Effects of mentally induced fatigue on balance control: a systematic review

The relationship between cognitive demands and postural control is controversial. Mental fatigue paradigms investigate the attentional requirements of postural control by assessing balance after a prolonged cognitive task. However, a majority of mental fatigue research has focused on cognition and sports performance, leaving balance relatively underexamined. The purpose of this paper was to systematically review the existing literature on mental fatigue and balance control. We conducted a comprehensive search on PubMed and Web of Science databases for studies comparing balance performance pre- to post-mental fatigue or between a mental fatigue and control group. The literature search resulted in ten relevant studies including both volitional (n = 7) and reactive (n = 3) balance measures. Mental fatigue was induced by various cognitive tasks which were completed for 20-90 min prior to balance assessment. Mental fatigue affected both volitional and reactive balance, resulting in increased postural sway, decreased accuracy on volitional tasks, delayed responses to perturbations, and less effective balance recovery responses. These effects could have been mediated by the depletion of attentional resources or impaired sensorimotor perception which delayed appropriate balance-correcting responses. However, the current literature is limited by the number of studies and heterogeneous mental fatigue induction methods. Future studies are needed to confirm these postulations and examine the effects of mental fatigue on different populations and postural tasks. This line of research could be clinically relevant to improve safety in occupational settings where individuals complete extremely long durations of cognitive tasks and for the development of effective fall-assessment and fall-prevention paradigms.

Effect of cognitive task complexity on dual task postural stability: a systematic review and meta-analysis

The dual task experimental paradigm is used to probe the attentional requirements of postural control. However, findings of dual task postural studies have been inconsistent with many studies even reporting improvement in postural stability during dual tasking and thus raising questions about cognitive involvement in postural control. A U-shaped non-linear relationship has been hypothesized between cognitive task complexity and dual task postural stability suggesting that the inconsistent results might have arisen from the use of cognitive tasks of varying complexities. To systematically review experimental studies that compared the effect of simple and complex cognitive tasks on postural stability during dual tasking, we searched seven electronic databases for relevant studies published between 1980 to September 2020. 33 studies involving a total of 1068 participants met the review's inclusion criteria, 17 of which were included in meta-analysis (healthy young adults: 15 studies, 281 participants; Stroke patients: 2 studies, 52 participants). Narrative synthesis of the findings in studies involving healthy old adults was carried out. Our result suggests that in healthy population, cognitive task complexity may not determine whether postural stability increases or decreases during dual tasking (effect of cognitive task complexity was not statistically significant; P > 0.1), and thus the U-shaped non-linear hypothesis is not supported. Rather, differential effect of dual tasking on postural stability was observed mainly based on the age of the participants and postural task challenge, implying that the involvement of cognitive resources or higher cortical functions in the control of postural stability may largely depends on these two factors.

Postural control during quiet standing and voluntary stepping response tasks in individuals post-stroke: a case-control study

Background: Postural control impairments following a stroke have an impact on mobility, reduce independence, and increase the risk of falls. Assessing these impairments during tasks representative of real-life situations, such as quiet standing (QS) and voluntary stepping response (VSR), will enhance our understanding of how the postural control system is impaired in individuals post-stroke (IPS). It will also inform the development of a more targeted and effective rehabilitation to prevent falls in IPS.Objectives: Identify the postural control impairments encountered by IPS during QS and VSR.Methods: Twenty IPS and 16 healthy controls were recruited to perform QS and VSR tasks, while ground reaction forces and whole-body motion were measured. Displacement and speed variation of the COM, center of pressure (COP) displacement and spatiotemporal data were calculated and compared between groups.Results: During QS, IPS exhibited greater maximal COP displacement in mediolateral direction, COM displacement in vertical direction and COM speed excursions compared to controls. During VSR, IPS exhibited smaller step length, braking force, posterior foot placement in relation to the pelvis and COM anteroposterior excursion compared to controls. IPS presented less static and dynamic postural stability compared to controls.Conclusions: Greater postural sway during QS, smaller anteroposterior COM displacement before losing balance and altered voluntary recovering steps during VSR could place IPS at more risk of falling when they face a postural challenge in the community. These novel results will improve the current knowledge base and should be considered in IPS rehabilitation.

Standing Balance Strategies and Dual-Task Interference Are Differentially Modulated Across Various Sensory Contexts and Cognitive Tests in Individuals With Chronic Stroke

BACKGROUND AND PURPOSE

Mental tracking and verbal fluency tests have been linked to poor mobility and falls; however, no studies have examined the influence of imposing these tests on standing balance post-stroke. The purpose of this study was to investigate standing balance and cognitive performance across sensory conditions and cognitive tests post-stroke.

METHODS

Ninety-two participants with chronic stroke stood on dual-force platforms while performing various sensory conditions (eyes open/fixed surface, eyes closed/fixed surface, eyes open/sway-referenced surface, and eyes closed/sway-referenced surface) and cognitive tests (no cognitive test, serial subtractions, and verbal fluency). Equilibrium scores were computed based on the anterior-posterior sway angle. The number of correct verbal responses was recorded.

RESULTS

Performing serial subtractions during eyes closed/sway-referenced surface revealed the highest equilibrium score (64.0 ± 13.5), followed by the same sensory condition with added verbal fluency test (59.6 ± 15.1), followed by the single-task condition with no cognitive test (52.6 ± 20.9). The number of correct serial subtractions between seated (7.7 ± 3.7) and standing (7.2 ± 3.5) conditions was similar. A trend for more correct verbal fluency responses emerged when seated (8.1 ± 2.9) than the eyes open/sway-referenced surface condition (7.5 ± 2.8). Greater correct verbal fluency responses emerged when seated (8.6 ± 3.1) than the eyes closed/sway-referenced surface condition (7.8 ± 2.7).

DISCUSSION AND CONCLUSIONS

Standing balance enhancement (higher equilibrium score/better balance) depends on the neuropsychological process targeted and the sensory input available post-stroke. Cognitive dual-task interference emerged for the verbal fluency test during the most attention demanding sensory condition. Cognitive tests that enhance standing balance should be considered in assessments and interventions to evaluate and improve dual-tasking post-stroke.Video Abstract available for more insights from the authors (see the Video, Supplemental Digital Content 1, available at: http://links.lww.com/JNPT/A321).

Effect of Mental Fatigue on Postural Sway in Healthy Older Adults and Stroke Populations

The aim of this study was to examine the effect of mental fatigue on postural sway under different sensory conditions in healthy older adults and in persons with chronic stroke (PwCS). Thirty healthy older adults (> 60 years old), randomly separated into experimental and control groups, as well as 15 PwCS participated in this study. Experimental groups were asked to stand on a force platform wearing seven inertial sensors while performing the Sensory Organization Test (SOT) under two cognitive conditions (single- and dual-task) before and after a mental fatigue task (stop-signal task for 60 min). The control group performed the same protocol before and after watching a movie for 60 min. Changes in subjective fatigue was assessed by the NASA Task Load Index and psychophysiological workload was assessed by heart rate variability (HRV). Postural sway was assessed by calculating the Jerk and root mean square (RMS) of center of mass (COM). Higher Jerk and RMS of COM (p < 0.05) were observed after the mental fatigue task in both healthy older adults and PwCS during SOT, which was not observed in the control group (p > 0.05). Additionally, postural sway increased in the three groups as the SOT conditions became more challenging. Our results indicate that mental fatigue, induced by sustained cognitive activity, can impair balance during SOT in older adult and stroke populations.

A planar neuro-musculoskeletal arm model in post-stroke patients

Mathematical modeling of the neuro-musculoskeletal system in healthy subjects has been pursued extensively. In post-stroke patients, however, such models are very primitive. Besides improving our general understanding of how stroke affects the limb motions, they can be used to evaluate rehabilitation strategies by computer simulations before clinical evaluations. A planar neuro-musculoskeletal arm model for post-stroke patients is developed. The main idea is to use a set of new coefficients, Muscle Significance Factors (MSF), to incorporate the effects of stroke in the muscle control performance. The model uses the optimal control theory to mimic the performance of the CNS and a two-link skeletal model with six muscles for the biomechanical part. The model was developed and evaluated using experimental data from six post-stroke patients with Brunnstrom levels of 4-6. The results show that MSFs are relatively distinct and independent from the arm motion which is used to determine their values. Its variation is in the range of 0-2.58% and decreases in higher Brunnstrom levels. The mean error of the model in predicting the path of motion varies from 0.9% in level 6 to 5.58% in level 4 subjects which can be considered a promising level of accuracy. Using the proposed model and the MSF to customize the model for each individual stroke patient seems a promising approach. It shows a reasonable level of robustness, i.e., independence from the type of motions and correlated with the severity of stroke, and accuracy in predicting the shape of the motion path.

Effects of cognitive load on the amount and temporal structure of postural sway variability in stroke survivors

This study aimed to investigate the variability in postural sway patterns during quiet standing in stroke survivors. The postural sway was measured in 19 stroke survivors, as well as 19 healthy demographically matched participants, at 3 levels of postural difficulty (rigid surface with closed and open eyes, and foam surface with closed eyes), and 3 levels of cognitive difficulty (without a cognitive task, easy and difficult cognitive tasks). Both linear analyses (the amount of postural sway variability, including the standard deviation of the COP velocity in both the anteroposterior (AP) and mediolateral (ML) directions), as well as non-linear analyses [the temporal structure of the COP variability, including % Recurrence, % Determinism, Shannon Entropy, Trend and the maximum diagonal line (D _max)] were employed. The results revealed that the amount of variability of the postural sway of stroke survivors was significantly greater than that of healthy participants, along both the ML and AP directions, with the highest obtained during standing on foam with closed eyes. All measures of the temporal structure of the COP variability were significantly greater in stroke survivors, as compared to the control group, along the ML direction, but not along the AP direction. The cognitive error was significantly higher during difficult cognitive tasks, although it was neither affected by postural difficulty nor by group. The different results obtained for the amount and temporal structure of the COP variability in the AP and ML directions shed light on the intricate mechanisms employed by the CNS in post-stroke balance control, and suggest that effective rehabilitative and therapeutic strategies should be patient-specific, taking both the environment/surface as well as the specific protocols into consideration.