Age-Related Changes of the Anticipatory Postural Adjustments During Gait Initiation Preceded by Vibration of Lower Leg Muscles.

Gait initiation (GI) challenges the balance control system, especially in the elderly. To date, however, there is no consensus about the age effect on the anticipatory postural adjustments (APAs). There is also a lack of research on APAs in older adults after proprioceptive perturbation in the sagittal plane. This study aimed to compare the ability of young and older participants to generate APAs in response to the vibratory-induced perturbation delivered immediately before GI. Twenty-two young and 22 older adults performed a series of GI trials: (1) without previous vibration; (2) preceded by the vibration of triceps surae muscles; and (3) preceded by the vibration of tibialis anterior muscles. The APAs magnitude, velocity, time-to-peak, and duration were extracted from the center of pressure displacement in the sagittal plane. Young participants significantly modified their APAs during GI, whereas older adults did not markedly change their APAs when the body vertical was shifted neither backward nor forward. Significant age-related declines in APAs were observed also regardless of the altered proprioception.The results show that young adults actively responded to the altered proprioception from lower leg muscles and sensitively scaled APAs according to the actual position of the body verticality. Contrary, older adults were unable to adjust their postural responses indicating that the challenging transition from standing to walking probably requires higher reliance on the visual input. The understanding of age-related differences in APAs may help to design training programs for the elderly specifically targeted to improve balance control in different sensory conditions, particularly during gait initiation.

The Effects of Virtual Height Exposure on Postural Control and Psychophysiological Stress Are Moderated by Individual Height Intolerance.

Virtual reality (VR) enables individuals to be exposed to naturalistic environments in laboratory settings, offering new possibilities for research in human neuroscience and treatment of mental disorders. We used VR to study psychological, autonomic and postural reactions to heights in individuals with varying intensity of fear of heights. Study participants (N = 42) were immersed in a VR of an unprotected open-air elevator platform in an urban area, while standing on an unstable ground. Virtual elevation of the platform (up to 40 m above the ground level) elicited robust and reliable psychophysiological activation including increased distress, heart rate, and electrodermal activity, which was higher in individuals suffering from fear of heights. In these individuals, compared with individuals with low fear of heights, the VR height exposure resulted in higher velocity of postural movements as well as decreased low-frequency (<0.5 Hz) and increased high-frequency (>1 Hz) body sway oscillations. This indicates that individuals with strong fear of heights react to heights with maladaptive rigidity of posture due to increased weight of visual input for balance control, while the visual information is less reliable at heights. Our findings show that exposure to height in a naturalistic VR environment elicits a complex reaction involving correlated changes of the emotional state, autonomic activity, and postural balance, which are exaggerated in individuals with fear of heights.

Postural stability after treadmill and overground walking in young and elderly.

BACKGROUND: Ageing 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 QUESTION: The 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. RESULTS: In 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.

Parkinson's disease versus ageing: different postural responses to soleus muscle vibration.

BACKGROUND: Impairments of postural stability occur with increasing age and in neurodegenerative diseases like the Parkinson's disease (PD). While changes in balance have been described in many studies under steady-state conditions, less is known about the dynamic changes in balance following sudden transition to different sensory inputs. RESEARCH QUESTION: The aim was to clarify different effects of age and Parkinson's disease on dynamic postural responses immediately after lower leg muscle stimulation offset. Sudden removing of active sensory input represents a transient period in balance control. METHODS: Postural responses of 13 young, 13 healthy elderly and 13 PD patients to proprioceptive bilateral vibration of soleus muscles during stance were assessed by a force platform and two accelerometers attached on the upper and the lower trunk. The experimental protocol consisted of 2 conditions of soleus muscle vibration with 1) eyes open and 2) eyes closed randomly repeated four times. RESULTS: During vibration period before stimulus offset, postural responses were similar in elderly and PD patients. Contrary, immediately after vibration offset significantly larger backward amplitude of centre of foot pressure (CoP) displacement and trunk tilts were observed in PD patients compared to healthy peers. In returning to vertical position, peak-to-peak amplitudes, maximal velocity of CoP and trunk tilts significantly increased in PD patients. Without vision, their postural responses were more enhanced. The differences between young and elderly were found in most parameters in transient period after vibration offset and also during vibration. SIGNIFICANCE: The PD patients showed more unstable transient postural responses to selective sensory stimulation switch off, which may reflect impairment of sensory reweighting in balance control. Understanding how early stages PD patients differ in balance control from neurologically intact peers may help researchers and clinicians to refine their intervention and fall prevention programs.

Effect of oral contraceptives intake on postural stability in young healthy women throughout the menstrual cycle.

The purpose of the study was to investigate the effect of oral contraceptives on static postural stability in young healthy women during their menstrual cycle. Twenty-three women with the regular menstrual cycle, using or not using oral contraceptives, participated in this study. Salivary progesterone and estradiol levels were measured during one menstrual cycle. Measurements of balance were performed during a quiet stance on a firm and foam surface by the force platform, with eyes either opened or closed, on day 2, 7, 14, 21 and 28 of the cycle. Results of stability on a firm surface with eyes opened showed a significant effect in the amplitude of body sway in the anterior-posterior direction since women using oral contraceptives had a lower amplitude compared to control women on day 28. During stance on a firm surface with eyes closed we showed only impact of the menstrual cycle on postural stability of women. In condition of stance on a foam surface with the eyes opened or closed no significant effects were found. Our results showed that oral contraceptives intake can improve the static postural stability before the onset of menstruation and decrease a risk of injury of young healthy women.

Breathing changes accompanying balance improvement during biofeedback.

The aim of this study was to determine whether respiration would be altered during visual biofeedback condition while standing on a foam surface. Fifty young, healthy subjects (24 men, 26 women) were divided into a spirometry group, in which additional spirometry analysis was performed, and a control group. All subjects were tested in two conditions: 1) standing on a foam surface and 2) standing on a foam surface with visual biofeedback (VF) based on the centre of pressure (CoP). CoP amplitude and velocity in anterior-posterior (Aap, Vap) and medial-lateral (Aml, Vml) directions were measured by the force platform. Breathing movements were recorded by two pairs of 3D accelerometers attached on the upper chest (upper chest breathing - UCB) and the lower chest (lower chest breathing - LCB). Results showed that significant decreases of CoP amplitude and velocity in both directions were accompanied by a significant decrease of lower chest breathing, and an increase of LCB frequency was seen during VF condition compared to control condition in both groups. Moreover, a significant decrease in tidal volume and increased breathing frequency during VF condition were confirmed by spirometric analysis. Reduced breathing movements and volumes as well as increased breathing frequency are probably part of an involuntary strategy activated to maximize balance improvement during VF condition.

Velocity dependence of vestibular information for postural control on tilting surfaces.

Vestibular information is known to be important for postural stability on tilting surfaces, but the relative importance of vestibular information across a wide range of surface tilt velocities is less clear. We compared how tilt velocity influences postural orientation and stability in nine subjects with bilateral vestibular loss and nine age-matched, control subjects. Subjects stood on a force platform that tilted 6 deg, toes-up at eight velocities (0.25 to 32 deg/s), with and without vision. Results showed that visual information effectively compensated for lack of vestibular information at all tilt velocities. However, with eyes closed, subjects with vestibular loss were most unstable within a critical tilt velocity range of 2 to 8 deg/s. Subjects with vestibular deficiency lost their balance in more than 90% of trials during the 4 deg/s condition, but never fell during slower tilts (0.25-1 deg/s) and fell only very rarely during faster tilts (16-32 deg/s). At the critical velocity range in which falls occurred, the body center of mass stayed aligned with respect to the surface, onset of ankle dorsiflexion was delayed, and there was delayed or absent gastrocnemius inhibition, suggesting that subjects were attempting to actively align their upper bodies with respect to the moving surface instead of to gravity. Vestibular information may be critical for stability at velocities of 2 to 8 deg/s because postural sway above 2 deg/s may be too fast to elicit stabilizing responses through the graviceptive somatosensory system, and postural sway below 8 deg/s may be too slow for somatosensory-triggered responses or passive stabilization from trunk inertia.

Diurnal changes of the postural control in young women without and with hormonal contraceptive treatment.

OBJECTIVES: The aim of this study was to investigate diurnal changes of the postural control in young women without and with hormonal contraceptive treatment. METHODS: The postural activity was assessed during stance from two accelerometers positioned at the level of the lumbar (L5) and thoracic (Th4) vertebra in twenty healthy young women non-using (13) and using (7) hormonal contraception. RESULTS: We observed a significant increase of trunk tilts in the morning in the group of women with hormonal contraception compared to control. Women with hormonal contraception showed the significant decrease of trunk tilts and their velocity in the evening in relation to increased morning data at the L5 in anterior-posterior direction during stance on foam. Measurements at Th4 showed higher variability of lateral trunk tilts in conditions with altered somatosensory inputs. Distinct reduction of velocity of lateral trunk tilts in the evening related to morning measurements were present in the control group at the L5 in conditions with altered somatosensory inputs and at the Th4 in all experimental conditions in both groups. CONCLUSION: We demonstrated diurnal changes of the postural control in young women. Women using hormonal contraceptives showed a weakened postural stability compared with the control group in the morning and the normalization of postural stability in the evening to the values of the control group. These findings suggest that the time of day and the use of hormonal contraception affect postural stability of women.

Effectiveness of different visual biofeedback signals for human balance improvement.

The aim of this study was to examine the effectiveness of visual biofeedback (VBF) signals from a force platform and accelerometer sensors placed on different body segments. The study was performed on 20 young subjects during standing on a firm and foam support surface with a VBF signal sensed from CoP, lower trunk (L5) and upper trunk (Th4). The VBF signal was controlled by 2D-movement of chosen body segment, which was presented as a red point on a monitor screen. Location of VBF signal had a significant effect on each postural parameter of CoP and trunk segments. RMS and amplitudes of postural sway in medial-lateral and anterior-posterior directions decreased during standing on both types of support surface due to VBF. L5-VBF and CoP-VBF significantly reduced CoP displacements and lower trunk tilts. Th4-VBF reduced upper trunk tilts. Frequency analysis of postural sway revealed a decrease of power spectral density (PSD) values in low frequency range (0.02-0.3Hz) and an increase of PSD values in higher frequency range (0.5-1.4 Hz) in the VBF conditions during the stance on the firm surface in anterior-posterior direction. Reduction of body sway was the most significant in the body segment from which the VBF signal was sensed. The CoP position and L5 position provided the best signals for VBF. Changes in frequency ranges of body sway suggest voluntary activation of balance control. The results open new opportunities to optimize VBF system for balance improvement using accelerometers.

The age-related changes of trunk responses to Achilles tendon vibration.

The contribution of different sensory modalities to balance control is modified by age. Postural responses to Achilles tendon vibration were investigated in order to understand the influence of age on proprioceptive input from lower legs in human stance. Postural responses to bilateral vibrations of Achilles tendon with 10s duration were recorded at three frequencies (40, 60 and 80 Hz) in 9 healthy young (range, 24-27 years) and in 9 healthy older adults (59-70 years). Subjects were instructed to keep standing on firm surface with eyes closed. They performed three trials in each of three vibration frequencies. Postural responses were characterized by displacement of the centre of foot pressure (CoP) and by kinematics of body segments in the anterior-posterior direction. Bilateral vibrations of Achilles tendon induced backward body lean increasing with frequency of vibration and with age. The leg angle response to vibration was found similar in both groups of subjects. Slight trunk tilts from vertical position were induced by vibration in young subjects while in older subjects the trunk tilted backward together with the whole body. This observation was supported also by the minimal change of hip angle in older subjects contrary to increased hip activity in young subjects. The findings showed that the trunk and hip angle responses to proprioceptive stimulation might be a good indicator of age-related destabilization in balance control.

Modification of human postural responses to soleus muscle vibration by rotation of visual scene.

Sensory interaction in posture control in 20 healthy subjects was investigated by postural responses to differently timed proprioceptive and visual stimulation. As proprioceptive stimulation was used both soleus muscles vibration. Visual stimulation was rotating disc moving in forward and backward direction. Centre of foot pressure (CoP) and trunk tilts in antero-posterior (AP) direction were measured during stance by two accelerometers on the upper and the lower trunk level. Subjects performed four series of eight trials lasted 20s: vibration with eyes open and closed, visual scene motion forward or backward, vibration together with scene motion forward or backward, scene motion forward or backward 3s before vibration. The results showed that early velocities and final angles of body tilt induced by soleus muscle vibration were modified by motion of visual scene. Early part of postural responses was changed slightly if sensory stimulation starts together. When visual stimulation started 3s before muscle vibration, the induced early CoP and trunk tilt records of postural response to vibration occurred with the faster slope similar for both directions of scene motion and for condition of vibration alone with eyes closed. Significantly different final CoP shifts and trunk angles were measured between condition with visual scene motion forward and backward. The effect of visual input on posture control was two times enhanced in paired sensory stimulation. The results indicated an important influence of unstable visual field on posture response to somatosensory stimulation where the early part of postural reaction indicated absence of visual influence.

Effects of linear versus sigmoid coding of visual or audio biofeedback for the control of upright stance.

Although both visual and audio biofeedback (BF) systems for postural control can reduce sway during stance, a direct comparison between the two systems has never been done. Further, comparing different coding designs of audio and visual BF may help in elucidating how BF information is integrated in the control of posture, and may improve knowledge for the design of innovative BF systems for postural control. The purpose of this paper is to compare the effects of linear versus sigmoid coding of trunk acceleration for audio and visual BF on postural sway in a group of eight, healthy subjects while standing on a foam surface. Results showed that sigmoid-coded audio BF reduced sway acceleration more than did a linear-coded audio BF, whereas a linear-coded visual BF reduced sway acceleration more than a sigmoid-coded visual BF. In addition, audio BF had larger effects on reducing center of pressure (COP) displacement whereas visual BF had larger effects on reducing trunk sway. These results suggest that audio and visual BF for postural control benefit from different types of sensory coding and each type of BF may encourage a different type of postural sway strategy.

Vestibular stimulation affects medium latency postural muscle responses.

This study explores whether galvanic vestibular stimulation can alter automatic postural muscle responses triggered 100 ms after surface translations. Our previous study concluded that a step of bipolar, galvanic vestibular stimulation delivered 500 ms prior to a platform translation tilted the internal representation of vertical because subjects' final center of foot pressure and center of mass equilibrium position shifted toward the anode and this tilt was larger than the sum of effects for platform translations and galvanic alone. In the current study, we show that 0.2-0.4 mA of galvanic vestibular stimulation produced significant changes in the tilt of the trunk in space that was realized by changes in the magnitude of the medium latency postural muscle responses at the ankle. The galvanic-induced changes in latency and magnitude of the first 50 ms of gastrocnemius muscle burst in response to the backward surface translation were consistent with changes in background muscle tone induced by the direct vestibulospinal effects of the galvanic current. However, the galvanic-induced changes in the second 50 ms of the gastrocnemius response were in the opposite direction and consistent with the forward- or backward-tilted, final postural equilibrium goal. Kinematic analysis showed that galvanic-induced tilt was first initiated in the trunk and that the shank and thigh segment angles were not altered by galvanic stimulation prior to platform translation such that changes in gastrocnemius and soleus postural responses to translations were not due to an effect of galvanic stimulation on initial ankle angle. More proximal muscles and antagonist muscles involved in the postural response were not altered by the galvanic stimulation. These results suggest that galvanic vestibulospinal stimulation can alter medium latency, automatic postural responses in prime movers by changing the postural equilibrium goal.