Adaptation to continuous perturbation of balance: progressive reduction of postural muscle activity with invariant or increasing oscillations of the center of mass depending on perturbation frequency and vision conditions

We investigated the adaptation of balancing behavior during a continuous, predictable perturbation of stance consisting of 3-min backward and forward horizontal sinusoidal oscillations of the support base. Two visual conditions (eyes-open, EO; eyes-closed, EC) and two oscillation frequencies (LF, 0.2 Hz; HF, 0.6 Hz) were used. Center of Mass (CoM) and Center of Pressure (CoP) oscillations and EMG of Soleus (Sol) and Tibialis Anterior (TA) were recorded. The time course of each variable was estimated through an exponential model. An adaptation index allowed comparison of the degree of adaptation of different variables. Muscle activity pattern was initially prominent under the more challenging conditions (HF, EC and EO; LF, EC) and diminished progressively to reach a steady state. At HF, the behavior of CoM and CoP was almost invariant. The time-constant of EMG adaptation was shorter for TA than for Sol. With EC, the adaptation index showed a larger decay in the TA than Sol activity at the end of the balancing trial, pointing to a different role of the two muscles in the adaptation process. At LF, CoM and CoP oscillations increased during the balancing trial to match the platform translations. This occurred regardless of the different EMG patterns under EO and EC. Contrary to CoM and CoP, the adaptation of the muscle activities had a similar time-course at both HF and LF, in spite of the two frequencies implying a different number of oscillation cycles. During adaptation, under critical balancing conditions (HF), postural muscle activity is tuned to that sufficient for keeping CoM within narrow limits. On the contrary, at LF, when vision permits, a similar decreasing pattern of muscle activity parallels a progressive increase in CoM oscillation amplitude, and the adaptive balancing behavior shifts from the initially reactive behavior to one of passive riding the platform. Adaptive balance control would rely on on-line computation of risk of falling and sensory inflow, while minimizing balance challenge and muscle effort. The results from this study contribute to the understanding of plasticity of the balance control mechanisms under posture-challenging conditions.     

Effects of plantar cutaneo-muscular and tendon vibration on posture and balance during quiet and perturbed stance

Modulation of lower limb somatosensory information by tendon or plantar vibration produces directionally specific, vibration-induced falling reactions that depend on the tendon or the region of the sole that is vibrated. This study characterized the effects of different patterns of plantar cutaneo-muscular vibration and bilateral Achilles tendon vibration (ATV) on the postural strategies observed during quiet and perturbed stance. Twelve healthy young participants stood barefooted, with their vision blocked, on two sets of plantar vibrators placed on two AMTI force plates embedded in a moveable support surface. Two other vibrators were positioned over the Achilles tendons. Participants were randomly exposed to different patterns of plantar cutaneo-muscular and ATV. Tilts of the support surface in the toes-up (TU) and toes-down (TD) directions were given 5-8 s after the beginning of vibration. Body kinematics in 3D and ground reaction forces were recorded. Bilateral ATV applied with or without rearfoot vibration (RFV) during quiet stance resulted in a whole-body backward leaning accompanied by an increase in trunk extension and hip and knee flexion. RFV alone produced a forward whole-body tilt with increased flexion in trunk, hip, and ankle. When stance was perturbed by TU tilts, the center of mass (CoM) and center of pressure (CoP) displacements were larger in the presence of RFV or ATV and associated with increased peak trunk flexion. TD tilts with or without ATV resulted in no significant difference in CoM and CoP displacements, while larger trunk extension and smaller distal angular displacements were observed during ATV. RFV altered the magnitude of the balance reactions, as observed by an increase in CoP displacements and variable response in trunk displacement. Significant interactions between ATV and RFV were obtained for the peak angular excursions for both directions of perturbations, where ATV either enhanced (for TU tilts) or attenuated (for TD tilts) the influence of RFV. Manipulating somatosensory information from the plantar cutaneo-muscular and muscle spindle Ia afferents thus results in altered and widespread postural responses, as shown by profound changes in body kinematics and CoM and CoP displacements. This suggests that the CNS uses plantar cutaneo-muscular and ankle spindle afferent inputs to build an appropriate reference of verticality that influences the control of equilibrium during quiet and perturbed stance.     

Influence of prolonged wearing of unstable shoes on upright standing postural control

ObjectiveTo study the influence of prolonged wearing of unstable shoes on standing postural control in prolonged standing workers.MethodsThe participants were divided into two groups: one wore unstable shoes while the other wore conventional shoes for 8 weeks. Stabilometry parameters related to centre of pressure (CoP), rambling (RM) and trembling (TR) as well as the total agonist/antagonist muscle activity, antagonist co-activation and reciprocal activation were evaluated during upright standing, before and after the 8 weeks period. In both moments, the subjects were evaluated wearing the unstable shoes and in barefoot.ResultsThe unstable shoe condition presented increased CoP displacement related variables and decreased co-activation command compared to barefoot before and after the intervention. The prolonged wearing of unstable shoes led to: (1) reduction of medial–lateral CoP root mean square and area; (2) decreased anteroposterior RM displacement; (3) increased anteroposterior RM mean velocity and mediolateral RM displacement; (4) decreased anteroposterior TR RMS; and (5) increased thigh antagonist co-activation in the unstable shoe condition.ConclusionThe unstable shoe condition is associated to a higher destabilising effect that leads to a selection of more efficient and accurate postural commands compared to barefoot. Prolonged wearing of unstable shoes provides increased effectiveness and performance of the postural control system, while wearing of unstable shoes in upright standing, that are reflected by changes in CoP related variables and by a reorganisation of postural control commands.     

Postural control after unexpected external perturbation: Effects of Parkinson’s disease subtype

Different clinical subtypes of Parkinson’s disease (PD) have long been recognized. Recent studies have focused on two PD subtypes: Postural Instability and Gait Difficulty (PIGD) and Tremor Dominant (TD). PIGD patients have greater difficulties in postural control in relation to TD. However, knowledge about the differences in reactive adjustment mechanisms following a perturbation in TD and PIGD is limited. This study aimed to compare reactive postural adjustments under unexpected external perturbation in TD, PIGD, and control group (CG) subjects. Forty-five individuals (15 TD, 15 PIGD, and 15 CG) participated in this study. Postural perturbation was applied by the posterior displacement of the support surface in an unexpected condition. The velocity (15 cm/s) and displacement (5 cm/s) of perturbation were the same for all participants. Center of pressure (CoP) and center of mass (CoM) were analyzed for two reactive windows after the perturbation (0–200 ms and 200–700 ms). The Bonferroni post hoc test indicated a higher range of CoP in the PIGD when compared to the CG (p = 0.021). The PIGD demonstrated greater time to recover the stable posture compared to the TD (p = 0.017) and CG (p = 0.003). Furthermore, the TD showed higher AP-acceleration peak of CoM when compared to the PIGD (p = 0.048) and CG (p = 0.013), and greater AP-acceleration range of CoM in relation to the CG (p = 0.022). These findings suggest that PD patients present worse reactive postural control after perturbation compared to healthy older individuals. CoP and CoM parameters are sensitive to understand and detect the differences in reactive postural mechanisms in PD subtypes.     

Smaller muscle mass is associated with increase in EMG–EMG coherence of the leg muscle during unipedal stance in elderly adults

Age-induced decline in the ability to perform daily activities is associated with a deterioration of physical parameters. Changes occur in neuromuscular system with age; however, the relationship between these changes and physical parameters has not been fully elucidated. Therefore, in this study, we aimed to determine the relationship between neuromuscular system evaluated using a coherence analysis of the leg muscles and physical parameters in community-dwelling healthy elderly adults. The participants were required to stand still in bipedal and unipedal stances on a force plate. Then, electromyography (EMG) was recorded from the tibialis anterior (TA) and medial and lateral gastrocnemius (MG/LG) muscles, and intermuscular coherence was calculated between the following pairs: TA and MG (TA–MG), TA and LG (TA–LG), and MG and LG (MG–LG). Furthermore, gait speed, unipedal stance time, and muscle mass were measured. EMG–EMG coherence for the MG–LG pair was significantly greater in the unipedal stance task than in the bipedal one (p = .001). Multiple linear regression analysis revealed that the muscle mass of the leg was negatively correlated with the change in the β-band coherence for the MG–LG pair from bipedal to unipedal stance (R² = 0.067, standard β = −0.345, p = .044). As the β-band coherence could reflect the corticospinal activity, the increased β-band coherence may be a compensation for the smaller muscle mass, or alternatively may be a sign of changes in the nervous system resulting in the loss of muscle mass.     

The 'extrapolated center of mass' concept suggests a simple control of balance in walking

Next to position x and velocity v of the whole body center of mass (CoM) the 'extrapolated center of mass' (XcoM) can be introduced: xi = chi + nu/omega 0, where omega 0 is a constant related to stature. Based on the inverted pendulum model of balance, the XcoM enables to formulate the requirements for stable walking in a relatively simple form. In a very simple walking model, with the effects of foot roll-over neglected, the trajectory of the XcoM is a succession of straight lines, directed in the line from center of pressure (CoP) to the XcoM at the time of foot contact. The CoM follows the XcoM in a more sinusoidal trajectory. A simple rule is sufficient for stable walking: at foot placement the CoP should be placed at a certain distance behind and outward of the XcoM at the time of foot contact. In practice this means that a disturbance which results in a CoM velocity change Deltav can be compensated by a change in foot position (CoP) equal to Deltav/omega 0 in the same direction. Similar simple rules could be formulated for starting and stopping and for making a turn.     

Four Weeks of Neuromuscular Training Improve Static and Dynamic Postural Control in Overweight and Obese Children: A Randomized Controlled Trial

Abstract

Thirty-two children with overweight or obesity were randomly divided into a neuromuscular training group (NTG) (n = 16) and a control group (CG) (n = 16). All individuals participated in the measurement of static postural control under two conditions: the double-leg stance with eyes open (EO) and eyes closed (EC). The center of pressure variables was obtained. mSEBT was used for dynamic postural control. Neuromuscular training was performed twice per week and lasted 4 weeks. The results of this study indicate that 4 weeks of neuromuscular training improve static and dynamic postural control in children with excess body weight.     

Ankle muscles activation and postural stability with Star Excursion Balance Test in healthy individuals

IntroductionThe ankle joint, a part of the kinetic chain of the lower limb, plays a significant role in the maintenance of postural stability during bipedal and unipedal balancing activities. This study aimed to evaluate the neuromuscular control of the ankle joint and the postural stability while executing the Star Excursion Balance Test (SEBT), by recording the EMG activity of the extrinsic ankle musculature and the displacement of the center of pressure (CoP).MethodsThe EMG activity of the tibialis anterior (TA), the peroneus brevis (PB) and the medial and lateral gastrocnemius (GM, GL), along with the anteroposterior and mediolateral displacements (APd and MLd) of CoP as well as the plantar pressure distribution of the supportive lower limb were recorded during reaching to the eight directions of SEBT in 29 healthy, physically active college students (15 males and 14 females; mean ± SD of age 25.6 ± 4.5 yrs.; height: 172.5 ± 8.2 cm; body weight: 67.7 ± 13.6 kg; and BMI: 22.6 ± 2.9 kg/m²).ResultsThe tibialis anterior muscle demonstrated the greatest EMG activity during SEBT, followed by the PB, GL and GM muscles. The increased EMG activity of TA and PB during the execution of all posterior-oriented and lateral directions coincided with a decreased APd of CoP and increased reaching distances. The opposite occurred during the execution of all the anterior-oriented and medial directions. The differences among the directions of SEBT regarding the EMG activity of GL, GM and the mediolateral displacement of CoP were, in general, not significant.ConclusionsThe neuromuscular control of the ankle joint and the associated postural stability during SEBT was highly depended upon the activation level of TA and PB, which should be considered by clinicians and sports specialists when using this test for screening and/or rehabilitation purposes.     

Unilateral Fatigue Affects the Unipedal Postural Balance in Individuals With Intellectual Disability

This study aimed to explore the effect of local muscle fatigue on the unipedal stance in men with intellectual disability (ID). The Centre of pressure (CoP) excursions and the isometric maximal voluntary contraction (MVC) were measured before and after a fatiguing exercise. Higher baseline values of CoP excursions and lower MVC values were recorded in the ID group. After the fatiguing exercise, this group showed higher MVC decrease and higher percentage of increase of the mean CoP velocity. In conclusion, men with ID are more vulnerable to the disturbing effects of fatigue during the unipedal stance compared to men without ID.     

Investigators' Affirmation of Ethical,Safeguard, and Scientific Commitments in Human Research

Little is known about how researchers view ethically salient aspects of human studies. As part of a National Institutes of Mental Health-funded study, the authors performed a confidential written survey to assess the attitudes, views, and experiences of researchers with institutional review board approved protocols at the University of New Mexico. A total of 363 researchers (57% response rate) participated. Investigators overall held favorable views of general ethical aspects of research and ethics-based safeguards, and they identified a positive role of ethics training. Investigators with more experience encountering ethical problems (p < .001), more ethics training (p = .001), and a PhD or MD/PhD (p = .003) held more favorable general ethical perspectives. Women investigators (p < .03), nonphysician investigators (p < .001), those whose training had been helpful in resolving ethical dilemmas (p = .006), and those for whom spirituality is important (p = .008) more strongly endorsed ethical safeguards. Investigators perceive the scientific and ethical aspects of their work as valuable and linked, and they affirm the role of safeguards in human studies. Formal ethics preparation and training initiatives were also viewed positively by investigators.     

Young adults use whole-body feedback and ankle proprioception to perceive small locomotor disturbances

To prevent a fall when a disturbance to walking is encountered requires sensory information about the disturbance to be sensed, integrated, and then used to generate an appropriate corrective motor response. Prior research has shown that feedback of whole-body motion (e.g., center-of-mass kinematics) drives this corrective response. Here, we hypothesized that young adults also use whole-body motion to perceive locomotor disturbances. 15 subjects performed a locomotor discrimination task in which the supporting leg was slowed during stance every 8–12 steps to emulate subtle slips. The perception threshold of these disturbances was determined using a psychometrics approach and found to be 0.08 ± 0.03 m/s. Whole-body feedback was examined through center-of-mass (CoM) kinematics and whole-body angular momentum (WBAM). Perturbation-induced deviations of CoM and WBAM were calculated in response to the two perturbation levels nearest each subject's perception threshold. Consistent with our hypothesis, we identified significantly higher perturbation induced deviations for perceived perturbations in sagittal-plane WBAM, anteroposterior CoM velocity, and vertical CoM velocity and acceleration. Because whole body motion is not sensed directly but instead arises from the integration of various sensory feedback signals, we also explored local sensory feedback contributions to the perception of locomotor disturbances. Local sensory feedback was estimated through kinematic analogues of vision (head angle), vestibular (head angular velocity), proprioception (i.e., sagittal hip, knee, and ankle angles), and somatosensation (i.e., anterior-posterior & mediolateral center-of-pressure, COP). We identified significantly higher perturbation induced deviations for perceived perturbations in sagittal-plane ankle angle. These results provide evidence for both whole-body feedback and ankle proprioception as important for the perception of subtle slip-like locomotor disturbances in young adults. Our interpretation is ankle proprioception is a dominant contributor to estimates of whole-body motion to perceive locomotor disturbances.     

Utility of center of pressure measures during obstacle crossing in prediction of fall risk in people with Parkinson’s disease

IntroductionPostural instability during walking and tripping over obstacles are the main causes of falls in people with Parkinson’s disease (PD). Preliminary limited evidence suggests that the length of the prospective follow-up period affects falls prediction in PD, with shorter periods leading to more accurate prediction. Thus, the primary aim of the present study was to test the performance of center of pressure (CoP) variables during obstacle crossing to predict fall risk in people with PD during subsequent periods of four, six, and 12 months. We also compared CoP variables during obstacle crossing between fallers and non-fallers.MethodsForty-two individuals with PD, in mild to moderate stages, completed the baseline obstacle crossing assessment and reported falls for 12 months. Participants walked at their self-selected pace and were instructed to cross an obstacle (half knee height) positioned in the middle of an 8-m long pathway. A force platform was used to analyze CoP parameters of the stance phase of the trailing limb (most affected limb). The ability of each outcome measure to predict fall risk at four, six, and 12 months was assessed using receiver operating characteristic curve analyses.ResultsTen individuals (23.8%) were considered fallers at four months, twelve individuals (28.5%) at six months, and twenty-one individuals (50%) at 12 months. CoP amplitude and CoP velocity in the mediolateral direction significantly predicted fall risk at four, six, and 12 months. As judged by the area under the curve, mediolateral CoP velocity showed the best performance at four months, while mediolateral CoP amplitude showed the best performance at six months. Fallers presented greater values of mediolateral CoP velocity and amplitude than non-fallers.ConclusionThese findings suggest that mediolateral CoP velocity and amplitude during obstacle crossing might be useful to predict fall risk in people with PD. Therefore, larger studies are encouraged.     

Immediate Effect of Training at the Onset of Reaching in Preterm Infants: Randomized Clinical Trial

The authors' aim was to investigate the immediate effect of a single specific training session (serial varied practice), of short duration on the kinematic parameters of reaching, in the period of the emergence of the skill in preterm and low birth weight infants. The study included 16 infants of both sexes, born at a mean gestational age of 32.13 (±1.36) weeks and mean birth weight of 1720.94 (±358.46) g. The infants were randomly divided into 2 groups: experimental and control. The experimental group was given a 5-min training session in reaching, while the control group received no training. The results showed significant differences in peak velocity in the intra (Z = –2.10, p = .036) and intergroup (U = 9.00, p = .016) evaluations, which decreased in the experimental group after training. Cohen's d test for clinical relevance suggested that the specific, short duration training proved effective in promoting slower reaches, with greater adjustment and lower number of units of movement. These results are positive for preterm infants given that these parameters more closely resemble the typical development of mature reaching behaviors in term infants, which suggests that this protocol of reaching training (serial varied practice) could be used as an evidence-based intervention strategy.     

What Cognitions Predict State Anger?

This study evaluated the relationship between specific cognitions and state anger. Clinical outpatients (23%), workplace recruits (37%), and college students (40%) (N = 236, mean age = 31.68) completed a questionnaire, the Anger Episode Record (AER), which asked them to describe an experience that elicited feelings of irritation, annoyance, anger, fury, or rage. They rated the intensity of their anger, cognitions, physiological responses, behaviors, and consequences for the specific episode. A stepwise multiple regression suggests that cognitions for revenge accounted for the greatest variance in predicting state anger, R = .26, p = .00, followed by demands on event, R = .32, p = .00, self-efficacy, R = .37, p = .00, and demands of others, R = .39, p = .04, respectively. The role of these cognitions in psychotherapy and interventions for angry clients is discussed.     

Saccade-Stepping Interactions Revise the Motor Plan for Obstacle Avoidance

The authors used a stimulus-response compatibility paradigm to assess the effect of changing the estimated time to obstacle contact. A limb-selection cue was presented in different phases of gait to young (n = 5) and to older (n = 4) adults while they were moving toward a foam obstacle in the walking path. A downward saccade was initiated after the cue; the saccade typically occurred during the stance phase of the target limb (the foot cued to lead the step over the obstacle). The mean saccade-step latency after the cue was on the order of ?500 ms in both young and elderly participants. On reaching the obstacle, both groups generated an upward saccade approximately ?300 ms before target footlift in both groups. Saccades following the limb-selection cue appeared to direct the gaze toward footfall targets just beyond the obstacle, whereas saccades generated just before obstacle footlift moved the gaze to the forward-looking direction. The elderly had significantly longer saccade-trailing-footlift latencies and prolonged gaze-fixation times than did the younger adults. Transient disruptions in optical flow appeared to be necessary for successful obstacle-avoidance behavior when there was an unexpected change in the estimated time to obstacle contact.     

Predictors of aerobic physical activity and resistance training among Canadian adults with type 2 diabetes: An application of the Protection Motivation Theory

BackgroundIt is well established that both aerobic physical activity (PA) and resistance training are essential in the treatment and management of type 2 diabetes (T2D), but few studies have examined the determinants of both modes of PA in the same sample.PurposeThe main objective was to investigate the utility of the Protection Motivation Theory (PMT) in predicting aerobic PA and resistance training in a population sample of T2D adults.MethodsA total of 244 individuals completed self-report PMT constructs of vulnerability, severity, fear, response efficacy, self-efficacy and intention, and a 3-month follow-up that assessed aerobic PA and resistance training.ResultsPMT explained 19% (p < .001) and 20% (p < .001) of the variance respectively for aerobic PA and resistance training behaviour. Significant associations were found between self-efficacy (β = 0.45, p < .001) and gender (β = 0.15, p < .05) for aerobic PA, and self-efficacy (β = 0.48, p < .001) and age (β = 0.17, p < .05) for resistance training. PMT accounted for 43% (p < .001) and 56% (p < .001) of the variance respectively for aerobic PA and resistance training intentions. For aerobic PA, response efficacy (β = 0.14, p < .05) and self-efficacy (β = 0.59, p < .001) were significantly associated with intention, while response efficacy (β = 0.23, p < .001), self-efficacy (β = 0.64, p < .001) and age (β = 0.10, p < .05) were significantly related with resistance training intention.ConclusionsNone of the unique constructs of the PMT (i.e., perceived vulnerability, severity and fear) were significant with either aerobic and resistance training intention. These results may guide the development of effective PA interventions in people with T2D.     

Effectiveness Evaluation of Search and Target Acquisition Training Prototype Using Performance Metrics With Eye-Tracking Data

Despite the increased use of sensor technologies, including unmanned vehicles, the vast majority of improvised explosive device (IED) detections are made by human vision. Thus, TRAC-Monterey developed a simulation-based training prototype called the perceptual learning trainer (PLT). Fourteen novice and 5 expert IED detectors participated in human-in-the-loop experiments in which all participants were trained using the PLT tool while their eye-movement and IED detection performance were tracked in real-time. A series of 100 IED images with various degrees of difficulty was used for the training session. Pre- and posttraining assessments were conducted. Both speed and accuracy improved after just 1 session of the PLT training: RT decreased by 3.7 s for novices (p < .001) and 3.4 s for experts (p = .031), and detection probability increased by 5.9% for novices (p = .001). The PLT tool improved IED detection performance more in novice IED detectors than in experts. Novices and experts showed different visual scan patterns.     

Broad stance conditions change postural control and postural sway

Intuitively, a broad stance (i.e., standing with the feet farther apart than usual) should significantly improve postural stability. However, this intuition was not confirmed in quiet stance. Hence, a motion analysis system (markers attached to the trunk and head) and a force platform were used to investigate 13 healthy, young adults who performed 8 trials in standard and broad stances. In broad stance, the medialateral center of pressure (COP) sway mean power frequency was expected to be greater, whereas the variability (standard deviation) of COP, head, and trunk sway and the mean velocity of head and trunk sway was expected to be significantly lower. Accordingly, adoption of a broad stance significantly increased the medialateral mean power frequency of COP sway; decreased the standard deviation of medialateral COP, trunk, and head sway; and decreased the medialateral mean velocity of head sway. A broad stance was also associated with lower variability for head and COP sways in the anteroposterior axis. Unexpectedly, an effect of trial repetition was found for the variability of medialateral trunk sway. This was probably due to the break halfway through the study. In practical terms, broad stance conditions can improve postural control in the medialateral and anteroposterior axes.     

Coordination of Eye and Leg Movements During Visually Guided Stepping

In the present study, 2 related hypotheses were tested: first, that vision is used in a feedforward control mode during precision stepping onto visual targets and, second, that the oculomotor and locomotor control centers interact to produce coordinated eye and leg movements during that task. Participants' (N = 4) eye movements and step cycle transition events were monitored while they performed a task requiring precise foot placement at every step onto irregularly placed stepping stones under conditions in which the availability of visual information was either restricted or intermittently removed altogether. Accurate saccades, followed by accurate steps, to the next footfall target were almost always made even when the information had been invisible for as long as 500 ms. Despite delays in footlift caused by the temporary removal (and subsequent reinstatement) of visual information, the mean interval between the start of the eye movement and the start of the swing toward a target did not vary significantly (p > .05). In contrast, the mean interval between saccade onset away from a target and a foot landing on that target (stance onset) did vary significantly (p < .05) under the different experimental conditions. Those results support the stated hypotheses.