Measuring age-related differences in kinematic postural strategies under yaw perturbation

The assessment of age-related postural strategies under external perturbation is of great interest for a better evaluation of the risk of falls in healthy humans. In the large majority of the studies, subjects have been perturbed with antero/posterior translations or rotations in the pitch and roll angles in order to investigate the postural strategies adopted for balance control. However, physiological mechanisms involved in the response to continuous yaw perturbation to maintain stable balance in healthy subjects are still unclear. Ten younger subjects (age: 28±3 years) and ten older adults (age: 61±4 years) were asked to stand on the RotoBiT 1D under two visual conditions: (a) eyes opened looking at a fixation point, and (b) eyes closed. The platform, driven by an ad-hoc control software, provided two sinusoidal rotations on the horizontal plane with fixed amplitude (±55°) and two different frequencies (0.2 Hz and 0.3 Hz). Kinematics of head, trunk, pelvis, arms, forearms, thighs and shanks body-segments was gathered using eleven inertial measurement units. Body-segment absolute rotations in the transverse plane were compared to platform absolute rotation after fast Fourier transform. The gain (G) and phase lag (φ) of all body-segments were computed and analyzed as a function of age, visual and frequency conditions. G values were statistically lower in older subjects than in younger subjects for all body-segments attesting prominent stiffness and limited ranges of movement. Regarding φ, our results demonstrated that in older subjects, lower limbs, trunk and pelvis anticipated platform movement probably compensating for postural perturbations. In both groups, G decreased and φ delayed progressively by increasing the frequency of the perturbation while similar results were obtained comparing the two visual conditions. In conclusion, to maintain balance under sinusoidal yaw perturbation, older subjects adopted a different motor control strategy compared with younger subjects mainly implying reduced body-movements and the anticipation of the movement.