Standing on unstable surface challenges postural control of tracking tasks and modulates neuromuscular adjustments specific to task complexity

Biomechanical optimization and reinforcement learning provide insight into transition from ankle to hip strategy in human postural control

Human postural control strategies, categorized as ankle or hip strategies, adapt to varying perturbation magnitudes and support surface sizes. While numerous studies have characterized these strategies, few have explored the underlying mechanisms driving the transition from ankle to hip strategy. This study investigated whether postural strategy transitions can be explained through an optimization mechanism incorporating biomechanical constraints. We analyzed postural strategy changes in human responses to backward perturbations and developed a reinforcement learning (RL)-based optimization model. The biomechanical constraint was defined as the center of pressure (CoP) range limitation to the metatarsal joint. The control objective function featured a novel CoP constraint penalty, complemented by terms for upright posture recovery and control effort minimization. The RL-based optimization model successfully reproduced the ankle-to-hip strategy transition observed in human postural responses. With increasing perturbation magnitude, the model demonstrated a pattern of limited ankle torque coupled with increased hip joint kinematics, closely aligning with observed human postural adaptations. These results suggest that the adaptive nature of human postural strategy transitions can be understood within an optimization framework incorporating biomechanical constraints. Additionally, this study supports the use of RL models, capable of implementing nonlinear optimization, as a valuable tool for comprehensively analyzing diverse adaptive characteristics in human movement.

A Comprehensive Understanding of Postural Tone Biomechanics: Intrinsic Stiffness, Functional Stiffness, Antagonist Coactivation, and COP Dynamics in Post-Stroke Adults

OBJECTIVE

To analyse the relationship between traditional stiffness and muscle antagonist coactivation in both stroke and healthy participants, using linear and non-linear measures of coactivation and COP during standing, stand-to-sit, and gait initiation.

METHODS

Participants were evaluated through a cross-sectional design. Electromyography, isokinetic dynamometer, and force plate were used to calculate coactivation, intrinsic and functional stiffness, and COP displacement, with both linear and non-linear metrics. Spearman's correlations and Mann-Whitney tests were applied (p < 0.05).

RESULTS

Post-stroke participants showed higher contralesional intrinsic stiffness (p = 0.041) and higher functional stiffness (p = 0.047). Coactivation was higher on the ipsilesional side during standing (p = 0.012) and reduced on the contralesional side during standing and transitions (p < 0.01). Moderate correlations were found between intrinsic and functional stiffness (p = 0.030) and between coactivation and intrinsic stiffness (standing and stand-to-sit: p = 0.048) and functional stiffness (gait initiation: p = 0.045). COP displacement was reduced in post-stroke participants during standing (p < 0.001) and increased during gait initiation (p = 0.001). Post-stroke participants exhibited increased gastrocnemius/tibialis anterior coactivation during gait initiation (p = 0.038) and higher entropy and stability across tasks (p < 0.001).

CONCLUSION

Post-stroke participants showed higher contralesional intrinsic and functional stiffness, reduced coactivation in static tasks, and increased coactivation in dynamic tasks. COP and coactivation analyses revealed impaired stability and random control, highlighting the importance of multidimensional evaluations of postural tone.

Postural control of sway dynamics on an unstable surface reduces similarity in activation patterns of synergistic lower leg muscles

Introduction

Diversity of activation patterns within synergistic muscles can be important for stability control in challenging conditions. This study investigates the similarity of activation patterns within the triceps surae and quadriceps femoris muscles and the effects of unstable surface during a visually guided postural task.

Methods

Eighteen healthy adults performed a visually guided anteroposterior tracking task on both stable and unstable surfaces. Electromyographic activity of triceps surae (gastrocnemius medialis, gastrocnemius lateralis, soleus) and quadriceps femoris (vastus medialis, vastus lateralis, rectus femoris) was recorded at 1,000 Hz. Cosine similarity (CS) between muscle pairs within each muscle group was calculated to assess the similarity of activation patterns of synergistic muscles for stable and unstable conditions. To compare the CS of the muscle pairs, a linear mixed model was used. For all tests the level of significance was set to α = 0.05.

Results

Across all surface conditions, CS values within the triceps surae muscles were lower than those of the quadriceps (p < 0.001), indicating a greater diversity in activation patterns of the distal muscles. The unstable surface reduced CS values for both muscle groups (p = 0.021). No significant interaction was observed between muscle pair and surface condition (p = 0.833).

Discussion

The reduced similarity of activation patterns within the synergistic triceps surae and quadriceps femoris muscles on the soft surface indicates an increased flexibility of neuromotor control for the unstable condition. The lower similarity within the synergistic triceps surae muscles suggests a higher diversity of activation patterns compared to the quadriceps femoris muscles, which may increase the flexibility of neuromotor control to meet specific joint stabilization challenges during the studied tracking task.

Exploratory Analysis of Unstable Surface Training: A Systematic Review and Meta-Analysis for Chronic Ankle Instability

Objective

To conduct an exploratory systematic review and meta-analysis to evaluate the effect of unstable surface training on balance and hop function in individuals with chronic ankle instability (CAI).

Data Sources

Four major electronic databases were searched, including Cochrane Library, PubMed, Embase, and Web of Science, from January 1, 2000 to June 20, 2024.

Study Selection

Randomized controlled trials that compare unstable surface training with either general intervention or no intervention in individuals with CAI were included.

Data Extraction

The physical therapy evidence database scale was used to assess the risk of bias and methodological quality of included studies. The mean differences (MDs) with 95% confidence intervals (CIs) were calculated using Review Manager 5.4 software.

Data Synthesis

The review ultimately included 9 studies involving 308 participants. Compared with the other exercises or no exercise, unstable surface training could improve the significant effects of the star excursion balance test (SEBT) in the direction of posterolateral (MD=5.80; 95% CI, 1.60-9.99; P=.007), posteromedial (MD=6.24; 95% CI, 2.32-10.16; P=.002), medial (MD=9.11; 95% CI, 6.42-11.80; P<.00001), anteromedial (MD=7.25; 95% CI, 2.33-12.17; P=.004), the time-in-balance test (MD=8.45; 95% CI, 1.50-15.40; P=.02), the foot-lift test (MD=-1.39; 95% CI, -2.49 to -0.28; P=.01). However, there was no significant difference in the anterior direction of the SEBT (MD=3.22; 95% CI, -0.66 to 7.10; P=.10), the side-hop test (MD=-1.94; 95% CI, -4.82 to 0.95; P=.19), and the figure-of-8 hop test (MD=-0.97; 95% CI, -2.39 to 0.46; P=.18) between groups.

Conclusions

Compared with the other exercises or no exercise, unstable surface training has potential benefits in improving balance in people with CAI but has no significant effect on hop function. However, the exploratory nature of this study highlights the need for further research to confirm these findings.

Intermittent hypoxia enhances voluntary activation and reduces performance fatigability during repeated lower limb contractions

Prior research has highlighted the therapeutic benefits of acute intermittent hypoxia (AIH) in enhancing motor performance after motor incomplete spinal cord injury and in able-bodied individuals. Although studies in rodents and humans indicate that AIH may facilitate motor excitability, the relationship between excitability changes and functional performance remains unclear. In addition, discrepancies in the effects of AIH on excitability in able-bodied individuals merit further investigation. Understanding the concurrent impact of repetitive AIH on voluntary activation and spinal reflex excitability may clarify the functional implications of AIH for muscle force production. High voluntary activation is vital for sustaining torque production during activities that require repeated muscle contractions. We hypothesized that repetitive AIH would attenuate decreases in both voluntary activation and maximum torque production typically observed during fatiguing contractions. To test this hypothesis, we examined the effects of four consecutive days of AIH on voluntary activation and torque generation during repeated maximal plantar flexion contractions. We assessed changes in voluntary activation using the central activation ratio by calculating the ratio of voluntary torque to the torque produced with supramaximal electrical stimulation. Consistent with our hypothesis, we show that repetitive AIH significantly increases both voluntary activation and peak torque during fatiguing contractions. We did not observe any changes in resting spinal reflex excitability or antagonist muscle coactivation during fatiguing contractions post-AIH. Together, these findings suggest that repetitive AIH reduces performance fatigability through enhanced descending neural drive. Optimizing voluntary activation is critical for facilitating the recovery of functional walking skills after neurological injury.NEW & NOTEWORTHY This study shows that repetitive acute intermittent hypoxia (AIH) significantly increases both voluntary activation and peak torque during fatiguing lower limb contractions. However, resting spinal reflex excitability and antagonist muscle coactivation during fatiguing contractions did not change following repetitive AIH. Together, these observations indicate that repetitive AIH reduces performance fatigability through enhanced descending neural drive. These findings underscore the therapeutic potential of AIH for promoting motor recovery after neurological injury.

Postural Control and Neuromuscular Activation in 11-13-Year-Old Athletic Boy Swimmers

OBJECTIVE

This study compared postural control and neuromuscular activation in athletic swimmers (A-S) and non-athletic swimmers (N-A-S) in older children.

METHODS

Ten A-S and ten N-A-S underwent assessments of center of pressure (CoP) parameters under static and dynamic surfaces in two directions (dynamic mediolateral (DML) and dynamic anteroposterior (DAP)) in eyes-open (EO) and eyes-closed (EC) conditions, and electromyography (EMG) parameters under DAP and DML directions in EO and EC conditions.

RESULTS

Results showed that A-S demonstrated significantly superior postural control (p < 0.05), with smaller CoP area and lower CoP mean velocity compared with N-A-S, particularly in static with EC, DAP with EO and EC, and DML with EO conditions. A-S exhibited significantly larger neuromuscular activation amplitudes (p < 0.05), especially in the AP direction.

CONCLUSIONS

These findings suggested that athletic swimming training may enhance postural control and neuromuscular activation in 11-13-year-old children, emphasizing the potential benefits of incorporating swimming exercises in these children.

Differences in left and right lower limb control strategies in coping with visual tracking tasks during bipedal standing

Introduction

Differences in motor control between the lower limbs may influence the risk of sports injury and recovery from rehabilitation. In this study, differences in the visual feedback ability of the left and right lower limbs were assessed using visual target tracking tasks.

Methods

Thirty-four healthy young subjects (aged 20.4 ± 1.2 years) were asked to move their bodies back and forth while tracking a visual target displayed on a monitor in front of them for 30 s. The two target motions were sinusoidal (i.e., predictable patterns) and more complex (random) patterns. To assess the ability of the lower limbs to follow visual target tracking, antero-posterior CoP (right limb, CoPap-r; left limb, CoPap-l) and medio-lateral CoP (right limb, CoPml-r; left limb, CoPml-l) data were measured using a stabilometer. Tracking ability by visual feedback ability was calculated as the difference in displacement between the target signal and the trajectories of the right and left pressure centers as trapezoidal areas, and a smaller sum of area (SoA) over the entire measurement time was defined as a greater tracking ability.

Results

Regarding the SoA in the anterior-posterior CoP, the mean SoA in the sinusoidal and random tasks was significantly lower in the CoP-r data than in the CoP-l data, indicating that the right lower limb had a more remarkable ability to follow visual target tracking. Regarding the SoA in the medial-lateral direction (CoP), the mean SoA in the sinusoidal and random tasks did not significantly differ between the two legs.

Discussion

The right lower limb may have a tracking function activated by the target signal when responding to visual stimuli. Identifying the motor strategies of each lower limb in response to visual stimuli will not only help identify potential differences between each lower limb but also suggest the possibility of enhancing the role of each lower limb in balance control.

Influence of postural control difficulty on changes in spatial orienting of attention after leftward prism adaptation

Prism adaptation (PA) affects visuospatial attention such as spatial orienting in both the right and left hemifields; however, the systematic after-effects of PA on visuospatial attention remain unclear. Visuospatial attention can be affected by non-spatial attentional factors, and postural control difficulty, which delays the reaction time (RT) to external stimulation, may be one such factor. Therefore, we aimed to investigate the influence of postural control difficulty on changes in spatial orienting of attention after leftward PA. Seventeen healthy young adults underwent 15-min and 5-min PA procedures for a leftward visual shift (30 diopters). Participants underwent the Posner cueing test immediately before (pre-evaluation) and in between and after the PA procedures (post-evaluations) while standing barefoot on the floor (normal standing condition) and on a balance-disc (balance standing condition). In the pre-evaluation, RTs in the balance standing condition were significantly longer compared to those in the normal standing condition for targets appearing in both the right and left hemifields. Leftward PA improved the RT for targets appearing in the right, but no left, hemifield in the balance standing condition, such that RTs for targets in the right hemifield in the post-evaluation were not significantly different between the two standing conditions. However, leftward PA did not significantly change RTs for targets in both hemifields in the normal standing condition. Therefore, postural control difficulty may enhance sensitivity to the features of the visuospatial cognitive after-effects of leftward PA.

Investigating the Influence of Varying Surface Conditions on Human Postural Control and Sensory Integration Strategies

This study investigated the effects of different surface conditions on postural stability in response to unexpected perturbations. Thirty healthy adults underwent balance assessments on flat, incline ramp, balance pad, and balance pad on incline ramp surfaces. The center of pressure (COP) displacement in the mediolateral (ML) and anteroposterior (AP) directions, the velocity, and the area were measured. We found that the flat and ramp conditions resulted in significantly lower COP ML (F(3, 87) = 38.272, p < 0.001, ηp2 = 0.569) and AP displacements (F(3, 87) = 89.177, p < 0.001, ηp2 = 0.755), velocity (F(3, 87) = 89.177, p < 0.001, ηp2 = 0.755), and area (F(3, 87) = 52.659, p < 0.001, ηp2 = 0.645) compared to the balance pad and balance pad on ramp conditions (p < 0.05). The use of a balance pad, particularly on a ramp, significantly increased all the COP measurements, suggesting greater challenges to postural control. Through these findings, we demonstrate the adaptability and limitations of the human postural control system in response to varying surface conditions and perturbations.

Effects of resistance training on muscular strength, endurance, body composition and functional performance among sarcopenic patients: a systematic review

Background

Sarcopenia, a gradual loss of muscle mass and strength associated with ageing, contributes to a decline in physical abilities, increase in disability and frailty and loss of functional independence. This functional deterioration which comes with ageing, can be slowed in pace with exercise.

Objective

The objective of the current review was to thoroughly search for literature assessing impact of RT on physical performance, body composition, muscle strength and endurance in sarcopenic elderly patients.

Methods

PubMed, Scopus, Web of Science, and PEDro databases were brought in use for a thorough search for articles published from 2010 to 2023. Two researchers independently retrieved data from studies that complied with the inclusion and exclusion criteria, while they also evaluated quality of the evidence.

Results

In total, 14 studies with 742 patients with mean age of 72.4 ± 9.22 years were included in the analysis for this review. Results indicate, RT improves body composition (p = 0.001), functional performance (p 0.001), postural stability (p = 0.005) and muscle strength (p 0.001) in elderly sarcopenic patients.

Conclusion

A promising intervention for the management of sarcopenia is RT. To yield RT's positive effects, a well-designed prescription is the need of the hour, just like it is with other treatment strategies.

Impaired Lumbar Extensor Force Control Is Associated with Increased Lifting Knee Velocity in People with Chronic Low-Back Pain

The ability of the lumbar extensor muscles to accurately control static and dynamic forces is important during daily activities such as lifting. Lumbar extensor force control is impaired in low-back pain patients and may therefore explain the variances in lifting kinematics. Thirty-three chronic low-back pain participants were instructed to lift weight using a self-selected technique. Participants also performed an isometric lumbar extension task where they increased and decreased their lumbar extensor force output to match a variable target force within 20-50% lumbar extensor maximal voluntary contraction. Lifting trunk and lower limb range of motion and angular velocity variables derived from phase plane analysis in all planes were calculated. Lumbar extensor force control was analyzed by calculating the Root-Mean-Square Error (RMSE) between the participants' force and the target force during the increasing (RMSEA), decreasing (RMSED) force portions and for the overall force error (RMSET) of the test. The relationship between lifting kinematics and RMSE variables was analyzed using multiple linear regression. Knee angular velocity in the sagittal and coronal planes were positively associated with RMSEA (R2 = 0.10, β = 0.35, p = 0.046 and R2 = 0.21, β = 0.48, p = 0.004, respectively). Impaired lumbar extensor force control is associated with increased multiplanar knee movement velocity during lifting. The study findings suggest a potential relationship between lumbar and lower limb neuromuscular function in people with chronic low-back pain.

External mechanical perturbations challenge postural stability in dogs

This study aimed to explore the effect of external mechanical perturbations on postural stability (PS) in dogs using the body center of pressure (COP). Thirteen sound adult dogs were included in this study. PS was tested during quiet standing on a pressure measurement plate. The conditions included a standard standing measurement and external mechanical perturbations conducted using six settings on a motorized training platform with different intensities of speed and amplitude. Measurement conditions were compared using linear mixed-effects models, followed by multiple comparisons using Sidak's alpha correction procedure. Compared with the standing measurement, external mechanical perturbations resulted in a significant increase in almost all COP parameters, indicating a challenge for the PS. Furthermore, an increase in amplitude had a greater effect than an increase in speed, whereas the combination of the highest intensities of amplitude and speed was not well tolerated by the dogs. The mediolateral COP displacement was significantly greater than the craniocaudal COP displacement during standing measurement and conditions with a small amplitude, whereas no significant difference was observed during settings with an increased amplitude. To the best of our knowledge, this is the first study to demonstrate the effects of a balance training device in dogs. Therefore, the intensity of the training programs on motorized platforms or similar devices can be controlled by the wobbling amplitude of the platform.

The 'Postural Rhythm' of the Ground Reaction Force during Upright Stance and Its Conversion to Body Sway-The Effect of Vision, Support Surface and Adaptation to Repeated Trials

The ground reaction force (GRF) recorded by a platform when a person stands upright lies at the interface between the neural networks controlling stance and the body sway deduced from centre of pressure (CoP) displacement. It can be decomposed into vertical (VGRF) and horizontal (HGRF) vectors. Few studies have addressed the modulation of the GRFs by the sensory conditions and their relationship with body sway. We reconsidered the features of the GRFs oscillations in healthy young subjects (n = 24) standing for 90 s, with the aim of characterising the possible effects of vision, support surface and adaptation to repeated trials, and the correspondence between HGRF and CoP time-series. We compared the frequency spectra of these variables with eyes open or closed on solid support surface (EOS, ECS) and on foam (EOF, ECF). All stance trials were repeated in a sequence of eight. Conditions were randomised across different days. The oscillations of the VGRF, HGRF and CoP differed between each other, as per the dominant frequency of their spectra (around 4 Hz, 0.8 Hz and <0.4 Hz, respectively) featuring a low-pass filter effect from VGRF to HGRF to CoP. GRF frequencies hardly changed as a function of the experimental conditions, including adaptation. CoP frequencies diminished to <0.2 Hz when vision was available on hard support surface. Amplitudes of both GRFs and CoP oscillations decreased in the order ECF > EOF > ECS ≈ EOS. Adaptation had no effect except in ECF condition. Specific rhythms of the GRFs do not transfer to the CoP frequency, whereas the magnitude of the forces acting on the ground ultimately determines body sway. The discrepancies in the time-series of the HGRF and CoP oscillations confirm that the body's oscillation mode cannot be dictated by the inverted pendulum model in any experimental conditions. The findings emphasise the robustness of the VGRF "postural rhythm" and its correspondence with the cortical theta rhythm, shed new insight on current principles of balance control and on understanding of upright stance in healthy and elderly people as well as on injury prevention and rehabilitation.

Deterioration of postural control due to the increase of similarity between center of pressure and smooth-pursuit eye movements during standing on one leg

Upright postural control is regulated by afferent and efferent/reafferent visual mechanisms. There are two types of efferent and conjugate eye movements: saccades and smooth pursuits. Although postural control is improved by saccades, the effects of smooth pursuits on postural control are still debated, because the difficulties of postural and visual tasks differ in the previous research. Additionally, the mechanisms that interfere with postural control and smooth pursuit are not fully understood. To address these issues, we examined the effects of different patterns of smooth-pursuit eye movement on the path length of the center of pressure (COP) displacement under bipedal and unipedal standing conditions. The relative frequency and amplitude of the COP displacement were remarkably increased when uniform linear visual targets were presented during unipedal standing. In addition, dynamic time warping analysis demonstrated that the similarity between the displacement of the COP and eye movements was increased by the presentation of uniform linear visual targets with orientation selectivity during unipedal standing but not during bipedal standing. In contrast, the attenuation of similarity between the displacement of the COP and eye movements significantly decreased the path length, relative frequency, and amplitude of the COP displacement. Our results indicate that postural stability is deteriorated by the increase of similarity between the displacement of the COP and smooth-pursuit eye movements under unstable conditions.

Vertical ground reaction force oscillation during standing on hard and compliant surfaces: The “postural rhythm”

When a person stands upright quietly, the position of the Centre of Mass (CoM), the vertical force acting on the ground and the geometrical configuration of body segments is accurately controlled around to the direction of gravity by multiple feedback mechanisms and by integrative brain centres that coordinate multi-joint movements. This is not always easy and the postural muscles continuously produce appropriate torques, recorded as ground reaction force by a force platform. We studied 23 young adults during a 90 s period, standing at ease on a hard (Solid) and on a compliant support (Foam) with eyes open (EO) and with eyes closed (EC), focusing on the vertical component of the ground reaction force (VGRF). Analysis of VGRF time series gave the amplitude of their rhythmic oscillations (the root mean square, RMS) and of their frequency spectrum. Sway Area and Path Length of the Centre of Pressure (CoP) were also calculated. VGRF RMS (as well as CoP sway measures) increased in the order EO Solid ≈ EC Solid < EO Foam < EC Foam. The VGRF frequency spectra featured prevailing frequencies around 4–5 Hz under all tested conditions, slightly higher on Solid than Foam support. Around that value, the VGRF frequencies varied in a larger range on hard than on compliant support. Sway Area and Path Length were inversely related to the prevailing VGRF frequency. Vision compared to no-vision decreased Sway Area and Path Length and VGRF RMS on Foam support. However, no significant effect of vision was found on VGRF mean frequency for either base of support condition. A description of the VGRF, at the interface between balance control mechanisms and sway of the CoP, can contribute information on how upright balance is maintained. Analysis of the frequency pattern of VGRF oscillations and its role in the maintenance of upright stance should complement the traditional measures of CoP excursions in the horizontal plane.

Incongruity of Geometric and Spectral Markers in the Assessment of Body Sway

Different measurements of body oscillations in the time or frequency domain are being employed as markers of gait and balance abnormalities. This study investigates basic relationships within and between geometric and spectral measures in a population of young adult subjects. Twenty healthy subjects stood with parallel feet on a force platform with and without a foam pad. Adaptation effects to prolonged stance were assessed by comparing the first and last of a series of eight successive trials. Centre of Foot Pressure (CoP) excursions were recorded with Eyes Closed (EC) and Open (EO) for 90s. Geometric measures (Sway Area, Path Length), standard deviation (SD) of the excursions, and spectral measure (mean power Spectrum Level and Median Frequency), along the medio-lateral (ML) and antero-posterior (AP) direction were computed. Sway Area was more strongly associated than Path Length with CoP SD and, consequently, with mean Spectrum Level for both ML and AP, and both visual and surface conditions. The squared-SD directly specified the mean power Spectrum Level of CoP excursions (ML and AP) in all conditions. Median Frequency was hardly related to Spectrum Level. Adaptation had a confounding effect, whereby equal values of Sway Area, Path Length, and Spectrum Level corresponded to different Median Frequency values. Mean Spectrum Level and SDs of the time series of CoP ML and AP excursions convey the same meaning and bear an acceptable correspondence with Sway Area values. Shifts in Median Frequency values represent important indications of neuromuscular control of stance and of the effects of vision, support conditions, and adaptation. The Romberg Quotient EC/EO for a given variable is contingent on the compliance of the base of support and adaptation, and different between Sway Area and Path Length, but similar between Sway Area and Spectrum Level (AP and ML). These measures must be taken with caution in clinical studies, and considered together in order to get a reliable indication of overall body sway, of modifications by sensory and standing condition, and of changes with ageing, medical conditions and rehabilitation treatment. However, distinct measures shed light on the discrete mechanisms and complex processes underpinning the maintenance of stance.

Increased 18F-FDG Uptake in the Axillary Lymph Nodes of the Vaccinated Side Associated with COVID-19 Vaccination

A 50-year-old female patient underwent (¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) following modified radical mastectomy for cancer of the left breast. Ten days before the PET/CT, the coronavirus disease-2019 (COVID-19) vaccine was injected intramuscularly into the right deltoid muscle. Increased (¹⁸F-FDG uptake of maximum standardized uptake value (11.0) was observed in the lymph nodes of the right axilla, which had not been observed in the previous PET/CT. The size of the oval-shaped lymph nodes was up to approximately 11×9 mm; however, it was larger than that observed on the previous PET/CT. We contemplate that the increased (¹⁸F-FDG uptake was a reactive change in the lymph nodes associated with the COVID-19 vaccine.

Balance Adaptation While Standing on a Compliant Base Depends on the Current Sensory Condition in Healthy Young Adults

Background

Several investigations have addressed the process of balance adaptation to external perturbations. The adaptation during unperturbed stance has received little attention. Further, whether the current sensory conditions affect the adaptation rate has not been established. We have addressed the role of vision and haptic feedback on adaptation while standing on foam.

Methods

In 22 young subjects, the analysis of geometric (path length and sway area) and spectral variables (median frequency and mean level of both total spectrum and selected frequency windows) of the oscillation of the centre of feet pressure (CoP) identified the effects of vision, light-touch (LT) or both in the anteroposterior (AP) and mediolateral (ML) direction over 8 consecutive 90 s standing trials.

Results

Adaptation was obvious without vision (eyes closed; EC) and tenuous with vision (eyes open; EO). With trial repetition, path length and median frequency diminished with EC (p < 0.001) while sway area and mean level of the spectrum increased (p < 0.001). The low- and high-frequency range of the spectrum increased and decreased in AP and ML directions, respectively. Touch compared to no-touch enhanced the rate of increase of the low-frequency power (p < 0.05). Spectral differences in distinct sensory conditions persisted after adaptation.

Conclusion

Balance adaptation occurs during standing on foam. Adaptation leads to a progressive increase in the amplitude of the lowest frequencies of the spectrum and a concurrent decrease in the high-frequency range. Within this common behaviour, touch adds to its stabilising action a modest effect on the adaptation rate. Stabilisation is improved by favouring slow oscillations at the expense of sway minimisation. These findings are preliminary to investigations of balance problems in persons with sensory deficits, ageing, and peripheral or central nervous lesion.

Proactive Modulation in the Spatiotemporal Structure of Muscle Synergies Minimizes Reactive Responses in Perturbed Landings

Stability training in the presence of perturbations is an effective means of increasing muscle strength, improving reactive balance performance, and reducing fall risk. We investigated the effects of perturbations induced by an unstable surface during single-leg landings on the mechanical loading and modular organization of the leg muscles. We hypothesized a modulation of neuromotor control when landing on the unstable surface, resulting in an increase of leg muscle loading. Fourteen healthy adults performed 50 single-leg landings from a 30 cm height onto two ground configurations: stable solid ground (SG) and unstable foam pads (UG). Ground reaction force, joint kinematics, and electromyographic activity of 13 muscles of the landing leg were measured. Resultant joint moments were calculated using inverse dynamics and muscle synergies with their time-dependent (motor primitives) and time-independent (motor modules) components were extracted via non-negative matrix factorization. Three synergies related to the touchdown, weight acceptance, and stabilization phase of landing were found for both SG and UG. When compared with SG, the motor primitive of the touchdown synergy was wider in UG (p < 0.001). Furthermore, in UG the contribution of gluteus medius increased (p = 0.015) and of gastrocnemius lateralis decreased (p < 0.001) in the touchdown synergy. Weight acceptance and stabilization did not show any statistically significant differences between the two landing conditions. The maximum ankle and hip joint moment as well as the rate of ankle, knee, and hip joint moment development were significantly lower (p < 0.05) in the UG condition. The spatiotemporal modifications of the touchdown synergy in the UG condition highlight proactive adjustments in the neuromotor control of landings, which preserve reactive adjustments during the weight acceptance and stabilization synergies. Furthermore, the performed proactive control in combination with the viscoelastic properties of the soft surface resulted in a reduction of the mechanical loading in the lower leg muscles. We conclude that the use of unstable surfaces does not necessarily challenge reactive motor control nor increase muscle loading per se. Thus, the characteristics of the unstable surface and the dynamics of the target task must be considered when designing perturbation-based interventions.

Specific Posture-Stabilising Effects of Vision and Touch Are Revealed by Distinct Changes of Body Oscillation Frequencies

We addressed postural instability during stance with eyes closed (EC) on a compliant surface in healthy young people. Spectral analysis of the centre of foot pressure oscillations was used to identify the effects of haptic information (light-touch, EC-LT), or vision (eyes open, EO), or both (EO-LT). Spectral median frequency was strongly reduced by EO and EO-LT, while spectral amplitude was reduced by all "stabilising" sensory conditions. Reduction in spectrum level by EO mainly appeared in the high-frequency range. Reduction by LT was much larger than that induced by the vision in the low-frequency range, less so in the high-frequency range. Touch and vision together produced a fall in spectral amplitude across all windows, more so in anteroposterior (AP) direction. Lowermost frequencies contributed poorly to geometric measures (sway path and area) for all sensory conditions. The same subjects participated in control experiments on a solid base of support. Median frequency and amplitude of the spectrum and geometric measures were largely smaller when standing on solid than on foam base but poorly affected by the sensory conditions. Frequency analysis but not geometric measures allowed to disclose unique tuning of the postural control mode by haptic and visual information. During standing on foam, the vision did not reduce low-frequency oscillations, while touch diminished the entire spectrum, except for the medium-high frequencies, as if sway reduction by touch would rely on rapid balance corrections. The combination of frequency analysis with sensory conditions is a promising approach to explore altered postural mechanisms and prospective interventions in subjects with central or peripheral nervous system disorders.