monopodal equilibri um on a narrow beam (task 1) or bipodal equilibrium on an unstable roc king platform (task 2) for 5 s. Each task was performed under four exp erimental conditions: (1) in light, (2) in darkness, (3) in light whil e subject had to hold a full cup of water, and (4) as in 3, but with a dditional instructions to fix the gaze on the cup. The movements of th e trunk and head in the frontal plane were recorded by means of a 50-H z TV image analyzer that computed the coordinates of small reflective markers glued on the skin of the subjects. On the beam the trunk was i nclined on the side of the supporting foot (13+/-9 degrees), on the ro cking platform the mean trunk orientation during the tests was nearly vertical (2+/-7 degrees). Nevertheless, in both tasks the mean head po sition was the same and close to vertical: 1.5+/-4 degrees on the rock ing platform and 1.5+/-5 degrees on the beam. For both tasks and all e xperimental conditions the head remained stabilized relative to vertic al, despite large translations in the frontal plane. Standard deviatio ns of head orientation from its mean value were 2.8+/-2 degrees for ta sk 1 and 2+/-1.5 degrees for task 2. The changes of trunk orientation were significantly higher: 6.2+/-3.8 degrees and 4.5+/-4 degrees, resp ectively. The differences in angular stability of head and trunk, meas ured through the standard deviations of angular displacements, were es pecially pronounced in trials with large trunk movements. It was concl uded that head angular stabilization, providing the central nervous sy stem with necessary visual and vestibular references, is essential for effective dynamic postural control in the frontal plane during comple x equilibrium tasks.