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

• Published in: Neurophysiology (New York) 2006-03, Vol.38 (2), p.134-141
• Main Authors: Smetanin, B. N., Popov, K. E., Kozhina, G. V.
• Format: Article
• Language: English
• Subjects: Eyes & eyesight; Joints; Motor ability; Neurosciences
• ISSN: 0090-2977; 1573-9007
• DOI: 10.1007/s11062-006-0036-8

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. In contrast to the median frequencies, the RMSs demonstrated greater variability depending on different visual conditions. At standing on a rigid support, paired comparisons showed significant differences between the RMSs of the spectra of the CPF-CG variable of the sagittal direction under CE and OE conditions (0.14 +/- 0.030 and 0.09 +/- 0.020 mm, respectively) and under DL and OE conditions (0.130 +/- 0.025 and 0.090 +/- 0.020 mm, respectively). The RMS of the CPF-CG variable calculated for the frontal stabilogram differed significantly from each other for the VI and OE conditions (0.115 +/- 0.020 and 0.075 +/- 0.015 mm, respectively). In case of standing on a pliable support, a greater variability of visual influences on the CPF-CG variable was found. The RMS for its sagittal motion was the greatest under CE conditions (0.19 +/- 0.03 mm); it was significantly greater than the respective values under OE, CV, and DL conditions (0.097 +/- 0.020, 0.110 +/- 0.020, and 0.140 +/- 0.030 mm, respectively). The means of RMSs of the spectra of the frontal CPF-CG was also the greatest under CE conditions (0.20 +/- 0.03 mm) and the smallest under OE conditions (0.095 +/- 0.020 mm). In