Dependence of joint stiffness on the conditions of visual control in upright undisturbed stance in humans

摘要

We recorded the sagittal and frontal components of the stabilogram of healthy humans in upright undisturbed stance under five conditions of visual control: (i) open eyes (OE); (ii) closed eyes (CE); (iii) visual inversion (VI); (iv) central vision (CV), and (v) diffused light (DL). Through a low-pass filter of trajectories of the center of pressure of feet (CPF), the vertical projection of the center of gravity (CG) and, consequently, the difference CPF-CG were estimated. The former represents the controlled variable, while the latter is proportional to the horizontal acceleration and assumed to express the resultant joint stiffness (mostly in the ankle joints). The stiffness was characterized through a method based on spectral analysis of the CPF-CG variable and subsequent calculations of the median frequency (MF) and the root mean square (RMS) of the spectra. The median frequencies of the spectra of the CPF-CG variable changed slightly under various visual conditions. At standing on a rigid support, they varied from 0.97 to 0.99 Hz and from 0.93 to 0.97 Hz for the CPF-CG, calculated from the sagittal and frontal components of the stabilogram, respectively. Under conditions of a pliable support, the corresponding frequencies varied within the limits of 0.79–0.83 Hz and 0.74–0.78 Hz.