Effects of Physical and Mental Fatigue on Postural Sway and Cortical Activity in Healthy Young Adults

Dynamic modulations of effective brain connectivity associated with postural instability during multi-joint compound movement on compliant surface

Random fluctuations in somatosensory signals affect the ability of effectively coordinating multimodal information pertaining to the postural state during movement. Therefore, this study aimed to investigate the impact of a compliant surface on cortico-cortical causal information flow during multi-joint compound movements. Fifteen healthy adults (7 female / 8 male, 25.9 ± 4.0 years) performed 5 × 20 repetitions of bodyweight squats on firm and compliant surface. Motor behavior was quantified by center of pressure (CoP) displacements, hip movement and the root mean square of the rectus femoris activity. Using source space analysis, renormalized partial directed coherence (rPDC) computed subject-level multivariate effective brain connectivity of sensorimotor nodes. Bootstrap statistics revealed significantly decreased medio-lateral CoP displacement (p < 0.001), significantly increased velocity of medio-lateral hip motion (p < 0.001) as well as significantly lower rectus femoris activity (p < 0.01) in the compliant surface condition. On the cortical level, rPDC showed significantly modulated information flow in theta and beta frequencies for fronto-parietal edges (p < 0.01) only during the concentric phase of the movement. The compliant surface led to increased difficulties controlling hip but not center of pressure motion in the medio-lateral plane. Moreover, a decreased activation of the prime movers accompanied by modulations of effective brain connectivity among fronto-central nodes may point to altered demands on sensorimotor information processing in presence of sensory noise when performing bodyweight squats on compliant surface. Further studies are needed to evaluate a potential benefit for athletic and clinical populations.

Sensor-Based Assessment of Mental Fatigue Effects on Postural Stability and Multi-Sensory Integration

OBJECTIVE

Mental fatigue (MF) induced by prolonged cognitive tasks poses significant risks to postural stability, yet its effects on multi-sensory integration remain poorly understood.

METHOD

This study investigated how MF alters sensory reweighting and postural control in 27 healthy young males. A 45 min incongruent Stroop task was employed to induce MF, validated via subjective Visual Analog Scale (VAS) scores and psychomotor vigilance tests. Postural stability was assessed under four sensory perturbation conditions (O-H: no interference; C-H: visual occlusion; O-S: proprioceptive perturbation; C-S: combined perturbations) using a Kistler force platform. Center of pressure (COP) signals were analyzed through time-domain metrics, sample entropy (SampEn), and Discrete Wavelet Transform (DWT) to quantify energy distributions across sensory-related frequency bands (visual: 0-0.1 Hz; vestibular: 0.1-0.39 Hz; cerebellar: 0.39-1.56 Hz; proprioceptive: 1.56-6.25 Hz).

RESULTS

MF significantly reduced proprioceptive energy contributions (p < 0.05) while increasing vestibular reliance under O-S conditions (p < 0.05). Time-domain metrics showed no significant changes in COP velocity or displacement, but SampEn decreased under closed-eye conditions (p < 0.001), indicating reduced postural adaptability. DWT analysis highlighted MF's interaction with visual occlusion, altering cerebellar and proprioceptive energy dynamics (p < 0.01).

CONCLUSION

These findings demonstrate that MF disrupts proprioceptive integration, prompting compensatory shifts toward vestibular and cerebellar inputs. The integration of nonlinear entropy and frequency-domain analyses advances methodological frameworks for fatigue research, offering insights into real-time sensor-based fatigue monitoring and balance rehabilitation strategies. This study underscores the hierarchical interplay of sensory systems under cognitive load and provides empirical evidence for optimizing interventions in high-risk occupational and clinical settings.

Control Mechanisms of Sensorimotor System on Manipulation of Proprioceptive Inputs During Balance Maintenance

Proprioceptive inputs have crucial roles in control of the posture. The aim of the present study was to assess the effect of interfering with these signals on postural stability by ice-induced anaesthesia and local calf muscle fatigue. Seventeen healthy young individuals participated in this study to stand quietly and on an unstable platform under normal, anaesthesia, and fatigue conditions. A force platform calculated excursions of centre of pressure. Stabilogram-diffusion analysis was utilised to evaluate how body controls the posture with and without proprioceptive inputs. Time intervals of using the sensory feedback is significantly increased by anaesthesia in quiet standing (430 ms, p = 0.034) to note more delayed use of sensory information in a closed-loop. Additionally, fatigue significantly increased the time intervals of using sensory feedback during standing on the unstable platform (290 ms, p = 0.016). Interestingly, sensory interventions had no effect on the stability of the open-loop control of posture (short-term control), but they significantly influenced the closed-loop control (long-term control) (p < 0.004). Specifically, fatigue led to increased instabilities when the body used sensory inputs during both quiet standing (p = 0.021) and standing on the unstable platform (p = 0.041). These findings highlight the importance of proprioception in balance control for healthy individuals. Interfering with proprioceptive inputs, either through anaesthesia or fatigue, resulted in instabilities during balance maintenance. Our study provides new insights into the mechanisms underlying postural control, emphasising the significance of proprioceptive inputs. Understanding how proprioception affects balance maintenance may have implications for rehabilitation strategies, injury prevention, and the development of interventions to improve postural stability.

Monitoring Cortical and Neuromuscular Activity: Six-month Insights into Knee Joint Position Sense Following ACL Reconstruction

Background

Changes in cortical activation patterns after rupture of the anterior cruciate ligament (ACL) have been described. However, evidence of these consequences in the early stages following the incident and through longitudinal monitoring is scarce. Further insights could prove valuable in informing evidence-based rehabilitation practices.

Purpose

To analyze the angular accuracy, neuromuscular, and cortical activity during a knee joint position sense (JPS) test over the initial six months following ACL reconstruction. Study design: Cohort Study.

Methods

Twenty participants with ACL reconstruction performed a JPS test with both limbs. The measurement time points were approximately 1.5, 3-4 and 6 months after surgery, while 20 healthy controls were examined on a single occasion. The active JPS test was performed seated with a target angle of 50° for two blocks of continuous angular reproduction (three minutes per block). The reproduced angles were recorded simultaneously by an electrogoniometer. Neuromuscular activity of the quadriceps muscles during extension to the target angle was measured with surface electromyography. Spectral power for theta, alpha-2, beta-1 and beta-2 frequency bands were determined from electroencephalographic recordings. Linear mixed models were performed with group (ACL or controls), the measurement time point, and respective limb as fixed effect and each grouping per subject combination as random effect with random intercept.

Results

Significantly higher beta-2 power over the frontal region of interest was observed at the first measurement time point in the non-involved limb of the ACL group in comparison to the control group (p = 0.03). Despite individual variation, no other statistically significant differences were identified for JPS error, neuromuscular, or other cortical activity.

Conclusion

Variation in cortical activity between the ACL and control group were present, which is consistent with published results in later stages of rehabilitation. Both indicate the importance of a neuromuscular and neurocognitive focus in the rehabilitation.

Level of Evidence

3.

Cortical activity in body balance tasks as a function of motor and cognitive demands: A systematic review

Technological tools, like electroencephalography and functional near-infrared spectroscopy, have deepened our understanding of cortical regions involved in balance control. In this systematic literature review, we aimed to identify the prevalent cortical areas activated during balance tasks with specific motor or cognitive demands. Our search strategy encompassed terms related to balance control and cortical activity, yielding 2250 results across five databases. After screening, 67 relevant articles were included in the review. Results indicated that manipulations of visual and/or somatosensory information led to prevalent activity in the parietal, frontal and temporal regions; manipulations of the support base led to prevalent activity of the parietal and frontal regions; both balance-cognitive dual-tasking and reactive responses to extrinsic perturbations led to prevalent activity in the frontal and central regions. These findings deepen our comprehension of the cortical regions activated to manage the complex demands of maintaining body balance in the performance of tasks posing specific requirements. By understanding these cortical activation patterns, researchers and clinicians can develop targeted interventions for balance-related disorders.

The relationship between dizziness and sleep: a review of the literature

Background

The relation between vestibular disorders and (quality of) sleep is underexplored scientifically and the complex interactions between vestibular and sleep disorders are far from being well understood. Some studies have been conducted on the association between patients with vestibular disorders and (the occurrence of) sleep disorders, other studies have been published on the prevalence of dizziness complaints in patients with sleep disorders. The quality of sleep in patients with vestibular disorders generally receives little attention in clinical practice.

Objective

To establish what is currently known about the mutual relationship between dizziness and sleep, and to assess whether or not there is evidence of causality with regard to this relationship.

Methods

After systematically searching four literature database up until 1 April 2024, selected studies were summarized and evaluated through a (critical) review.

Results

Ultimately, 42 studies were selected and evaluated. Patients with dizziness in general and patients with a specific vestibular disorder like Benign Paroxysmal Positional Vertigo, Vestibular Migraine, Meniere's disease, and vestibular hypofunction were significantly more likely to have sleep disorders than control groups. A causal relationship is not supported due to the nature of the studies. In patients with obstructive sleep apnea syndrome there were significantly more vestibular complaints, and more disorders in the vestibular system were identified.

Conclusion

Dizziness complaints often co-exist with a sleep disorder. To what extent this sleep disorder influences dizziness is not clear. Paying attention to the quality of sleep in patients with a vestibular disorder seems to be important. In patients with OSAS, consideration should be given to vestibular complaints and dysfunction.

Patients with chronic ankle instability exhibit increased sensorimotor cortex activation and correlation with poorer lateral balance control ability during single-leg stance: a FNIRS study

Introduction

Chronic Ankle Instability (CAI) is a musculoskeletal condition that evolves from acute ankle sprains, and its underlying mechanisms have yet to reach a consensus. Mounting evidence suggests that neuroplastic changes in the brain following ankle injuries play a pivotal role in the development of CAI. Balance deficits are a significant risk factor associated with CAI, yet there is a scarcity of evidence regarding the sensorimotor cortical plasticity related to balance control in affected individuals. This study aims to evaluate the differences in cortical activity and balance abilities between patients with CAI and uninjured individuals during a single-leg stance, as well as the correlation between these factors, in order to elucidate the neurophysiological alterations in balance control among patients with CAI.

Methods

The study enrolled 24 patients with CAI and 24 uninjured participants. During single-leg stance, cortical activity was measured using a functional near-infrared spectroscopy (fNIRS) system, which included assessments of the pre-motor cortex (PMC), supplementary motor area (SMA), primary motor cortex (M1), and primary somatosensory cortex (S1). Concurrently, balance parameters were tested utilizing a three-dimensional force platform.

Results

Independent sample t-tests revealed that, compared with the uninjured individuals, the patients with CAI exhibited a significant increase in the changes of oxyhemoglobin concentration (ΔHbO) during single-leg stance within the left S1 at Channel 5 (t = 2.101, p = 0.041, Cohen's d = 0.607), left M1 at Channel 6 (t = 2.363, p = 0.022, Cohen's d = 0.682), right M1 at Channel 15 (t = 2.273, p = 0.029, Cohen's d = 0.656), and right PMC/SMA at Channel 11 (t = 2.467, p = 0.018, Cohen's d = 0.712). Additionally, the center of pressure root mean square (COP-RMS) in the mediolateral (ML) direction was significantly greater (t = 2.630, p = 0.012, Cohen's d = 0.759) in the patients with CAI. Furthermore, a moderate positive correlation was found between ML direction COP-RMS and ΔHbO2 in the M1 (r = 0.436; p = 0.033) and PMC/SMA (r = 0.488, p = 0.016), as well as between anteroposterior (AP) direction COP-RMS and ΔHbO in the M1 (r = 0.483, p = 0.017).

Conclusion

Patients with CAI demonstrate increased cortical activation in the bilateral M1, ipsilateral PMC/SMA, and contralateral S1. This suggests that patients with CAI may require additional brain resources to maintain balance during single-leg stance, representing a compensatory mechanism to uphold task performance amidst diminished lateral balance ability in the ankle joint.

Balance Training Under Fatigue: A Randomized Controlled Trial on the Effect of Fatigue on Adaptations to Balance Training

ABSTRACT

Keller, M, Lichtenstein, E, Roth, R, and Faude, O. Balance training under fatigue: a randomized controlled trial on the effect of fatigue on adaptations to balance training. J Strength Cond Res 38(2): 297-305, 2024-Balance training is an effective means for injury prevention in sports. However, one can question the existing practice of putting the balance programs at the start of a training session (i.e., train in an unfatigued state) because the occurrence of injuries has been associated with fatigue. Therefore, the aim of this study was to assess the influence of balance training in a fatigued or an unfatigued state on motor performance tested in fatigued and unfatigued conditions. Fifty-two, healthy, active volunteers (28.0 years; 19 women) were randomly allocated to 1 of 3 different training groups. The BALANCE group completed 6 weeks of balance training. The other 2 groups completed the identical balance tasks either before (BALANCE-high-intensity interval training [HIIT]) or after (HIIT-BALANCE) a HIIT session. Thus, these groups trained the balance tasks either in a fatigued or in an unfatigued state. In PRE and POST tests, balance (solid ground, soft mat, wobble board) and jump performance was obtained in fatigued and unfatigued states. Balance training resulted in reduced sway paths in all groups. However, the linear models revealed larger adaptations in BALANCE-HIIT and BALANCE when compared with HIIT-BALANCE ( d = 0.22-0.71). These small to moderate effects were-despite some uncertainties-consistent for the "unfatigued" and "fatigued" test conditions. The results of this study revealed for the first time that balance training under fatigue results in diminished adaptations, even when tested in a fatigued state. Therefore, the data indicate that balance training should be implemented at the start of a training session or in an unfatigued state.

Age and Type of Task-Based Impact of Mental Fatigue on Balance: Systematic Review and Meta-Analysis

The role of cognition in balance control suggests that mental fatigue may negatively affect balance. However, cognitive involvement in balance control varies with the type or difficulty of the balance task and age. Steady-state balance tasks, such as quiet standing, are well-learned tasks executed automatically through reflex activities controlled by the brainstem and spinal cord. In contrast, novel, and challenging balance tasks, such as proactively controlling balance while walking over rugged terrain or reacting to unexpected external perturbations, may require cognitive processing. Furthermore, individuals with preexisting balance impairments due to aging or pathology may rely on cognitive processes to control balance in most circumstances. This systematic review and meta-analysis investigated the effect of mental fatigue on different types of balance control tasks in young and older adults. A literature search was conducted in seven electronic databases and 12 studies met eligibility criteria. The results indicated that mental fatigue had a negative impact on both proactive (under increased cognitive load) and reactive balance in young adults. In older adults, mental fatigue affected steady-state and proactive balance. Therefore, mentally fatigued older individuals may be at increased risk of a loss of balance during steady-state balance task compared to their younger counterparts.

Do visual and step height factors cause imbalance during bipedal and unipedal stances? A plantar pressure perspective

Objective: The plantar pressure analysis technique was used to explore the static balance ability and stability of healthy adult males under the influence of visual and step height factors during bipedal and unipedal stances. Methods: Thirty healthy adult males volunteered for the study. Experiments used the F-scan plantar pressure analysis insoles to carry out with eyes open (EO) and eyes closed (EC) at four different step heights. The plantar pressure data were recorded for 10 s and pre-processed to derive kinematic and dynamic parameters. Results: For unipedal stance, most of kinematic parameters of the subjects' right and left feet were significantly greater when the eyes were closed compared to the EO condition and increased with step height. The differences in toe load between right and left feet, open and closed eyes were extremely statistically significant (p < 0.001). The differences in midfoot load between the EO and EC conditions were statistically significant (p = 0.024) and extremely statistically significant between the right and left feet (p < 0.001). The difference in rearfoot load between EO and EC conditions was extremely statistically significant (p < 0.001) and statistically significant (p = 0.002) between the right and left feet. For bipedal stance, most of kinematic parameters of the subjects' EO and EC conditions were statistically significant between the right and left feet and increased with step height. The overall load's difference between EO and EC states was statistically significant (p = 0.003) for both feet. The overall load's difference between the right and left feet was extremely statistically significant (p < 0.001) in the EC state. The differences between the right and left feet of the forefoot and rearfoot load with EO and EC suggested that the right foot had a smaller forefoot load, but a larger rearfoot load than the left foot (p < 0.001). The differences between the forefoot and rearfoot load of the subjects' both feet with EO and EC were extremely statistically significant (p < 0.001). Conclusion: Both visual input and step height factors, even the dominant foot, act on kinematic and dynamic parameters that affect the maintenance of static balance ability.

Mental fatigue does not affect static balance under both single and dual task conditions in young adults

The ability to control balance and prevent falls while carrying out daily life activities may require a predominantly controlled (cognitive) or automatic processing depending on the balance challenge, age, or other factors. Consequently, this process may be affected by mental fatigue which has been shown to impair cognitive abilities. Controlling static balance in young adults is a relatively easy task that may proceed automatically with minimal cognitive input making it insusceptible to mental fatigue. To investigate this hypothesis, static single and dual task (while concurrently counting backward by seven) balance was assessed in 60 young adults (25.2 ± 2.4 years) before and after 45 min of Stroop task (mental fatigue condition) and watching documentary (control), presented in a randomized counterbalanced order on separate days. Moreover, because mental fatigue can occur due to task underload or overload, participants carried out two different Stroop tasks (i.e., all congruent, and mainly incongruent trials) on separate days in the mental fatigue condition. Results of the study revealed a significantly higher feeling of mental fatigue after the mental fatigue conditions compared to control (p < 0.001). Similarly, the performance on congruent Stroop trials decreases with time indicating objective mental fatigue (p < 0.01). However, there was no difference in balance or concurrent task performance under both single and dual task assessments between the three conditions (p  > 0.05) indicating lack of effect of mental fatigue on static balance in this population. Therefore, future studies investigating this phenomenon in occupational or sport settings in similar population should consider using more challenging balance tasks.

Concussed Neural Signature is Substantially Different than Fatigue Neural Signature in Non-concussed Controls

Traumatic brain injuries can result in short-lived and long-lasting neurological impairment. Identifying the correct recovery timeframe is challenging, as balance-based metrics may be negatively impacted if testing is performed soon after exercise. Thirty-two healthy controls and seventeen concussed individuals performed a series of balance challenges, including virtual reality optical flow perturbation. The control group completed a backpacking protocol to induce moderate fatigue. Concussed participants had lower spectral power in the motor cortex and central sulcus when compared to fatigued controls. Moreover, concussed participants experienced a decrease in overall theta band spectral power while fatigued controls showed an increase in theta band spectral power. This neural signature may be useful to distinguish between concussed and non-concussed fatigued participants in future assessments.