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

Abstract

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 addition, the value of the RMS fluctuations under CE conditions (0.150 ± 0.025 mm) differed significantly from the respective values under OE conditions (0.095 ± 0.020 mm) and CV conditions (0.110 ± 0.020 mm). Thus, our findings support the statement that the influence of visual conditions on the maintenance of vertical stance is mediated (at least partially) by the mechanisms controlling the ankle joint stiffness. This regulation is mostly manifested in changes of a single parameter, the amplitude of fluctuations of the CPF-CG variable. We also found that the joint stiffness can...

Postural reactions of humans in upright stance: Dependence on different visual conditions and the level of stiffness in the ankle joints

“…Collins and De Luca [2] were probably the first to assume that a nonspecific stabilizing effect of visual control is realized via regulation of the muscle/joint stiffness. Earlier [1], we found confirmations of the statement that the effect of different visual conditions on the maintenance of an upright undisturbed stance is, in particular, realized via the mechanisms controlling the stiffness level in the ankle joints. It is obvious that these joints are the main joint structure that provides, by changing the joint angles, the maintenance of an upright stance.…”

“…To check in more detail the importance of nonspecific changes in the background joint stiffness, we used light tactile contact of the forefinger tip of the subject with a rigid support. As was shown earlier, such a mild contact provides slight additional stabilization of the vertical posture in undisturbed stance by decreasing, under different visual conditions, the joint stiffness to the level of the OE stance [1].…”

“…In an earlier study [1], we examined in humans the contribution of specific and nonspecific visual influences to the control of the vertical posture in the undisturbed stance, including those to changes in stiffness in the ankle joints observed in this case. Let us recall that visual influences based on detailed information on the position of the body and displacements of the latter with respect to external objects and involved, correspondingly, in the current control of postural sways are considered specific.…”

“…As was done earlier [1], we induced changes in the background joint stiffness by varying the intensity and quality of illumination of the retina. For this purpose, the subject stood with CE or stood with matt spectacles (pale milk glasses) on, when the eyes perceive only diffused light (DL).…”

“…For this purpose, the subject stood with CE or stood with matt spectacles (pale milk glasses) on, when the eyes perceive only diffused light (DL). It is known that under the latter conditions the subject stood more stably (despite the fact that, similarly to CE conditions, visual orientation with respect to the environment is impossible) [1,6]. Examination of the maintenance of upright stance under the latter two visual conditions allows one to reveal possible contributions of nonspecific visual influences not only to changes in the background joint stiffness but also to the development of evoked postural reactions.…”