Effects of natural shoe wear on traction performance: a longitudinal study.

Footwear outsole design is an important factor for shoe-floor friction and for preventing slipping. Shoes with small, uniformly-separated tread blocks (often included on slip-resistant shoes) have decreased slip risk due to their increased friction and better under-shoe fluid drainage. However, these traction performance metrics (friction and fluid drainage) diminish with wear. This study quantifies shoe traction performance in response to natural wear and compares the relationship between common wear metrics: time, distance walked, and worn region size (WRS). Participants wore two pairs of shoes in the workplace for up to 11 months and the distance walked was tracked with a pedometer. After each month of wear, traction performance and WRS of each shoe were measured. Traction performance was quantified by the under-shoe available coefficient of friction and fluid force during a simulated slip condition. Increased wear (months worn, distance walked, and WRS) was associated with decreased traction performance. A WRS of 800 mm2 was associated with reductions in friction of 16-38% and increases in fluid force by 286-528%. Three and six months of wear were associated with WRS values of 251 mm2 and 462 mm2 and distances of 203 km and 519 km, respectively. A walking distance of 500 km was associated with a WRS of 406 mm2. This study showed that all these wear metrics are good indicators of shoe traction performance loss. Thus, the most practical metric in a particular application can be selected. We argue that WRS may be the best indicator due to variations in wear rate from the user and environment. Therefore, tracking footwear usage and monitoring outsole wear can aid in shoe replacement recommendations to reduce slips and falls.

Dynamic postural stability in pregnant fallers and non-fallers.

OBJECTIVE: To compare dynamic postural stability in pregnant women who have fallen during their pregnancies with those who have not, and with a group of non-pregnant women. DESIGN: The study was both longitudinal and cross-sectional. A cohort of pregnant women were followed through their second and third trimesters. A non-pregnant control group was used for comparison. SETTING: University-based laboratory. POPULATION: A total of 81 women (41 pregnant and 40 controls) participated. Twenty-nine pregnant women completed the protocol. METHODS: Data were collected on the pregnant women in the middle of their second and third trimesters. Pregnant women were surveyed about their daily activities, exercise participation, and fall history. Postural reaction time and centre of pressure (COP) movement data, in response to translational perturbations, were collected using a force plate. A mixed-model analysis of variants (ANOVA) was performed on each of the dependent variables (alpha = 0.05). Chi-square analysis was performed to determine if exercise participation altered the likelihood of a subject experiencing a fall (alpha = 0.05). MAIN OUTCOME MEASURES: Reaction time, initial sway, total sway, and sway velocity. RESULTS: Fifty-two percent of our pregnant subjects experienced a fall. Initial sway response, total sway, and sway velocity were smaller in the pregnant fallers than in the non-fallers and control participants (P < 0.05). Thirty-one of the pregnant subjects participated in regular exercise. Sedentary pregnant women were more likely to experience a fall than those who exercised. CONCLUSIONS: Dynamic balance is altered in pregnant women who have fallen compared with non-fallers and controls. Exercise may play a role in fall prevention in pregnant women.

Space and motion discomfort and abnormal balance control in patients with anxiety disorders.

OBJECTIVE: Previous research suggested that panic disorder with agoraphobia is associated with abnormalities on vestibular and balance function tests. The purpose of this study was to further examine psychiatric correlates of vestibular/balance dysfunction in patients with anxiety disorders and the specific nature of the correlated vestibular abnormalities. The psychiatric variables considered included anxiety disorder versus normal control status, panic disorder versus non-panic anxiety disorder diagnosis, presence or absence of comorbid fear of heights, and degree of space and motion discomfort (SMD). The role of anxiety responses to vestibular testing was also re-examined. METHODS: 104 subjects were recruited: 29 psychiatrically normal individuals and 75 psychiatric patients with anxiety disorders. Anxiety patients were assigned to four subgroups depending on whether or not they had panic disorder and comorbid fear of heights. SMD and anxiety responses were measured by questionnaires. Subjects were examined for abnormal unilateral vestibular hypofunction on caloric testing indicative of peripheral vestibular dysfunction, asymmetric responses on rotational testing as an indicator of an ongoing vestibular imbalance and balance function using Equitest dynamic posturography as an indicator of balance control. Logistic regression was used to establish the association between the psychiatric variables and vestibular or balance test abnormalities. RESULTS: Rotational test results were not significantly related to any of the psychiatric variables. The presence of either panic attacks or fear of heights increased the probability of having caloric hypofunction in a non-additive fashion. SMD and anxiety responses were independently associated with abnormal balance. Among specific posturography conditions, the association with SMD was significant for a condition that involved the balance platform tilting codirectionally with body sway, suggesting an abnormal dependence on somatosensory cues in the control of balance. CONCLUSION: In patients with anxiety disorders, higher SMD is indicative of somatosensory dependence in the control of balance. The absence of both panic and fear of heights reduces the probability of having peripheral vestibular dysfunction. Future research should examine if vestibular rehabilitation can be of value for patients with anxiety disorders complicated by SMD.

Interpersonal interactions for haptic guidance during balance exercises.

BACKGROUND: Caregiver-patient interaction relies on interpersonal coordination during support provided by a therapist to a patient with impaired control of body balance. RESEARCH QUESTION: The purpose of this study was to investigate in a therapeutic context active and passive participant involvement during interpersonal support in balancing tasks of increasing sensorimotor difficulty. METHODS: Ten older adults stood in semi-tandem stance and received support from a physical therapist (PT) in two support conditions: 1) physical support provided by the PT to the participant's back via an instrumented handle affixed to a harness worn by the participant ("passive" interpersonal touch; IPT) or 2) support by PT and participant jointly holding a handle instrumented with a force-torque transducer while facing each other ("active" IPT). The postural stability of both support conditions was measured using the root-mean-square (RMS) of the Centre-of-Pressure velocity (RMS dCOP) in the antero-posterior (AP) and medio-lateral (ML) directions. Interpersonal postural coordination (IPC) was characterized in terms of cross-correlations between both individuals' sway fluctuations as well as the measured interaction forces. RESULTS: Active involvement of the participant decreased the participant's postural variability to a greater extent, especially under challenging stance conditions, than receiving support passively. In the passive support condition, however, stronger in-phase IPC between both partners was observed in the antero-posterior direction, possibly caused by a more critical (visual or tactile) observation of participants' body sway dynamics by the therapist. In-phase cross-correlation time lags indicated that the therapist tended to respond to participants' body sway fluctuations in a reactive follower mode, which could indicate visual dominance affecting the therapist during the provision of haptic support. SIGNIFICANCE: Our paradigm implies that in balance rehabilitation more partnership-based methods promote greater postural steadiness. The implications of this finding with regard to motor learning and rehabilitation need to be investigated.

Postural control, attention and sleep deprivation.

We investigated the effect of sleep deprivation on postural control during a simple reaction time task (SRT), during a task requiring the intermittent inhibition of a reaction (IRT), and in the absence of a concurrent information processing task. Postural sway, i.e. changes in center of pressure on a force platform, was recorded in three increasingly difficult standing conditions (fixed platform, sway-referenced platform and sway-referenced platform with sway-referenced visual scene) during the three information-processing task conditions. Five healthy subjects performed the tasks either after normal sleep or following 24 h of sustained wakefulness. As hypothesized, sleep deprivation significantly increased postural sway only in the IRT condition. Within the IRT condition, sleep deprivation significantly increased sway across all postural conditions.

Spectral characteristics of visually induced postural sway in healthy elderly and healthy young subjects.

Spectral analysis and time-frequency analysis were applied to anterior-posterior (A-P) center-of-pressure (COP) data collected during quiet stance and sinusoidal (0.25 Hz) moving visual scene perturbations from sixteen healthy elderly subjects and thirteen healthy young subjects. While the total energy of COP was larger in the elderly subjects than in the young subjects, energy-normalized spectra of the entire COP time series were similar between the two populations. Further time-dependent analysis of the spectral characteristics showed that the time-varying spectra were also similar, as revealed by time-frequency analysis, although some differences were observed. Specifically, time-dependent mean frequency and bandwidth responses to the stimulus showed similar increases in both young and elderly subjects with the introduction of the stimulus. Furthermore, time-frequency analysis showed that both groups exhibited an initial increase in sway at the stimulus frequency that then declined as the sinusoidal scene movement progressed. This reduction in sway during visual stimulation may indicate an "adaptation" to constant frequency visual perturbation that is invariant with age. This finding is in contrast to results reported recently for patients with vestibular impairments, who appear to lack such adaptation.

Lower extremity corrective reactions to slip events.

A significant number of injuries in the workplace is attributed to slips and falls. Biomechanical responses to actual slip events determine whether the outcome of a slip will be recovery or a fall. The goal of this study was to examine lower extremity joint moments and postural adjustments for experimental evidence of corrective strategies evoked during slipping in an attempt to prevent falling. Sixteen subjects walked onto a possibly oily vinyl tile floor, while ground reaction forces and body motion were recorded at 350 Hz. The onset of corrective reactions by the body in an attempt to recover from slips became evident at about 25% of stance and continued until about 45% into stance, i.e. on average between 190 and 350 ms after heel contact. These reactions included increased flexion moment at the knee and extensor activity at the hip. The ankle, on the other hand, acted as a passive joint (no net moment) during fall trials. Joint kinematics showed increased knee flexion and forward rotation of the shank in an attempt to bring the foot back towards the body. Once again, the ankle kinematics appeared to play a less dominant role (compared to the knee) in recovery attempts. This study indicates that humans generate corrective reactions to slips that are different than previously reported responses to standing perturbations translating the supporting surface.

A model of foot placement during gait.

Foot placement is known to affect balance during gait; however, how foot placements are chosen is unknown. Objectives of this research were to analyze swing trajectories of the foot during gait with respect to the pelvis, and to propose a model of foot placement control which provides a stable base of support. Effects of gait speed and vision on this model of foot placement were then examined. Foot trajectories were analyzed using spherical coordinates referenced to the pelvis, termed the pelvic spherical coordinates (PSCs). A model was developed based on this coordinate system which predicts foot placement in terms of position and velocity with respect to the pelvis. It is proposed that foot placements are chosen to minimize the sum of PSC coordinates of the stance and swing feet. Foot velocity at heel contact is proposed to minimize the sum of the PSC stance and swing angular velocities. Experimental data were collected to test this model during walking trials of different speeds, both with and without vision. Results showed that sums of stance and swing feet PSCs were very close to zero at heel contact, supporting the positional control hypothesis. Sums of PSC velocities, however, were not zero at heel contact. Rather, individual swing leg PSC velocities were zero at heel contact, suggesting independent swing leg velocity control. Vision did not have an effect on position or velocity variables at heel contact. Gait speed did have an effect, particularly on PSC velocities at heel contact.(ABSTRACT TRUNCATED AT 250 WORDS)

Time-frequency analysis of postural sway.

Postural sway during quiet stance has been used to characterize the postural control system. Most studies have used center of pressure (COP) measurements and have assumed stationarity, however, recent research has indicated that COP is not stationary. The purpose of this study is to introduce and demonstrate a nonstationary spectral estimation technique to examine the time-varying nature of postural sway. Data from two experiments were used to verify the usefulness of the spectral estimator for the analysis of COP. The first data set contains COP recorded from normal subjects swaying about their ankles in response to a metronome as it was gradually changed from 2 to 1 Hz. The time-frequency distribution reveals time-varying spectral changes corresponding to frequency changes made by the subjects. The second set consists of COP from normal subjects and vestibularly impaired patients standing quietly on a force plate with eyes closed for 100 s. The time-frequency distributions for the COP were estimated for both sets of data. The COP's appear to be nonstationary with the energies at a given frequency modulating through time.

Galvanic-induced postural movements as a test of vestibular function in humans.

Galvanic stimulation produces a postural sway and eye movements in humans. Since galvanic currents are thought to exert their effect at the trigger zone of the vestibular nerve, an intact vestibular nerve should be necessary to produce a response. We have used galvanic stimulation in humans to test the hypothesis that intact vestibular nerve fibers are required to obtain a postural away response. Experimental subjects included normal subjects, patients who had undergone resection of an acoustic neuroma, and patients who had undergone vestibular neurectomy and surgical labyrinthectomy. Our results support the hypothesis that an intact vestibular nerve is necessary to produce a response. Moreover, two patients with recurrent vertigo following vestibular neurectomy and labyrinthectomy, who had absent ice-water caloric test responses in the operated ears, were found to have a positive galvanic response. This result suggested that their recurrent vertigo was based on intact residual vestibular nerve fibers. Although previous research has not yielded a routine clinical use for galvanic stimulation, our results suggest that galvanic stimulation of the vestibular system can provide unique and valuable diagnostic information.

Attention influences sensory integration for postural control in older adults.

This study investigated the influence of attention on the sensory integration component of postural control in young and older adults. Eighteen young and 18 older healthy subjects performed information-processing tasks during different postural challenge conditions. Postural conditions included seated, standing on a firm surface, standing on a sway-referenced floor, and standing on a sway-referenced floor while viewing a sway-referenced scene. During each condition, reaction time (RT) was measured during two simple and one inhibitory RT tasks. For the inhibitory task, the time required to inhibit an action was derived, termed the inhibitory time (IT). Performing a RT task was associated with increased postural sway in older subjects, but not in young subjects. The greatest influence of RT task on sway of older subjects was found during the sway-referenced floor/sway-referenced scene condition. Conversely, postural condition had an influence on RT task performance in both young and older subjects. The IT was increased in both young and older subjects only during the sway-referenced floor/scene condition. These results suggest that the sensory integration component of postural control in particular seems to require attention. Further, our data suggest that attentional processes related to inhibitory control are engaged when sensory integration requirements are high.

Acceleration threshold detection during short anterior and posterior perturbations on a translating platform.

Balance control systems have usually been studied under two conditions, during quiet standing or under large postural perturbations of a magnitude that requires a postural adjustment to prevent falling. Between these two extremes lie perturbations that can be repeated and measured while not forcing adaptive strategies from the postural control system. Unlike other studies of postural control, we employed very short translations with varying accelerations at the edge of psychophysical detectability. These perturbations were vibration-free anterior or posterior translations of the platform on which a subject stood. Using a full Latin-square design set of perturbations in the forward or backward direction, with a smooth or jerk acceleration profile, and of length 4 or 20 mm, were presented to five subjects. Perceptual peak acceleration thresholds were determined by an iterative psychophysical method that forced the subjects to choose in which of two sequential intervals that they perceived a stimulus to have been presented. The only factor found that significantly correlated with detection was perturbation length. The 4 mm peak thresholds averaged 14.51 mm/s2 while 20 mm thresholds averaged 8.55 mm/s2. For the short perturbations employed in this study, detection of motion thus was dependent upon the magnitude of the acceleration, but it was independent of the acceleration profile (jerk versus smooth) or movement direction. By understanding the influences on the ability to perceptually detect motion underfoot, we can begin to understand what elements of the postural control system might be involved in the second-to-second control of balance.

The steady-state postural response to continuous sinusoidal galvanic vestibular stimulation.

Galvanic vestibular stimulation (GVS) applied between the mastoids during quiet standing elicits postural sway. The aim of this study was to characterize the postural sway response to continuous sinusoidal GVS across various stimulus frequencies and amplitudes. Binaural bipolar sinusoidal GVS was applied to the skin overlying the mastoid processes of 10 subjects while they stood on a force plate with eyes closed. The position of the center of pressure (COP) at the feet was recorded from a forceplate, while the head displacement was measured with a magnetic position tracking system. The stimulus conditions included four frequencies (0.1, 0.25, 0.45, and 1.1 Hz) and five peak amplitudes (0.05, 0.1, 0.25, 0.5, and 1.0 mA). Each subject experienced one trial at each amplitude-frequency pair. Additionally, each subject underwent three trials in which a dual-frequency stimulus (0.1 plus 0.45 Hz at a peak of 0.5 mA each) was presented. The stimuli elicited sway in the frontal plane in all subjects, as evidenced by changes in the displacement of the COP and head. Sway magnitude decreased with increasing stimulus frequency and increased with increasing stimulus amplitude. However, the response magnitude saturated at higher stimulus amplitudes. Phase lag increased with increasing stimulus frequency. The response to the dual-frequency stimulus was reduced at 0.1 Hz and nearly equal at 0.45 Hz in comparison with the single-frequency responses. This study suggests that the postural sway response is nonlinear due to saturation and violation of the principle of superposition.

Psychological factors influencing recovery from balance disorders.

This article reviews evidence for three mechanisms whereby psychological factors may aggravate dizziness and retard recovery from balance disorders. Firstly, a common behavioral response to dizziness is to avoid activities and environments that provoke symptoms, yet such avoidance deprives the individual of the exposure necessary to promote psychological and neurophysiological adaptation. Secondly, anxiety arousal and hyperventilation may add to, amplify, and disinhibit the somatic symptoms induced by balance disorder. Thirdly, attention and cognitive load may influence the central processing of information required for the perception and control of orientation. The need to combine physiotherapy for dizziness with psychotherapy is discussed.

Visual influences on balance.

This paper discusses the impact of vision on balance and orientation in patients with vestibular disorders and in anxiety patients with space and motion discomfort (SMD). When the vestibular system is impaired, vision has a greater influence on standing postural control, resulting in greater sway when individuals are presented with erroneous or conflicting visual cues. Studies have shown that individuals with other motion sensitivities, such as motion sickness, also tend to rely on vision for balance and do not disregard erroneous visual cues. Recently, patients with anxiety disorders that include SMD also have been shown to have increased postural sway in conflicting visual environments, similar to patients with vestibular disorders. Thus, while specific vestibular deficits are not always directly associated with SMD, data regarding the impact of vision on balance suggest that some patients with SMD may have an underlying balance disorder.

Visual-induced postural sway in children with and without otitis media.

Children with otitis media with effusion (OME) have been shown to have a significantly higher velocity of sway than normal children. To further evaluate the effect of OME on balance, we studied visual dependency for balance by investigating the influence of optic flow on postural sway. The results of this preliminary study suggest that children with OME may be more visually dependent for balance than healthy age-matched controls. This was particularly evident for higher-frequency stimulus conditions (0.25 Hz) as opposed to lower-frequency stimulus conditions (0.10 Hz). These findings indicate that OME may affect vestibular function in children, thereby causing excessive reliance on other, nonvestibular sensory cues to maintain balance. Further studies are needed to define the role of vestibular function in the management of children with OME.

Lumbar muscle size and locations from CT scans of 96 women of age 40 to 63 years.

Computed tomography scans of 96 women aged between 40 and 63 years were systematically measured to determine torso muscle moment arms and cross-sectional areas at L 2L 3, L 3L 4 and L 4L 5 disc levels. The major findings were as follows: (1) the mean muscle moment arm and area data were not different bilaterally; (2) psoas, quadratus lumborum, and latissimus dorsi muscle moment arms consistently changed at the three disc levels, while erector spinae, rectus abdominis, transverse abdominis and the oblique muscles remained about the same distance from the three disc centroids; (3) psoas and quadratus lumborum muscles increased in mean size at the lower levels and (4 gross torso anthropometry and body weight had a significant (P < 0·01) but varied correlation (r(2) from 0·12 to 0·65) with the size of the erector spinae and psoas muscles, and with the moment arms of the rectus abdominis, transverse abdominis, latissimus dorsi, and oblique muscles.

Paraspinal muscle response to electrical vestibular stimulation.

Galvanic (electrical) vestibular stimulation (GVS) has been used to study the role of the vestibular system in postural control by inducing postural sway in standing subjects. The purpose of this study was to determine the timing and pattern of activation in the paraspinal muscles in response to GVS and to compare these responses with those in the muscles of the lower leg. Binaural-bipolar GVS was applied to the skin overlying the mastoid processes of 10 subjects while they stood on a force plate with their eyes closed. The stimulus consisted of a 0.6 mA 5-pulse sequence. Each pulse lasted for 2 s, followed by 4 s of rest. The centre of pressure (COP) vs. time for each trial was calculated from the reaction forces and moments. Surface electromyographic (EMG) signals from the paraspinal and gastrocnemius muscles were recorded bilaterally. The EMG signals were rectified and integrated (iEMG). The iEMG from the muscles on the cathodal side of the body were then subtracted from the iEMG of the anodal side muscles, to yield a differential EMG (dEMG). Both the paraspinal and gastrocnemius muscles became activated in response to the stimulus. The pattern of activation was consistent with the changes observed in the centre of pressure. The primary response in both muscles acted to move the body toward the anode. This primary response began at 74 +/- 20 ms in the paraspinal muscles and at 118 +/- 18 ms in the gastrocnemius. A second component of the response began at 232 +/- 27 ms in the paraspinal muscles and 262 +/- 54 ms in the gastrocnemius muscles. This second phase of the response was opposite in direction to the primary response and was responsible for decelerating the body and maintaining the deviated position of the centre of mass over the base of support. Following the termination of the stimulus, the opposite pattern of muscle activation in both the paraspinal and the gastrocnemius muscles was observed. The results of this study suggest that the paraspinal muscles may play a significant role in the frontal plane response to vestibular stimulation during stance in humans.

Visual-vestibular interaction during OVAR in the elderly.

This study assessed visual-otolith interaction in healthy older humans and compared responses from older subjects to those of younger subjects. Using off-vertical axis rotation (OVAR) to stimulate the otolith organs, eye movement responses, measured using electro-oculography, were recorded during rotation in the dark, rotation with an earth-fixed lighted visual surround, and rotation with a subject-fixed fixation target. Results indicated that older subjects, like young subjects, exhibit a modulation component that was as large during rotation with a lighted earth-fixed visual surround as that seen in the dark and a modulation component during rotation with a subject-fixed visual target that was incompletely suppressed. The modulation component was, in general, larger in the older subjects. This study confirms findings from a previous study of visual-otolith interaction in young subjects and suggests that older subjects, like young subjects, have difficulty visually suppressing the modulation component induced by off-vertical axis rotation.

Effect of aging on the otolith-ocular reflex.

We assessed the influence of age on the otolith-ocular reflex and semicircular canal-otolith interaction. Healthy young (n=30) and healthy older (n=60) subjects were rotated about an earth vertical axis, and about a 30 degree off-vertical axis. Eye movements during and following rotation were recorded using electro-oculography. Results indicated that there were statistically significant changes in the otolith-ocular reflex and semicircular canal-otolith interaction as a function of age. The modulation component during off-vertical axis rotation (OVAR) was greater in the older group compard to the young adults, whereas the bias component was smaller with advanced age. The foreshortening of the vestibulo-ocular reflex time constant induced by post-rotatory head tilt following cessation of rotation was less prominent in the older group. There were no consistent changes in the semicircular canal-ocular reflex. Overall, response parameters showed more variability in the older subjects. We conclude that age related changes in the otolith-ocular reflex and semicircular canal-otolith interaction are a result primarily of a degradation of central vestibular processing of otolith signals rather than a decline of peripheral vestibular function.