ECG voltage modifications as response to gravity changes

The aim of the study was to analyze ECG (QRS) voltage responses to body fluid shift due to gravity chances. Acute changes in gravity were created by two ways: 1) changes in gravity value during parabolic flights (within 27 subjects 45 ECG have been analyzed); 2) changes in gravity direction due to rotation of the body during postural tests (within 11 subjects 14 ECG have been analyzed). Results and conclusions. Gravity change leads to body fluid shift and changes of intrathoracic organs and tissues electroconduction. It influences on ECG voltage. During parabolic flights in up-right position: R amplitude in Z axis increases in hypergravity (+0.19 mV) and decreases in microgravity (-0.24 mV). During postural tests, R amplitude in Z axis increases in orthostatic position (+0.09 mV) and decreases in antiorthostatic position (-0.025 mV). Changes in QRS voltage during parabolic flights are more important than during postural tests. This could be due to more effective blood redistribution during parabolic flights.     

Does whole-body vibration training have acute residual effects on postural control ability of elderly women?

The purpose of this study was to investigate acute residual effects of a single vibration session on balance control in a group of elderly women. Several studies, in fact, have shown that whole-body vibration (WBV) training may improve balance in the elderly, but possible side effects of acute exposure to WBV, such as temporary reduction of balance control ability because of perturbations of the vestibular system, have not been investigated. Twenty-two healthy elderly women (71.8 ± 4.7 years of age) were trained with a 9.5-minute bout of static and dynamic knee-extensor exercises executed on a vibrating platform (Well-net Vibe Revolution). The vibration frequency was set at 35 Hz. A subgroup of 14 subjects performed the same exercise protocol also without the vibrations to discriminate between vibration and exercise effects. Balance control ability was assessed through computerized posturography: a force plate (Bertec Co, Columbus, OH, USA) was used to measure the center of pressure trajectories during 4 different experimental trials: before, immediately after, 15 minutes after, and 60 minutes after the training. A set of postural parameters, typically adopted to assess elderly subjects, was then computed and 2-way analysis of variance was used to determine differences between values found in the 4 postural tests (level of significance p = 0.05) in the 2 groups. The results showed no significant variations in the postural parameters recorded during the 4 sessions. A significant group effect was found for 2 postural parameters, with no interaction between the 2 factors. In conclusion, the proposed single bout of WBV does not induce dangerous acute effects on elderly women balance control ability and could be safely administered as part of a long-term intervention program.     

Changes in nasal resistance as a result of posture in normal subjects

The variations of nasal resistance, after lateral right, lateral left and supine recumbency have been studied in 20 normal subjects. These variations were in order of constant succession by rhinomanometry. In lateral right recumbency (first recumbency), the mean increase of nasal resistance in lower nasal cavity was 35%; on the contrary in the following recumbency the increase was twice as large as the first one. In supine recumbency the results were abnormal. In fact, with respect to the same nasal cavity, some of our subjects showed a congestion, while some showed a decongestion. It may be concluded that utilization of postural tests, like index vasomotor reactivity, must keep the increase of nasal resistance in normal subject, following postural variations in mind, in order to express a judgement of significative pathological difference.     

Stance stability with unilateral and bilateral light touch of an external stationary object

Unilateral light fingertip touch of a stationary object has a significant stabilizing effect on postural sway during stance. The purpose of this study was to find out if this effect is enhanced by bilateral light touch of parallel stationary objects. The postural sway of 54 healthy subjects was tested in four stance conditions: no touch; unilateral left light touch of the left handle of a walker; unilateral right light touch of the right handle of the same walker; and bilateral light touch of the two handles. During testing, subjects stood blindfolded on two foam pads placed on the left and right force plates of the Tetrax balance system. Testing in each condition lasted 45 s and was executed twice in a random order. As expected, postural sway was significantly reduced by unilateral left or right light fingertip touch. It was significantly further decreased by bilateral light touch. In addition, light touch conditions were associated with a reduction in pressure fluctuations between the heel and forefoot of the same foot as well as those of the contralateral foot, with a concomitant increase in weight shift fluctuations between the two feet. The decrease in postural sway with bilateral light touch suggests cortical modulation of the bilateral touch inputs, with enhancement of the stabilizing response.     

Modeling balance control during sustained waking allows posturographic sleepiness testing

We develop a method to quantify sleepiness. Sleepiness is a major risk factor in traffic and occupational accidents, but lack of convenient tests precludes monitoring impending sleepiness. Posturographic balance testing could address this need because sleepiness increases postural sway. It is, however, unclear how sleepiness influences balance control. Our results, for 12 subjects, show that balance control is more susceptible to increasing time awake (TA) compared to neuromuscular processes. This conclusion is reached since during sustained waking the control process slows down by 3.4% per hour of increased TA. This slowdown accounts for 65% of the variance in diurnal balance. We quantified balance control by modeling the body as an inverted pendulum and by expressing the control as the critical time interval for open-loop control (Deltat(c)) of the center-of-mass movements of this pendulum. To estimate the subjects' TA, we regressed the Deltat(c) scores recorded during sustained waking against increasing TA, and equated separate Deltat(c) test scores with the diurnal Deltat(c) scores. We estimated TA with 68% positive predictive value. The results encourage implementing balance modeling into a device that performs clinical or industrial balance testing because the model-based Deltat(c) score responded to increasing TA.     

Effect of local and general fatiguing exercises on disturbed and static postural control

This study compared the effect of local and general fatiguing exercise on disturbed and static postural control performances. Surface electromyography and center of pressure signals were respectively recorded during self-initiated perturbation test and static postural stability test from 7 young male subjects. Local fatiguing exercise was performed using intermittent isometric knee extensions at the level of 40% of maximal voluntary torques. General fatiguing exercise was implemented with rowing ergometer at a speed of 200 ± 5 m/min. Results of disturbed postural tests showed no significant change of anticipatory postural adjustment (APAs) organizations in individual muscles following both fatiguing exercises, but observed larger APAs coactivations in trunk and dorsal muscle pairs following local than general fatiguing exercise, and larger compensatory postural adjustments (CPAs) coactivation in dorsal muscle pair after both fatiguing exercises. In addition, the results of static postural tests indicated efficient static postural stability accompanying the down-weighting of visual input and the up-weighting of vestibular/somatosensory component following both fatiguing exercises. These findings evidenced a general compensation in the central nervous system in response to the neuromuscular deficiencies induced by local fatiguing exercise and put forward the function of sensory recalibration in maintaining postural stability under fatigue conditions.     

Postural changes after sustained neck muscle contraction in persons with a lower leg amputation

Lower leg amputation generally induces asymmetrical weight-bearing, even after rehabilitation treatment is completed. This is detrimental to the amputees’ long term quality of life. In particular, increasing strains on joint surfaces that receive additional weight load causes back and leg pain, premature wear and tear and arthritis. This pilot study was designed to determine whether subjects with lower leg amputation experience postural post-effects after muscle contraction, a phenomenon already observed in healthy subjects, and whether this could improve the weight-bearing on their prosthesis.Fifteen subjects with a unilateral lower leg amputation and 17 control subjects volunteered to participate in this study. Centre of pressure (CP) position was recorded during standing posture, under eyes closed and open conditions. Recordings were carried out before the subjects performed a 30-s voluntary isometric lateral neck muscle contraction, and again 1 and 4 min after the contraction.Postural post-effects characterized by CP shift, occurred in the medio-lateral plane in the majority of the amputated (7/15 eyes closed, 9/15 eyes open) and control (9/17 eyes closed, 11/17 eyes open) subjects after the contraction. Half of these subjects had a CP shift towards the side of the contraction and the other half towards the opposite side. In four amputated subjects tested 3 months apart, shift direction remained constant. These postural changes occurred without increase in CP velocity.Thus, a 30-s voluntary isometric contraction can change the standing posture of persons with lower leg amputation. The post-effects might result from the adaptation of the postural frame of reference to the proprioceptive messages associated with the isometric contraction.     

Biomechanical evaluation of the relationship between postural control and body mass index

Postural stability is crucial in maintaining body balance during quiet standing, locomotion, and any activities that require a high degree of balance performance, such as participating in sports and dancing. Research has shown that there is a relationship between stability and body mass. The aims of this study were to examine the impact that two variables had on static postural control: body mass index (BMI) and gender. Eighty healthy young adults (age=21.7±1.8 yr; height=1.65±0.09 m; mass=67.5±19.0 kg) participated in the study and the static postural control was assessed using the Biodex Balance System, with a 20 Hz sampling rate in the bipedic stance (BLS) and unipedic stance (ULS) for 30s. Five test evaluations were performed for each balance test. Postural control was found to be negatively correlated with increased adiposity, as the obese BMI group performed significantly poorer than the underweight, normal weight and overweight groups during BLS and ULS tests. The underweight, normal weight and overweight groups exhibited greater anterior-posterior stability in postural control during quiet stance. In addition, female displayed a trend of having a greater postural sway than male young adults, although it was evidenced in only some BMI groups. This study revealed that BMI do have an impact on postural control during both BLS and ULS. As such, BMI and gender-specific effects should be taken into consideration when selecting individuals for different types of sporting activities, especially those that require quiet standing.     

Postural Stability in Young Adults with Down Syndrome in Challenging Conditions

To evaluate postural control and performance in subjects with Down syndrome (SwDS), we measured postural sway (COP) in quiet stance in four 20-second tests: with eyes open or closed and on hard or foam surface. Ten SwDS and eleven healthy subjects participated, aged 29.8 (4.8) and 28.4 (3.9), respectively. The time-series recorded with the sampling rate of 100 Hz were used to evaluate postural performance (COP amplitude and mean velocity) and strategies (COP frequency, fractal dimension and entropy). There were no intergroup differences in the amplitude except the stance on foam pad with eyes open when SwDS had larger sway. The COP velocity and frequency were larger in SwDS than controls in all trials on foam pad. During stances on the foam pad SwDS increased fractal dimension showing higher complexity of their equilibrium system, while controls decreased sample entropy exhibiting more conscious control of posture in comparison to the stances on hard support surface. This indicated that each group used entirely different adjustments of postural strategies to the somatosensory challenge. It is proposed that the inferior postural control of SwDS results mainly from insufficient experience in dealing with unpredictable postural stimuli and deficit in motor learning.     

Postural control system

The postural control system has two main functions: first, to build up posture against gravity and ensure that balance is maintained; and second, to fix the orientation and position of the segments that serve as a reference frame for perception and action with respect to the external world. This dual function of postural control is based on four components: reference values, such as orientation of body segments and position of the center of gravity (an internal representation of the body or postural body scheme); multisensory inputs regulating orientation and stabilization of body segments; and flexible postural reactions or anticipations for balance recovery after disturbance, or postural stabilization during voluntary movement. The recent data related to the organization of this system will be discussed in normal subjects (during ontogenesis), the elderly and in patients with relevant deficits.     

Stance stability with unilateral and bilateral light touch of an external stationary object

Unilateral light fingertip touch of a stationary object has a significant stabilizing effect on postural sway during stance. The purpose of this study was to find out if this effect is enhanced by bilateral light touch of parallel stationary objects. The postural sway of 54 healthy subjects was tested in four stance conditions: no touch; unilateral left light touch of the left handle of a walker; unilateral right light touch of the right handle of the same walker; and bilateral light touch of the two handles. During testing, subjects stood blindfolded on two foam pads placed on the left and right force plates of the Tetrax balance system. Testing in each condition lasted 45?s and was executed twice in a random order. As expected, postural sway was significantly reduced by unilateral left or right light fingertip touch. It was significantly further decreased by bilateral light touch. In addition, light touch conditions were associated with a reduction in pressure fluctuations between the heel and forefoot of the same foot as well as those of the contralateral foot, with a concomitant increase in weight shift fluctuations between the two feet. The decrease in postural sway with bilateral light touch suggests cortical modulation of the bilateral touch inputs, with enhancement of the stabilizing response.     

Effect of ageing and vision on limb load asymmetry during quiet stance

Although the identification and characterization of limb load asymmetries during quiet standing has not received much research attention, they may greatly extend our understanding of the upright stance stability control. It seems that the limb load asymmetry factor may serve as a veridical measure of postural stability and thus it can be used for early diagnostic of the age-related decline in balance control. The effects of ageing and of vision on limb load asymmetry (LLA) during quiet stance were studied in 43 healthy subjects (22 elderly, mean age 72.3+/-4.0 yr, and 21 young, mean age 23.9+/-4.8 yr). Postural sway and body weight distribution were recorded while the subject was standing on two adjacent force platforms during two 120 s trials: one trial was performed with the eyes open (EO), while the other trial was with the eyes closed (EC). The results indicate that LLA was greater in the old adults when compared with the young control subjects. The LLA values were correlated with the postural sway magnitudes especially in the anteroposterior direction. Eyes closure which destabilized posture resulted in a significant increase of body weight distribution asymmetry in the elderly but not in the young persons. The limb load difference between EO and EC conditions showed a significantly greater effect of vision on LLA in the elderly compared to the young subjects. The observed differences in the LLA may be attributed to the decline of postural stability control in the elderly. Ageing results in the progressive decline of postural control and usually the nervous system requires more time to complete a balance recovery action. To compensate for such a deficiency, different compensatory strategies are developed. One of them, as evidenced in our study, is preparatory limb unload strategy (a stance asymmetry strategy) which could significantly shorten reaction time in balance recovery.     

A new method to analyze postural stability during a transition task from double-leg stance to single-leg stance

Time to stabilization (TTS) has been introduced as a method to analyze dynamic postural stability during jump and landing tasks, but has also been applied during the transition task from double-leg stance (DLS) to single-leg stance (SLS). However, the application of the originally described TTS technique during the latter task has some important limitations. The first goal of this study was to present an adapted version of the TTS technique to provide an effective alternative method to better analyze postural stability during the transition from DLS to SLS. The second goal was to study the influence of pathology and different speeds on postural stability outcomes. Fifteen healthy control subjects and 15 subjects with chronic ankle instability (CAI) performed the transition task on their preferred speed and as fast as possible, with eyes open and with eyes closed. Subjects with CAI performed the transition significantly slower when moving at their preferred speed with eyes closed. The time subjects needed to reach a new stability point was not discriminative between groups and largely dependent on movement speed. However, the amount of sway after this new stability point was significantly increased in the CAI group and when eyes were closed. The results of this study suggest that subjects with CAI have a decreased ability to overcome the postural perturbation created by the voluntary movement from DLS to SLS. Focusing only on TTS during the transition from DLS to SLS may lead at least in some cases to misinterpretations when assessing postural stability.     

Enhancing human balance control with galvanic vestibular stimulation

 With galvanic vestibular stimulation (GVS), electrical current is delivered transcutaneously to the vestibular afferents through electrodes placed over the mastoid bones. This serves to modulate the continuous firing levels of the vestibular afferents, and causes a standing subject to lean in different directions depending on the polarity of the current. Our objective in this study was to test the hypothesis that the sway response elicited by GVS can be used to reduce the postural sway resulting from a mechanical perturbation. Nine subjects were tested for their postural responses to both galvanic stimuli and support-surface translations. Transfer-function models were fit to these responses and used to calculate a galvanic stimulus that would act to counteract sway induced by a support-surface translation. The subjects' responses to support-surface translations, without and with the stabilizing galvanic stimulus, were then measured. With the stabilizing galvanic stimulus, all subjects showed significant reductions in both sway amplitude and sway latency. Thus, with GVS, subjects maintained a more erect stance and followed the support-surface displacement more closely. These findings suggest that GVS could possibly form the basis for a vestibular prosthesis by providing a means through which an individual's posture can be systematically controlled. Received: 11 May 2000 / Accepted in revised form: 20 November 2000     

Postural Responses Evoked by Vibrational Stimulation of the Shin Muscles under Conditions of Virtual Visual Environment

We studied the effects of unexpected shifts of the visually perceived artificial surroundings (virtual visual environment, VVE) on postural reactions evoked by vibrational stimulation of proprioceptors of the shin muscles; tests were performed in a standing position of the subject. The VVE possessed two planes, a mobile foreground, whose displacements correlated with oscillations of the body, and a stationary background. The subjects were asked to use the latter as a reference system in corrections of the posture. The VVE parameters were controlled by a computer; shifts of the VVE foreground were combined with similar, in their duration and profile, stimulation-evoked displacements of the body. Despite the fact that the subjects had a possibility to use the stationary background as the reference system, the magnitudes of the evoked postural responses under conditions of perception of the VVE significantly exceeded the respective magnitudes upon standing with the eyes open in front of a completely stationary visual image. Postural responses progressively increased with increases in relative values of the shifts of the VVE foreground but always remained smaller than the responses under conditions of testing with the eyes closed. Augmentation of the postural responses at a synphase pattern of interrelations between the body movements and VVE shifts was more significant than at antiphase relations. Thus, shifts of the VVE foreground, on the one hand, destabilized the maintenance of the vertical posture, which resulted in intensification of the postural responses. On the other hand, such shifts allowed the subject to use them as feedback signals and to modulate the magnitude of postural responses when there was a change in the direction of interrelations between the body movements and the perceived visual image.     

Balance study in asymptomatic subjects: Determination of significant variables and reference patterns to improve clinical application

Postural control is essential when carrying out everyday activities and its possible disorders have a very significant impact on personal autonomy. To provide the means to accurately measure postural control in the clinical environment, this study checks and discusses the suitability of procedures for a new balance assessment system with a stabilometric platform (MoveHuman-Dyna © UZ-IDERGO), which meets the criteria of clinical stabilometric standardisation established by the International Society for Posture and Gait Research (ISPGR) at the Bologna meeting (2009). The study was applied to a sample of 30 healthy volunteers (12 women, 18 men) aged between 18 and 30 years. A total of six balance tests were performed: four variations of the Romberg test, one test for a study of the limits of stability (LoS) and one test for rhythmic weight shift (RWS). Analysis of the results confirms that the variables assessed yielded similar values to other studies, the consistency of values between tests was checked, and preliminary reference values were obtained for asymptomatic subjects. We propose the following variables as the most significant for balance diagnosis: CoP mean speed, RMS, Range of CoP displacement and area. As a result of the study, the system is considered of interest in the medical/legal and forensic settings to assess the balance control and degree of collaboration during the tests.     

Effects of reflex delays on postural control during unstable seated balance

Patients with low-back pain (LBP) exhibit longer trunk muscle reflex latencies and poorer postural control than healthy individuals. We hypothesized that balance during a simulated postural control task would become impaired when the delays exhibited by LBP patients were incorporated into neuromuscular control. The task chosen for this investigation was seated balancing, which emphasizes trunk muscles’ contribution in postural control. This task was modeled in Simulink? as a fourth order linearized dynamic system with feedback delays. Optimization (minimizing error between experimental and model data) of state variables was used to determine neuromuscular control parameters. Experimental data were obtained from 7 subjects during 5 perturbation trials while balancing on the seat with eyes closed. Model accuracy, reflecting the ability of the model to capture the dynamics of seated balance, was correlated with seated balance performance (r=0.91, p<0.001). To minimize the risk of erroneous findings from inaccurate modeling, only the best five balancers’ data were used for hypothesis testing. In these five subjects, feedback delays in modeled neuromuscular control were increased to determine their effect on task stability, trunk displacement and trunk moment. Simulations showed that longer delays found in LBP, in general, did not produce unstable balancing, but did result in increased trunk displacement (p<0.001) and trunk moment (p=0.001). This impairment in neuromuscular control in chronic LBP patients could possibly exacerbate their condition by increasing tissue strain (more spinal displacement) and stress (more spinal loading).     

Design and validation of a machine for reproducible precision insertion of femoral hip prostheses for preclinical testing

Preclinical testing of orthopaedic implants is becoming increasingly important to eliminate inferior designs before animal experiments or clinical trials are begun. Preclinical tests can include both laboratory bench tests and computational modeling. One problem with bench tests is that variability in prosthesis insertion can significantly influence the failure rate; this makes comparison of prostheses more difficult. To solve this problem an insertion method is required that is both accurate and reproducible. In this work, a general approach to the insertion of hip prostheses into femoral bones is proposed based on physically replicating an insertion path determined using computer animation. As a first step, the seated prosthesis position is determined from templates and femur radiographs. Three-dimensional images of the prosthesis and bone are then imported into computer animation software and an insertion path in the coronal plane is determined. The insertion path is used to determine the profile of a cam. By attaching the prosthesis to a carriage, which is pneumatically moved along this cam, the required insertion motion of the prosthesis in the coronal plane can be achieved. This paper describes the design and validation of the insertion machine. For the validation study, a nonsymmetric hip prosthesis design (Lubinus SPII, Waldemar Link, Germany) is used. It is shown that the insertion machine has sufficient accuracy and reproducibility for preclinical mechanical testing.     

Diabetic neuropathy and surface sway-referencing disrupt somatosensory information for postural stability in stance

In order to determine the type of somatosensory information for postural control that is most affected by neuropathy, we compared the relative effects of three methods of sway-referencing the surface in a group of subjects with profound loss of somatosensory function associated with sensory polyneuropathy from diabetes with age-matched control subjects. Sway-referencing disrupted somatosensory feedback for postural control by servo-controlling the dorsi- and plantar-flexion rotation of the support surface in proportion to anterior-posterior excursion of (1) ankle angle, (2) center of body mass (CoM) angle or (3) filtered center of pressure (CoP). Postural sway in subjects with somatosensory loss was significantly larger than normal on a firm surface but not on the sway-referenced surfaces, suggesting that sway-referencing disrupts somatosensory information for postural control already disrupted by neuropathy. Control subjects standing on any sway-referenced surface swayed significantly more than neuropathy subjects who stood on a firm surface, suggesting that sway-referencing disrupts more somatosensory information than disrupted by severe neuropathy. CoP sway-referencing was less sensitive than ankle or CoM sway-referencing for distinguishing postural sway in subjects with somatosensory loss from age-matched control subjects. Given that filtered CoP sway-referencing disrupts the ability to utilize somatosensory information related to surface reactive force to a greater extent than the other two methods of sway-referencing, then these results support the hypothesis that subjects with diabetic peripheral neuropathy have lost more CoP information, than ankle or CoM angle information, for controlling postural sway in stance.