An analysis of directional changes in the center of pressure trajectory during stance

This study proposes a new approach of posturography analysis, which enables the evaluation of directional changes in the center of pressure trajectory. The concept is similar to that of so-called “stabilogram diffusion analysis”. Instead of calculating the distance traveled by the center of pressure in a certain time interval, this new method calculates the cosine of the instantaneous velocity vectors of the center of pressure, which corresponds to the amount of change in the sway direction (cosine = 1 corresponds to the identical direction; cosine = ?1 corresponds to the opposite direction). This method was applied to the analysis of the experimental data in which postural sway was collected under four conditions: open eyes, closed eyes, and two auditory biofeedback conditions. In the biofeedback conditions, auditory signal was given to the subjects when the center of pressure swayed out of a certain area. As results, the differences in the postural reaction under these conditions were clearly shown using the new method. The results indicated that the subjects reacted to the auditory signals by swaying in the opposite direction with biofeedback. It was also found that the eyes open condition exhibited a more random-like profile. As this method analyzes the directional change in the postural sway, this method can be utilized cooperatively together with such a method as stabilogram diffusion analysis, which analyzes the magnitude of sway.     

Postural stability after treadmill and overground walking in young and elderly

BackgroundAgeing commonly disrupts the balance control and compensatory postural responses that contribute to maintaining balance and preventing falls during perturbation of posture. Improvement of compensatory postural responses during walking is one of the main goals in fall prevention programs which often include treadmill walking training. However, during treadmill walking, there is a sensory (visualsomatosensory and vestibular-somatosensory) conflict that can evoke aftereffects of self-motion sensation and could alter postural stability after training.Research questionThe aim of this study was to compare the effect of overground and treadmill walking on postural stability in healthy young and elderly subjects. Methods: Postural responses of 31 Young and 19 healthy Elderly before and after overground and treadmill walking were assessed by a force platform in four stance conditions: firm and foam surface with eyes open and eyes closed.ResultsIn Elderly group, velocity parameters significantly increased after treadmill walking but not after overground walking. This increase was found particularly in the conditions with eyes open in both types of surfaces (firm, foam). The velocity parameters values (expect Vx) were significantly increased in Elderly compared to Young almost in all four conditions after treadmill and overground walking. Significance: Our study suggests that Elderly become more unstable after treadmill walking and have greater difficulties to adapt to new balance circumstances caused by sensory conflict associated with treadmill walking. It seems that during treadmill walking and subsequent stance, vision is the major factor contributing to posture stabilization. Thus, the suitability of treadmill walking as a part of training programs for elderly adults with higher fall risk should be seriously considered.     

Measurement of the visual contribution to postural steadiness from the COP movement: methodology and reliability

We studied the reliability of different measures of the visual contribution to postural steadiness by recording the postural sway during standing with eyes open (EO) or eyes closed (EC). The COP trajectory was recorded in 21 subjects aged 42-61 standing on a firm or foam support. The improvement of postural steadiness due to vision was measured with a higher reliability (i.e. lower intra- and inter-subject variabilities) with the sway velocity V, than with the position RMS. Due to the increase of the variability of V and RMS with their own mean values, we quantified the visual contribution to posture by the stabilization ratio (SR), based on a logarithm transform of V or RMS. As compared to the Romberg quotient (EC/EO), SR improved the reliability of the measurement of the visual contribution to posture within individuals, across subjects, and even across different studies in the literature. Our method led to decrease the inter-subject coefficient of variation of this measurement to about 25%, using a foam support. It leads to a similar accuracy in binocular and monocular vision, and it also applies to the quantification of other non-visual sensory contributions to posture.     

The assessment of body sway and the choice of the stability parameter(s)

This methodological study aims at comparison of the practical usefulness of several parameters of body sway derived from recordings of the center of pressure (CoP) with the aid of a static force platform as proposed in the literature. These included: mean displacement velocity, maximal range of movement along x- and y-co-ordinates, movement area, planar deviation, phase plane parameter of Riley and the parameters of the diffusion stabilogram according to Collins. They were compared in over 850 experiments in a group of young healthy subjects (n = 10, age 21-45 years), a group of elderly healthy (n = 38, age 61-78 years) and two groups of elderly subjects (n = 10 and n = 21, age 65-89 years) with stability problems under different conditions known to interfere with stability as compared to standing with open eyes fixing a visual anchoring point: closing the eyes, standing on plastic foam in stead of a firm surface and performing a cognitive task: the modified stroop test. A force platform (Kistler) was used and co-ordinates of the body's center of pressure were recorded during 60 s of quiet barefoot standing with a sampling frequency of 10 Hz. In general, the results show important overlapping among groups and test conditions. Mean displacement velocity shows the most consistent differences between test situations, health conditions and age ranges, but is not affected by an extra cognitive task in healthy old people. Mean maximal sideways sway range is different among groups and test conditions except for the cognitive task in young and elderly subjects. Standardised displacement parameters such as standard deviations of displacements and planar deviation discriminate less well than the actual range of motion or the velocity. The critical time interval derived from the diffusion stabilogram according to Collins et al. seems to add a specific type of information since it shows significant influence from addition of a cognitive task in old subjects standing on a firm surface but not when standing on plastic foam. The critical time interval shows no consistent relation to any other parameter. The influence of cognitive activity on balance merits further exploration. A new parameter, sum of maximal deviation time (SDT) was proposed showing complete discrimination between frail elderly and other old subjects when obtained while visual input was suppressed. It was concluded that mean displacement velocity seems to be the most informative parameter in most situations.     

Physiological characteristics of posture maintenance by dogs during immediate reaction to irradiation

The ability of dogs to maintain a vertical posture during the first hours after irradiation at a dose of 10 or 80 Gy was investigated. Two experiments were performed. In the first experiment, during 5 hours after irradiation the vector stabilogram area, peak muscle efforts and sensorimotor coordination were measured 10 times every 30 min. In the second experiment, postural responses to perturbations were measured before and after irradiation at a dose of 10 Gy. During exposure to 10 Gy maximum values of the vector stabilogram area reflected initial excitation and motor unrest, that preceded vomiting, while minimum values of the parameter were related to hypodynamia during 3-5 hours after exposure. During exposure to 80 Gy minimum values of the vector stabilogram area reflected the hypodynamic phase while maximum values were related to the phase of distinct cerebral lesions. A reliable indicator of the quality of vertical posture maintenance, that correlates with the post-irradiation quality of purposeful actions, including intermittent physical acts, or actions, requiring highly precise coordination, is the rate of posture recovery after instantaneous platform displacement. This parameter declined after irradiation, the maximum decrease being during the 4-5th hour.     

Balance control in unstable sitting in individuals with an acute episode of low back pain

BackgroundLow back pain (LBP) is associated with altered postural control, mostly observed at later stages in the LBP trajectory. It is unclear whether postural control differs in the acute phase of LBP.Research questionIs postural control different in the acute phase of LBP (<2 weeks) and do differences depend on pain intensity, psychological features and/or availability of vision to control posture?MethodsCross-sectional study design. An unstable sitting paradigm (to reduce the contribution of the legs) assessed postural control of participants with acute LBP (n = 133) and pain-free controls (n = 74). Centre of pressure (CoP) reflected seat movements. Participants balanced with eyes closed, open, or with visual feedback of the anteroposterior CoP position. Balance performance was expressed by CoP displacement and velocity, and stabilogram diffusion analysis. Generalised estimating equations (GEEs) including body mass index, sex, and safety bar touch, tested differences between groups and between balance conditions. Separate GEEs were used to model performance measures and bar touch (yes/no) including pain intensity, disability and psychological features.ResultsCoP displacement and critical point coordinates (time and distance where CoP diffusion rate or spread slows) were larger in LBP than pain-free controls independent of balance condition. Long-term diffusion rate was greater in LBP than controls with eyes closed. CoP velocity measures (RMS, short term diffusion rate) were not different between groups. Pain intensity and psychological features were not linearly related to balance performance in participants with acute LBP. Higher pain catastrophizing was associated with touching the safety bar.SignificancePostural control differs in acute LBP than pain-free controls. Findings might be explained by altered sensory processing, lesser ability to reweight proprioceptive information and/or less accurate trunk muscle control. Although not linearly related to pain-intensity or psychological features in the acute stage, reduced balance performance could potentially have impact on LBP recovery.     

The Multiple Tasks Test: development and normal strategies

Simultaneous challenge of posture and cognition (‘dual tasks’) may predict falls better than tests of isolated components of postural control. We describe a new balance test (the Multiple Tasks Test, MTT) which (1) is based upon simultaneous assessment of multiple (>2) postural components; (2) represents everyday situations; and (3) can be applied by clinicians. Relevant risk factors for falls and actual fall circumstances (identified from a prospective survey in Parkinson's disease) were used to design functional tests (or postural ‘components’) that resembled everyday situations. We distinguished a ‘cognitive’ component (answering serial questions) from largely ‘motor’ components (standing up, sitting down, turning around, walking, avoiding obstacles, and touching the floor). Four additional components included carrying an empty or loaded tray, wearing shoes with slippery soles and reduced illumination. These components were combined to yield eight separate tasks of increasing complexity that were executed sequentially. The first and simplest task consisted of standing up, undisturbed walking, turning around and sitting down. For each of the next tasks, a new component was added to the earlier and otherwise identical task. All components within each task had to be performed simultaneously. Errors were defined as Hesitations (slowed performance) or Blocks (complete cessation), which were scored separately for execution of motor and cognitive components. Speed of performance was not stressed, but was measured for all tasks. The MTT was administered to 50 young healthy subjects (mean age 27.6 years) and 13 elderly subjects (mean age 62.0 years). To study learning effects, 20 different young subjects (mean age 21.0 years) received the MTT in order of gradually decreasing complexity. For subjects who received the MTT in order of increasing difficulty, 62% in both age groups performed all eight tasks without any Errors in the motor components. Among those making Errors, the proportion of subjects that made motor Errors increased significantly as the tasks became more complex (F(1,7)=2.66, P<0.05). This increase differed across the two groups (significant interaction of Group by Task; F(1,7)=3.07, P=0.01) because more elderly subjects produced motor Errors during the most complex tasks. Cognitive Errors increased even more than motor Errors with task complexity, and this increase was most pronounced in young subjects (significant interaction of Group by Error Type by Task; F(1,1,7)=3.85, P=0.001). Only eight young (16%) and four elderly subjects (30.8%) performed the MTT without any motor or cognitive Errors, again suggesting that more young subjects made cognitive Errors. Among subjects who received the MTT in reverse order, motor errors were more common than among subjects who received the MTT in order of increasing complexity (F(1,7)=5.90, P<0.05), particularly during the two most difficult tasks. The elderly performed all tasks slower than the young subjects. We conclude that the MTT is a new balance test based upon a multiple task design that resembles everyday situations. Performance by healthy subjects revealed interesting insights into normal postural strategies. For complex postural tasks, healthy subjects favour execution of motor components over execution of a cognitive component (‘posture first’ strategy). Young subjects were more inclined than elderly subjects to use this strategy. Motor learning influenced performance among subjects who received the MTT in order of increasing difficulty. Further studies must determine whether the MTT can be used to evaluate balance disorders.     

Does forward head posture affect postural control in human healthy volunteers?

Proprioceptive afferent input from neck muscles plays an important role in postural control. Forward head posture has the potential to impair proprioceptive information from neck muscles and contribute to postural control deficits in patients with neck pain. This study investigated whether induced forward head posture affects postural control in healthy participants when compared to natural head posture. Centre of pressure sway area, distance covered and mean velocity were measured during 30 s of static standing using a force platform with 25 healthy individuals (mean age ± SD = 20.76 ± 2.19 years) in 8 different conditions. Base of support, eyes open or closed and natural or forward head posture varied within these testing conditions. The majority of comparisons between natural and forward head posture were not statistically significant (p > 0.05). This suggests that induced forward head posture in young healthy adults does not challenge them enough to impair postural control. Future studies should evaluate whether forward head posture affects postural control of individuals with chronic neck pain.     

Postural balance in low back pain patients: Intra-session reliability of center of pressure on a portable force platform and of the one leg stand test

Low back pain (LBP) patients have poorer postural control compared to healthy controls, and the importance of assessing and addressing balance is a matter of debate. In the clinic, balance is often tested by means of the one leg stand test (OLST) while research often employs center of pressure (CoP) on a force platform. Portable force platforms might be of clinical relevance, but their reliability for LBP patients in a clinical setting has not been demonstrated. As LBP patients are more dependent on vision compared to healthy controls, the ratio of tests performed with eyes open and eyes closed (Romberg Ratio) might be of clinical interest. This study aimed to assess postural balance in LBP patients by analyzing intra-session reliability of CoP parameters on a portable force platform, the Romberg Ratio, and the OLST. Furthermore, we aimed to determine whether CoP parameters and OLST measure identical aspects of postural stability. We examined 49 LBP patients and found acceptable reliability of the CoP parameters’ trace length and velocity, whereas reliability regarding C90 area, the Romberg Ratio, and the OLST was poor. Correlations between the CoP parameters and OLST were insignificant. Reliability of trace length and velocity is acceptable and can be used as parameters when assessing CoP in LBP patients.     

Visual availability,balance performance and movement complexity in dancers

Research regarding the complex fluctuations of postural sway in an upright standing posture has yielded controversial results about the relationship between complexity and the capacity of the system to generate adaptive responses. The aim of this study is to compare the performance and complexity of two groups with different levels of expertise in postural control during a balance task. We examined the balance ability and time varying (dynamic) characteristics in a group of 18 contemporary dancers and 30 non-dancers in different visual conditions. The task involved maintaining balance for 30 s on a stability platform with opened or closed eyes. The results showed that dancers exhibited greater balance ability only in open eyes task than non-dancers. We also observed a lower performance in both groups during the test with closed eyes, but only dancers reduced their complexity in closed eyes task. The main conclusion is that the greater postural control exhibited by dancers depends on the availability of visual information.     

Visual availability,balance performance and movement complexity in dancers

Research regarding the complex fluctuations of postural sway in an upright standing posture has yielded controversial results about the relationship between complexity and the capacity of the system to generate adaptive responses. The aim of this study is to compare the performance and complexity of two groups with different levels of expertise in postural control during a balance task. We examined the balance ability and time varying (dynamic) characteristics in a group of 18 contemporary dancers and 30 non-dancers in different visual conditions. The task involved maintaining balance for 30 s on a stability platform with opened or closed eyes. The results showed that dancers exhibited greater balance ability only in open eyes task than non-dancers. We also observed a lower performance in both groups during the test with closed eyes, but only dancers reduced their complexity in closed eyes task. The main conclusion is that the greater postural control exhibited by dancers depends on the availability of visual information.     

The influence of the aquatic environment on the control of postural sway

Balance training in the aquatic environment is often used in rehabilitation practice to improve static and dynamic balance. Although aquatic therapy is widely used in clinical practice, we still lack evidence on how immersion in water actually impacts postural control. We examined how postural sway measured using centre of pressure and trunk acceleration parameters are influenced by the aquatic environment along with the effects of visual information. Our results suggest that the aquatic environment increases postural instability, measured by the centre of pressure parameters in the time-domain. The mean velocity and area were more significantly affected when individuals stood with eyes closed in the aquatic environment. In addition, a more forward posture was assumed in water with eyes closed in comparison to standing on land. In water, the low frequencies of sway were more dominant compared to standing on dry land. Trunk acceleration differed in water and dry land only for the larger upper trunk acceleration in mediolateral direction during standing in water. This finding shows that the study participants potentially resorted to using their upper trunk to compensate for postural instability in mediolateral direction. Only the lower trunk seemed to change acceleration pattern in anteroposterior and mediolateral directions when the eyes were closed, and it did so depending on the environment conditions. The increased postural instability and the change in postural control strategies that the aquatic environment offers may be a beneficial stimulus for improving balance control.     

Effects of the removal of vision on body sway during different postures in elite gymnasts

The aim of this study was to analyse the effects of the removal of vision on postural performance and postural control in function of the difficulty and specificity of the posture. Twelve elite gymnasts were instructed to be as stable as possible with eyes open and eyes closed in three postures: bipedal, unipedal, and handstand ranked from the less difficult and less specific to the more difficult and more specific. The ratios eyes closed on eyes open, computed on CP surface and CP mean velocity, which respectively represents postural performance and postural control, were similar in the bipedal and handstand postures. They were highly increased in the unipedal one. The effect of the removal of vision and so the role of vision on body sway was not directly linked to the difficulty or specificity of the posture; other tasks' characteristics like the segments configuration also played a role.     

Improvement of dynamic postural stability by an exercise program

BackgroundCentral processing of multi-sensory feedback and motor commands responsible for force production are critical for postural control. An exercise program was developed to realign spinal curvature, but its effect on postural control is unknown.Research QuestionTo what extent would the exercise program influence on center of pressure (CoP) sway on stable and unstable surfaces?MethodsSubjects (n = 30) were randomly assigned into one of three groups: exercise on a cylinder-shaped tube (98-cm length, 15-cm diameter, n = 10), exercise on a flat surface (n = 10), and a control group that laid supine on a flat surface (n = 10). Standing posture of each subject was quantified using anterior-, posterior-, and lateral-view photography. Each subject’s CoP sway was measured while standing on a static and dynamic platform with eyes open and eyes closed. Subjects were instructed to stand still when the platform was held stationary (e.g., no tilt) during the static condition. During the dynamic condition the platform was allowed to tilt in response to changes of CoP and subjects were instructed to maintain the platform in a horizontal position.ResultsOnly when subjects performed the exercise program on the tube, the angles of neck flexion and pelvis tilt decreased, and CoP sway in the sagittal, but not frontal plane, decreased during the dynamic platform conditions with both eyes open and eyes closed (p < 0.05).SignificanceIt is speculated that performing the exercise program on the tube might enhance a) central processing of somatosensory and vestibular inputs, b) motor commands responsible for force production in postural control, and c) biomechanical advantage by the realigned posture. The exercise program can be used by a variety of populations as home-exercise to realign the neck and pelvic posture and improve dynamic postural stability.     

Postural control in quiet standing in patients with psychotic disorders

There is evidence that patients with psychotic conditions display greater center of pressure (CoP) displacement during quite standing than healthy subjects, but the underlying impairments in the control mechanisms are uncertain. The aim of this study was to identify the nature of possible impairments in the control of posture by modulation of visual and kinesthetic information during quiet standing. Center of pressure (CoP) data and whole-body kinematics of the center of mass (CoM) were recorded during quite standing on a firm surface with eyes open and with eyes closed, and standing with eyes open on a yielding surface. During all three conditions, patients displayed greater migration of CoM and CoP–CoM, a measure related to ankle joint torque, whereas CoP-frequency (MPF) was similar in patients and healthy subjects. Our results suggested that greater postural sway in patients may depend on disproportionally large ankle joint torque without corresponding increase in frequency. Furthermore, interactions between groups and conditions suggested that the patients made less use of visual information for postural control than the healthy subjects.     

Are there specific conditions for which expertise in gymnastics could have an effect on postural control and performance?

The first aim of this study was to analyse the effect of elite training, linked to expertise, in gymnastics on postural performance and control. For this purpose, body sway of expert gymnasts was compared to other sportsmen, non-experts and non-gymnasts, in two different postures: bipedal (easy and unspecific to gymnasts) and unipedal (difficult and fairly specific). The second aim was to compare the groups in the same tasks but in a visual condition for which they were not trained, i.e. with eyes closed. Postural performance was assessed by centre of gravity motion, which was computed from centre of pressure motion, estimating postural control. A significant difference between the two groups was observed for postural performance in the unipedal posture and with eyes open only. Regardless of their posture, the groups were similarly affected by removal of vision. Expertise in gymnastics seemed to improve postural performances only in situations for which their practise is related to, i.e. unipedal with eyes open. These reveal the importance of choosing a relevant postural configuration and visual condition according to the people's training or by extension experience.     

Validation of a smartphone embedded inertial measurement unit for measuring postural stability in older adults

BackgroundIdentifying older adults with increased fall risk due to poor postural control on a large scale is only possible through omnipresent and low cost measuring devices such as the inertial measurement units (IMU) embedded in smartphones. However, the correlation between smartphone measures of postural stability and state-of-the-art force plate measures has never been assessed in a large sample allowing us to take into account age as a covariate.Research questionHow reliably can postural stability be measured with a smartphone embedded IMU in comparison to a force plate?MethodsWe assessed balance in 97 adults aged 50–90 years in four different conditions (eyes open, eyes closed, semi-tandem and dual-task) in the anterio-posterior and medio-lateral directions. We used six different parameters (root mean square and average absolute value of COP displacement, velocity and acceleration) for the force plate and two different parameters (root mean square and average absolute value of COM acceleration) for the smartphone.ResultsTest-retest reliability was smaller for the smartphone than for the force plate (intra class correlation) but both devices could equally well detect differences between conditions (similar Cohen’s d). Parameters from the smartphone and the force plate, with age regressed out, were moderately correlated (robust correlation coefficients of around 0.5).SignificanceThis study comprehensively documents test-retest reliability and effect sizes for stability measures obtained with a force plate and smartphone as well as correlations between force plate and smartphone measures based on a large sample of older adults. Our large sample size allowed us to reliably determine the strength of the correlations between force plate and smartphone measures. The most important practical implication of our results is that more repetitions or longer trials are required when using a smartphone instead of a force plate to assess balance.     

Preparatory adjustments during gait initiation in 4-6-year-old children

The preparatory adjustments related to gait initiation in a group of six children (4–6 years old) were studied in comparison to a group of six adults (50–61 years old). Muscle activity, ground reaction forces and body kinematics were recorded during the initiation of gait in subjects standing with one foot on each of two forceplates. Like adults, children had consistent anticipatory activations of the tibialis anterior (TA) muscle accompanied by center of pressure (COP) displacements, but the relative magnitude of both the backward COP displacement and concomitant initial TA activation burst were lower (P<0.03 and P<0.001 respectively) in children. The latter findings were associated with a lower rate of forward progression in children (P=0.01), and a lack of significant covariance between the backward shift and forthcoming velocity. In contrast, the preparatory adjustments in the medio-lateral (M/L) direction were prominent in children. Larger M/L peak force rates in children (P=0.01) were associated with an earlier (P=0.007) weight transfer to the stance limb; moreover, children initiated gait from a wider (P=0.04) base of support and had a trajectory of the total COP that was lateral rather than posterior like in adults. The consistent preparatory adjustment responses indicate that the motor program for initiating gait is functional at this age. The prominence of the preparatory adjustments in the M/L direction together with a reduced magnitude of the responses in the antero-posterior direction suggest that the anticipatory behavior for initiating gait develops first in the frontal plane and that more walking experience and better postural stability are required to fully achieve the adult-like control of the gait initiation process.     

The effect of age on the attentional demands of postural control

A dual-task paradigm was used to determine the attentional demands of several postural control tasks in 16 older women (age, 71.5+/-3.4 years) and 14 young women (age, 25.5+/-2.4 years). Older women had slower verbal reaction times (VRT) compared with the younger women and increased reaction time more from a sitting to standing posture. Compared with the younger women, older women required more cognitive resources to maintain a simple eyes open standing posture versus an eyes open seated posture. Further, older women had significantly greater VRT during the dual-task conditions compared with younger women. However, VRT did not significantly change as the difficulty of the primary task increased in either group. These data have implications for older adults who may be at risk for falls in situations where they may be engaged in concurrent tasks, even when those tasks are considered automated and/or lower order operations.