Evaluation of inertial inputs on vestibular system during quick inversions of walking using a 6-DOF accelerometric device

Head attitude, velocity and acceleration are relevant variables in the estimation of the capability of the equilibrium, because they represent the actual inputs to the vestibular system, which essentially performs an inertial feedback for the dynamic postural control. In the present paper, a multi-accelerometer based device, capable to directly measure the linear and angular acceleration of the head, is described. The mentioned device consists in a cyclist helmet equipped with ten biaxial linear accelerometer and measures its own inertial components. In this study we also developed the procedure needed to determine the inertial input applied to the semicircular canals (SCC) and to the otolith system by means of a postprocessing of the accelerometer outputs. The procedure consists in a single static estimation of the inclination of the Frankfort Plane in respect to the reference horizontal plane of the helmet. As an example of the possible uses of the instrumented helmet, we tested the case of a quick step inversion during level walking. In particular a healthy subject was asked to walk for about 5 m, to quickly turn of 180 deg and to turn back to the starting position. The task was repeated three times and the entire trial was recorded by an optoelectronic system to keep also track of the full body motion data. The main outcomes deduced by the collected data are as follows. Firstly, in the turning phase the maximum value of angular acceleration was greater in the Lateral canal (approximate to 740 deg/s(2)) rather than in the other organs (approximate to 500 deg/s(2)) due to the specific orientation of the Lateral Canal in the head. Secondly, the maximum value of vertical bar a-g vertical bar was greater for the Saccule (12.6 m/s(2)) than for the Utricle (6.8 m/s(2)). During the inversion phase, the peak of vertical bar a-g vertical bar occurred in proximity of the footstrike event, as it happens during normal walking, but the amplitude is lower than the other phases due to the global deceleration of the subject's body.