Slowed sensory reweighting and postural illusions in older adults: the moving platform illusion

Effect of age on postural performance and control strategies during changes in visual input and dual-tasking stances

Background

With age, people begin to experience deterioration in standing balance, especially when sensory input is suddenly removed or added. Here, we sought to explore the effects of age on postural performance and postural control strategies.

Methods

The convenience sample consisted of 15 young, 10 middle-aged, and 14 elderly healthy adults. They were instructed to stand with their feet together in four randomly administered conditions involving visual input removal/addition and single-/dual-tasking. Dual-tasking involved continuous subtraction by 3s.

Results

Postural sway displacement in the two older groups seemed larger than that in the younger group; however, neither the main effect of group (F_{2, 36} = 1.152, p = .327) nor the group × time interaction effect (F_{4, 27} = 0.229, p = .922) was significant. Greater stiffness of the lower leg muscles was observed in the vision-addition condition than in the vision-removal condition in only the elderly group (t₁₃ = -2.755, p = .016). The dual-tasking condition resulted in smaller sway displacement (F_{1, 36} = 7.690, p = .009) and greater muscle stiffness (F_{1, 36} = 5.495, p = .025). In the vision-removal condition, the increase in muscle stiffness due to dual-tasking was significantly larger in the middle-aged (t₉ = -3.736, p = .005) and elderly groups (t₁₃ = -2.512, p = .026).

Conclusions

In healthy older individuals, age-related changes were observed in control strategies used to maintain standing balance upon changes in visual input. The dual-task paradigm induced the use of an ankle-stiffening strategy in middle-aged and elderly adults.

Induced effects of electrical muscle stimulation and visual stimulation on visual sensory reweighting dynamics during standing on a balance board

Providing instruction cues on body motions using stimulations has the potential to induce sensory reweighting dynamics. However, there are currently very few quantitative investigations on the difference in the induced effects on the sensory reweighting dynamics between stimulation methods. We therefore investigated the difference in the induced effects of electrical muscle stimulation (EMS) and visual sensory augmentation (visual SA) on sensory reweighting dynamics during standing on a balance board. Twenty healthy participants controlled their posture to maintain the board horizontally in the balance-board task, which included a pre-test without stimulation, a stimulation test, and a post-test without stimulation. The EMS group (n = 10) received EMS to the tibialis anterior or soleus muscle based on the board tilt. The visual SA group (n = 10) received visual stimuli via a front monitor based on the board tilt. We measured the height of the board marker and calculated the board sway. Before and after the balance-board task, the participants performed static standing with their eyes open and closed. We measured postural sway and calculated the visual reweighting. The visual reweighting showed a strong negative correlation with the balance board sway ratio between the pre- and stimulation tests in the EMS group and a strong positive correlation with that in the visual SA group. Moreover, for those who reduced the balance board sway in the stimulation test, the visual reweighting was significantly different between the stimulation methods, demonstrating that the induced effect on sensory reweighting dynamics is quantitatively different depending on which method is used. Our findings suggest that there is an appropriate stimulation method to change to the targeted sensory weights. Future investigations on the relationship between sensory reweighting dynamics and stimulation methods could contribute to the proposal and implementation of new training methods for learning to control the target weights.

Dual-task walking on real-world surfaces: Adaptive changes in walking speed, step width and step height in young and older adults

OBJECTIVES

Age-related changes in dual-task walking are well established, but research in this topic is based on evidence from laboratory rather than real-world studies. We investigated how dual-task walking on real-world surfaces affects young and older adults' gait characteristics and cognitive resource allocation.

METHOD

Sixteen young (aged 19-35, 12 female) and fifteen older adults (aged 70-85, 7 female) with no major neurological or musculoskeletal disorders walked at a self-selected speed on forty-metre outdoor paths that had asphalt or grass surface. They walked with or without a cognitive task (counting backwards). Cognitive task difficulty was individually adjusted at 80 % accuracy. Participants performed the three tasks in Single Task (ST Asphalt, ST Grass, ST Cognitive) and Dual Task context (DT Asphalt-Cognitive, DT Grass-Cognitive).

RESULTS

The two groups showed similar dual task effects in cognition and walking speed, both of which were slower when dual-task walking. Older adults' steps were wider overall but only young adults widened their step width when dual-task walking on grass compared to asphalt. Similarly, young adults' step height increased from single to dual-task walking when on grass, where older adults' did not.

DISCUSSION

The lack of adaptation of step width and height when dual-task walking may leave older adults vulnerable to tripping or falling in common real-world conditions, such as while walking on grass, gravel, or uneven city sidewalks. Considering this, the built environment should be made more accessible to facilitate older adults' safe walking.

The effects of the pressure on the coccygeal skin on the perception of backward-leaning sitting positions in stroke patients

The present study investigated the effects of an additional pressure stimulus on coccygeal skin using an original tool to evaluate the perceptibility of sitting while leaning backward in 13 chronic stroke patients who were able to walk independently and 12 age-matched healthy subjects. Each participant's perception of the trunk reference angle at which they felt the highest-pressure stimulation of the coccygeal skin while leaning backward from a quiet sitting position was evaluated based on the accuracy of each reproduction under both normal and additional pressure conditions. The absolute error under the pressure condition was significantly smaller than that under the normal condition in the control group, while no marked difference between conditions was found in the stroke group. The relationship between the absolute error under the normal condition and the pressure effect index showed a significant negative correlation in the stroke group. In stroke patients with a high trunk position perceptibility under the normal condition, the additional pressure information may have functioned as a disturbance and reduced the position perceptibility. In contrast, stroke patients with a low perceptibility in the normal condition may have been able to re-weight and prioritize the additional pressure information in the reference frame. In the control group, the added pressure information may have been re-weighted as prior position information in the reference frame.