The underlying mechanisms of improved balance after short- and long-term training in older adults

With training older adults can improve balance control, but the time course and neural mechanisms underlying these improvements are unclear. We studied changes in balance (robustness and performance), as well as in H-reflex gains, paired reflex depression (PRD) and co-contraction duration (CCI) in ankle muscles after short-term (1 session; STT) and long-term (3 weeks; LTT) balance training in 22 older adults. Mediolateral balance robustness during unipedal stance (time to balance loss in unipedal standing on a robotic platform with decreasing rotational stiffness) improved (33%) after STT, with no further improvement after LTT. Balance performance (mean absolute mediolateral center of mass velocity) improved (18.75%) after STT in perturbed unipedal standing and after LTT (18.18%) in unperturbed unipedal standing. CCI of soleus/tibialis anterior did not change after STT but increased (16%) after LTT. H-reflex gain and PRD excitability did not change with training. Cross-correlations showed that H-reflex gains in unipedal stance were lower and CCI was higher in participants with a more robust balance at the last time-point measurement and, CCI was higher in participants with better balance performance at several time-points. However, changes in robustness and performance were uncorrelated with changes in CCI, H-reflex gain, or PRD. Our results indicate that balance robustness improves over a single session, while balance performance improves more gradually over multiple sessions. Changes in co-contraction and motor neuron excitability of ankle muscles are not exclusive causes of improved balance performance and robustness.

Effects of Back Support Exoskeleton Use on Postural Stability

While occupational back support exoskeletons (BSEs) are being considered as a potential intervention to reduce physical demands in tasks such as repetitive lifting (e.g., Frost, Abdoli-E, & Stevenson, 2009; Koopman, Kingma, Faber, de Looze, & van Dieën, 2019), BSE use in practice may introduce some unexpected or unintended safety challenges (Baltrusch, van Dieën, van Bennekom, & Houdijk, 2018; de Looze, Bosch, Krause, Stadler, & O’Sullivan, 2016). One potential adverse effect is a decrease in postural balance and stability due to the extra weight and the rigid structure of a BSE. However, there is limited empirical evidence on how the use of a BSE affects postural balance and stability. In this study, we investigated the effects of using different BSEs on postural balance and stability during quiet upright stance. A total of 20, gender-balanced, healthy participants were included [males = 25.2 (3.8) years, 176.4 (7.4) cm, and 76.6 (8.8) kg; females = 27.5 (2.7) years, 166.5 (5.4) cm, and 61.2 (8.6) kg]. Each completed multiple trials of quiet upright stance at different levels of difficulty (i.e., bipedal and unipedal stance; each with eyes open and closed), while wearing two different BSEs (SuitX™ AC version, Laevo™ V2) and in a control (no-exoskeleton) condition. Respective masses of the SuitX™ and Laevo™ were 4.5 and 2.8 kg, and both devices were designed to provide external torque parallel to that created by the torso extensor muscles via three body contact points (i.e., thighs, pelvis, and chest). Center-of-pressure (COP) time series were measured using a force-plate during each trial, and traditional COP-based postural sway parameters were used as outcome measures: median frequency (MF), mean velocity (MV), root-mean-square distance (RMSD), and sway area. MF, MV, and RMSD values were obtained in both the anteroposterior (AP) and mediolateral (ML) directions. As MV, RMSD, and SwayAREA were significantly correlated with participant height, these measures were normalized using respective linear regression models to prevent undesired bias. Results from repeated measures analyses of variance indicated that wearing BSEs may adversely affect postural stability during bipedal stance. Compared to the control condition, wearing the Laevo™ increased MFAP by 50% ( p=0.015) with eyes open, and wearing the SuitX™ increased MVAP by 7% ( p=0.029) with eyes closed. However, specific to the unipedal standing with eyes closed, wearing the Laevo™ appeared to positively affect postural stability among males, whereas wearing the SuitX™ appeared to negatively affect postural stability among females. Specifically, males had a 24% decrease in RMSDAP ( p=0.003) and a 31% decrease in SwayAREA ( p=0.012) when wearing Laevo™, while females had a 15% increase in RMSDAP ( p=0.038) when wearing the SuitX™. In conclusion, we found evidence that wearing a BSE can affect postural balance and stability during quiet upright stance. More importantly, such effects appeared dependent on task conditions (i.e., bipedal and unipedal, and with eyes open/closed), and to be BSE-design and gender-dependent. Our findings may suggest complex interactive dynamics between a user and a BSE, potentially due to a change in total center of mass (i.e., body + BSE), restricted motion, and external supportive torques of the BSE. Furthermore, males and females may be differentially affected by such interactive dynamics, depending on the BSE, particularly in more challenging tasks. Future studies need to investigate the specificity of BSE effects on postural control while considering different demographic and individual factors, and also the effects of wearing BSEs on postural stability in dynamic conditions (e.g., walking, negotiating an obstacle, or recovering from an external perturbation).

Effects of Interrupting Sitting with Use of a Treadmill Desk Versus Prolonged Sitting on Postural Stability

High amounts of sitting increase the risk of non-communicable disease and mortality. Treadmill desks make it possible to reduce sitting during the desk-based workerʼs day. This study investigated the acute effect on postural stability of interrupting prolonged sitting with an accumulated 2-h of light-intensity treadmill desk walking. Twenty-one sedentary adults participated in this randomized acute crossover trial, with two 6.5 h conditions: 1) uninterrupted sitting and 2) interrupted sitting with accumulated 2 h light-intensity treadmill desk walking. Pre- and post-condition, participants performed four postural stability tests on a pressure plate (bipedal and unipedal standing stance, eyes open and eyes closed). Anteroposterior center of pressure amplitude showed a significant condition x time interaction in bipedal eyes closed (F(1,20)=4.62, p=0.046) and unipedal eyes open (F(1,20)=9.42, p=0.006) tests, and mediolateral center of pressure amplitude in bipedal eyes closed (F(1,20)=6.12, p=0.023) and bipedal eyes open (F(1,12)=5.55, p=0.029) tests. In the significant interactions, amplitude increased pre to post condition in the uninterrupted sitting condition. The accumulated 2 h light-intensity treadmill desk walking ameliorated the negative effect of 6.5 h prolonged sitting on postural sway, supporting workplace treadmill desk use.

Facilitation of Hoffmann Reflexes of Ankle Muscles in Prone but Not Standing Positions by Focal Ankle-Joint Cooling

Context:Focal ankle-joint cooling (FAJC) has been shown to increase Hoffmann (H) reflex amplitudes of select leg muscles while subjects lie prone, but it is unknown whether the neurophysiological cooling effects persist in standing.Objective:To assess the effects of FAJC on H-reflexes of the soleus and fibularis longus during 3 body positions (prone, bipedal, and unipedal stances) in individuals with and without chronic ankle instability (CAI).Design:Crossover.Setting:Laboratory.Participants:15 young adults with CAI (9 male, 6 female) and 15 healthy controls.Intervention:All subjects received both FAJC and sham treatments on separate days in a randomized order. FAJC was accomplished by applying a 1.5-L plastic bag filled with crushed ice to the ankle for 20 min. Sham treatment involved room-temperature candy corn.Main Outcome Measures:Maximum amplitudes of H-reflexes and motor (M) waves were recorded while subjects lay prone and then stood in quiet bipedal and unipedal stances before and immediately after each treatment. Primary outcome measures were Hmax:Mmax ratios for the soleus and fibularis longus. Three-factor (group × treatment condition × time) repeated-measures ANOVAs and Fisher LSD tests were performed for statistical analyses.Results:Significant interactions of treatment condition by time for prone Hmax:Mmax ratios were found in the soleus (P = .001) and fibularis longus (P = .003). In both muscles, prone Hmax:Mmax ratios moderately increased after FAJC but not after sham treatment. The CAI and healthy groups responded similarly to FAJC. In contrast, there were no significant interactions or main effects in the bipedal and unipedal stances in either muscle (P > .05).Conclusions:FAJC moderately increased H-reflex amplitudes of the soleus and fibularis longus while subjects were prone but not during bipedal or unipedal standing. These results were not different between groups with and without CAI.

Relationship between Mental Rotation of Body Parts and Postural Stability during Quiet Stance

The present study was designed to investigate a relationship between the ability to quickly perform a mental rotation (MR) task using body (particularly foot) stimuli and postural stability during unipedal and bipedal quiet stance. Twenty-four healthy young adults participated in this study to measure reaction times for the MR (stimuli: foot, hand, and car), postural sway values during unipedal and bipedal standings, and lower extremity functions. Results showed significant correlations between the reaction time for the MR of the foot stimuli (but not for hand and car stimuli) and some of postural sway values (total length of sway and mean velocity in the anterior–posterior direction) during unipedal standing (but not for bipedal standing). Consistently, participants who performed the MR task quickly showed significantly smaller sway values during unipedal standing than those who performed the task slowly. These findings suggest that the ability to mentally imagine the foot movement is likely to relate to postural stability, while involving a challenging postural task, such as unipedal standing. The reaction time for the MR of foot stimuli was also correlated with two-point discrimination (TPD) distance on the plantar skin. Given that the TPD distance not only represents cutaneous acuity but also reflects participants’ body image relating to their feet, MR performance may have been related to postural stability because both involve cognitive processes used for both motor imagery and motor execution of the foot movement.

EFFECTS OF LATERALITY, ANKLE INVERSION AND STABILIZERS ON THE PLANTAR PRESSURE DISTRIBUTION DURING UNIPEDAL STANDING

The purpose of this research was to evaluate the standing stability with plantar pressure distribution (PPD), and to assess the effects of limb laterality, ankle inversion and stabilizer on PPD during unipedal standing. Nineteen young healthy adults were requested to stand on different inclined surfaces (level 0° and inclined 10° and 20°) with each foot. Three ankle stabilizer conditions were designed as barefoot control, elastic ankle tape and semi-rigid brace. Statistically analyzed with multivariate analysis of variance, the anterior/posterior (A/P) and medial/lateral (M/L) force ratios and total contact area (TPCA) were the main parameters to evaluate the stability. Compared with non-dominant side, the dominant foot was safer during unipedal standing with significantly greater TPCA, and especially when standing with ankle inversion due to additional significantly greater A/P force ratio. Ankle inversion harmed the stability while standing with the non-dominant foot due to significantly decreased A/P force ratio and local contact areas. Ankle brace improved the standing stability with significantly increased TPCA.