Changes in Vocal Fold Morphology During Singing Over Two Octaves.

OBJECTIVE: Vocal folds are widely assumed to only elongate to raise vocal pitch. However, the mechanisms seem to be more complex and involve both elongation and tensioning of the vocal folds in series. The aim of the present study was to show that changes in vocal fold morphology depend on vocal fold elongation and tensioning during singing. STUDY DESIGN: This was a prospective study. METHODS: Forty-nine professional female singers (25 sopranos, 24 altos) were recruited and three-dimensional laryngeal images analyzed in a coronal view derived from high-resolution computed tomography scans obtained at the mean speaking fundamental frequency (ƒ0) and one (2ƒ0) and two octaves (4ƒ0) above ƒ0. RESULTS: The vocal fold angle, defined by a tangent above and below the vocal folds, was 58° at ƒ0, 47° at 2ƒ0, and 59° at 4ƒ0. CONCLUSION: The decreased caudomedial angle of the vocal fold from ƒ0 to 2ƒ0 (change in muscle belly from ";fat" to "thin") and increased angle from 2ƒ0 to 4ƒ0 (from "thin" to "fat") strongly supports the hypothesis that the vocal folds elongate and then tension when singing from ƒ0 to 4ƒ0. This is the first study to show this relationship in vivo.

Mental body transformation deficits in patients with chronic balance disorders.

BACKGROUND: Movements may be generated consistent with imagining one's own body transformed or "disembodied" to a new position. Based on this concept we hypothesized that patients with objective balance deficits (obj-BD) would have altered neural transformation processes executing own body transformation (OBT) with functional consequences on balance control. Also we examined whether feeling unstable due to dizziness only (DO), without an obj-BD, also lead to an impaired OBT. METHODS: 32 patients with chronic dizziness were tested: 16 patients with obj-BD as determined by balance control during a sequence of stance and gait tasks, 16 patients with dizziness only (DO). Patients and 9 healthy controls (HCs) were asked to replicate roll trunk movements of an instructor in a life size video: first, with spontaneously copied (SPO) or "embodied" egocentric movements (lean when the instructor leans); second, with "disembodied" or "transformed" movements (OBT) with exact replication - lean left when the instructor leans left. Onset latency of trunk roll, rise time to peak roll angle (interval), roll velocity, and amplitude were measured. RESULTS: SPO movements were always mirror-imaged. OBT task latencies were significantly longer and intervals shorter than for SPO tasks (p < 0.03) for all groups. Obj-BD but not DO patients had more errors for the OBT task and, compared to HCs, had longer onset latencies (p < 0.05) and smaller velocities (p < 0.003) and amplitudes (p < 0.001) in both the SPO and OBT tasks. Measures of DO patients were not significantly different from those of HCs. CONCLUSIONS: Mental transformation (OBT) and SPO copying abilities are impaired in subjects with obj-BD and dizziness, but not with dizziness only. We conclude that processing the neuropsychological representation of the human body (body schema) slows when balance control is deficient.

Movement strategies and sensory reweighting in tandem stance: differences between trained tightrope walkers and untrained subjects.

Does skill with a difficult task, such as tightrope walking, lead to improved balance through altered movement strategies or through altered weighting of sensory inputs? We approached this question by comparing tandem stance (TS) data between seven tightrope walkers and 12 untrained control subjects collected under different sensory conditions. All subjects performed four TS tasks with eyes open or closed, on a normal firm or foam surface (EON, ECN, EOF, ECF); tightrope walkers were also tested on a tightrope (EOR). Head, upper trunk and pelvis angular velocities were measured with gyroscopes in pitch and roll. Power spectral densities (PSDs) ratios, and transfer function gains (TFG) between these body segments were calculated. Center of mass (CoM) excursions and its virtual time to contact a virtual base of support boundary (VTVBS) were also estimated. Gain nonlinearities, in the form of decreased trunk to head and trunk to pelvis PSD ratios and TFGs, were present with increasing sensory task difficulty for both groups. PSD ratios and TFGs were less in trained subjects, though, in absolute terms, trained subjects moved their head, trunk, pelvis and CoM faster than controls, and had decreased VTVBS. Head roll amplitudes were unchanged with task or training, except above 3Hz. CoM amplitude deviations were not less for trained subjects. For the trained subjects, EOR measures were similar to those of ECF. Training standing on a tightrope induces a velocity modification of the same TS movement strategy used by untrained controls. More time is spent exploring the limits of the base of support with an increased use of fast trunk movements to control balance. Our evidence indicates an increased reliance on neck and pelvis proprioceptive inputs. The similarity of TS on foam to that on the tightrope suggests that the foam tasks are useful for effective training of tightrope walking.

Coordination of the head with respect to the trunk, pelvis, and lower leg during quiet stance after vestibular loss.

This study examined the relationship between head and trunk sway and between pelvis and leg sway during quiet stance in subjects with long-standing bilateral peripheral vestibular loss (BVLs) comparing these relationships to those of age-matched healthy controls (HCs). All subjects performed three different stance tasks: standing quietly on a firm or foam support surface, with eyes closed (ECF or eyes closed on normal) and on foam with eyes open. Data were recorded with four pairs of body-worn gyroscopes to measure roll and pitch angular velocities at the head, upper trunk, pelvis and lower-leg. These velocities were spectrally analysed and integrated for angle correlation analysis in three frequency bands: below 0.7Hz (low pass, LP), above 3 Hz (high pass, HP) and in between (band pass, BP). For both groups head motion was greater than trunk and pelvis motion except for BVL subjects (BVLs) under ECF conditions. BVLs had greater motion than HCs at all measurement locations for ECF conditions. Angle correlation analysis indicated that the head was almost "locked" to the trunk for BVLs over the LP and BP frequency bands. Head movements for both groups were relatively independent of the trunk in the HP band. Power spectral density ratios, and transfer functions showed a similar result - head relative to trunk movements were less up to 3 Hz in all tests for BVLs. The resonant frequency of head-on-trunk motion was shifted to a higher frequency for BVLs: from 3.2 to 4.3 Hz in pitch, 4.6 to 5.4 Hz in roll. Both groups show greater lower-leg than pelvis motion. These data indicate that during quiet stance BVLs change the characteristics of their head on shoulder motion, reducing relative motion of the head below 3 Hz and increasing head resonant frequency. Presumably these changes are accomplished with increased use of proprioceptive neck reflexes.

Movement patterns underlying first trial responses in human balance corrections.

BACKGROUND: We investigated whether the "first trial effect" (FTE) in responses to support surface tilt has directional characteristics, or is simply due to a startle-like response. The FTE is the difference between the first (unpractised) trial response (FTR) and subsequent responses. METHODS: Each group of 10 young adults received a series of identical support surface tilts (7.5°, 60°/s) in one of five leftward tilt directions or pure backward or forward. These were followed by randomly selected tilts in at least eight equally spaced directions. Only in-place responses were possible as the feet were strapped to the support surface. Body kinematics were collected and EMG activity was recorded from several trunk, leg and arm muscles. RESULTS: The centre of mass (CoM) vector displacement showed a FTE in all tilt directions. It was equally large for all directions of backward tilt but smaller for forward and lateral tilts. A similar effect was noted for the CoM anterior-posterior FTE. FTRs of lateral CoM movements were small for all tilt directions except in the backward left direction. A constant amplitude trunk flexion FTE was observed in all tilt directions, and pelvis backward motion for backward tilts, preceded by a FTE in the abdominal muscles for forward (and lateral) tilts and in the soleus for backward (and lateral) tilts. Hip flexion FTEs were largest in backward left direction and preceded by increased gluteus medius and deltoid FTR activity. FTRs in sternocleidomastoïdeus muscles, generally associated with startle activity, were largest in lateral and forward tilt directions. CONCLUSIONS: FTRs appear to consist of either a forward, backward or lateral movement strategy each imposed on an adapted response strategy. Only the lateral response shows a strong directional sensitivity. We hypothesise that FTR amplitudes result from a failure of the CNS to weight properly the stimulus metrics present in lower leg proprioceptive and vestibular inputs.

Coordination of the head with respect to the trunk and pelvis in the roll and pitch planes during quiet stance.

This study examined the relationship between head and trunk sway during quiet stance and compared this relationship with that of the pelvis to the trunk. Sixteen younger and 14 elderly subjects participated, performing four different sensory tasks: standing quietly on a firm or foam support surface, with eyes open or closed. Roll and pitch angular velocities were recorded with six body-worn gyroscopes; a set of two mounted at the upper trunk, an identical set at the hips, and another set on a head band. Angle correlation analysis was performed in three frequency bands: below 0.7 Hz (LP), above 3 Hz (HP) and in between (BP) using the integrated angle velocity signals. Angular velocities were spectrally analysed. Greater head than trunk motion was observed in angle correlations, power spectral density (PSD) ratios, and transfer functions (TFs). Head on trunk motion could be divided for all sensory conditions into a low-frequency (<0.7 Hz) "head locked to trunk" inverted pendulum mode, a mid-frequency (ca. 3 Hz), resonant mode, and a slightly anti-phasic head motion on stabilised trunk, high-frequency (>3 Hz) mode. There was coherent motion between head and trunk but not between head and pelvis. Trunk and pelvis data were consistent with previously reported in-phase and anti-phase movements between these segments. Significant age differences were not found. These data indicate that during quiet stance body motion increases in the order of pelvis, trunk, head and quiet stance involves control of at least two separate links: trunk on pelvis and head on trunk dominated by head resonance. The head is locked to the trunk for low-frequency motion possibly because motion is just supra-vestibular threshold. The head is not stabilised in space during stance, rather the pelvis is.

The effect of voluntary lateral trunk bending on balance recovery following multi-directional stance perturbations.

Stabilising shifts of the centre of mass (COM) are observed during balance recovery when subjects simultaneously execute voluntary unilateral knee flexion or unilateral arm raising. Here, we examined whether voluntary lateral trunk bending provided more beneficial stabilising effects, and how motor programs of balance corrections are combined with those of the focal voluntary action. The upright balance of 24 healthy young subjects (19-33 years of age) was perturbed using multi-directional rotations of the support-surface. The perturbations consisted of combined pitch and roll rotations (7.5 degrees and 60 degrees/s) presented randomly in six different directions. Three conditions were tested: perturbation of stance only (PO); combined balance perturbation and cued uphill bending of the trunk (CONT); and combined perturbation and cued downhill bending of the trunk (IPS). For comparison, subjects were required to perform trunk bending alone (TO). Outcome measures were biomechanical responses and surface EMG activity of several muscles. Calculated predicted outcomes (PO + TO) were compared with combined measures (CONT or IPS). CONT trunk bending uphill showed two phases of benefit in balance recovery for laterally but, in contrast to voluntary knee bending, not for posterior directed components of the perturbations. IPS trunk bending had negative effects on balance. Early balance correcting muscle responses were marginally greater than PO responses. Prominent secondary balance correcting responses, having a similar timing as voluntary responses observed under TO conditions, were seen under CONT only in trunk muscles. These, and later stabilising, responses had amplitudes as expected from PO + TO conditions being significantly greater than PO responses. The ability with which different muscle synergies for balance corrections and voluntary trunk bending were integrated into one indicates a flexible adjustment of the CNS programs to the demands of both tasks.

Vestibular and proprioceptive contributions to human balance corrections: aiding these with prosthetic feedback.

Movement strategies controlling quiet stance and rapid balance corrections may have common characteristics. We investigated this assumption for lower leg proprioceptive loss (PL), peripheral vestibular loss (VL), and healthy controls. Our underlying hypothesis was that changes in movement-strategy modulation following sensory loss would improve with prosthetic biofeedback. Quiet stance was measured under different sensory conditions and compared to corrections induced by multidirection support-surface tilts. Response synergies were assessed using electromyography recordings from several muscles. Biofeedback of trunk sway during gait and stance tasks used lower trunk rotations to drive head-band-mounted vibro-tactile and auditory actuators. Strategies of quiet stance were different for roll and pitch, depending on sensory conditions. Simultaneously acting strategies were observed for low- and high-frequency sway. PL induced strategies different from those of VL and controls. VL strategies were identical to those of controls but with greater amplitudes. Tilt perturbation movement strategies were similar to high-frequency strategies of quiet stance--multisegmental. VL induced increased trunk pitch and roll responses with hypermetric trunk muscle responses and hypometric knee responses but unchanged synergies. Increasing PL up the legs caused changed synergies. Biofeedback reduced stance body sway in VL and elderly subjects. In conclusion, several movement strategies underlie quiet stance with high-frequency strategies being common to those of perturbed stance. PL changes both movement strategies and synergies, whereas VL only causes pathological changes to the modulation depth. Thus, VL is more easily rectified using trunk sway positional biofeedback.

Incorporating voluntary unilateral knee flexion into balance corrections elicited by multi-directional perturbations to stance.

Positive effects on lateral center of mass (CoM) shifts during balance recovery have been seen with voluntarily unilateral arm raising but not with voluntarily bilateral knee flexion. To determine whether unilateral voluntary knee movements can be effectively incorporated into balance corrections we perturbed the balance of 30 young healthy subjects using multi-directional rotations of the support surface while they simultaneously executed unilateral knee flexion. Combined pitch and roll rotations (7.5 degrees and 60 degrees/s) were presented randomly in six different directions. Subjects were tested in four stance conditions: balance perturbation only (PO); cued flexion of one knee only (KO); combined support surface rotation and cued (at rotation onset) flexion of the uphill knee, contralateral to tilt (CONT), or of the downhill knee, ipsilateral to tilt (IPS). Outcome measures were CoM motion and biomechanical and electromyography (EMG) responses of the legs, arms and trunk. Predicted measures (PO+KO) were compared with combined measures (CONT or IPS). Unilateral knee flexion of the uphill knee (CONT) provided considerable benefit in balance recovery. Subjects rotated their pelvis more to the uphill side than predicted. Downhill knee bending (IPS) also had a positive effect on CoM motion because of a greater than predicted simultaneous lateral shift of the pelvis uphill. KO leg muscle activity showed anticipatory postural activity (APA) with similar profiles to early balance correcting responses. Onsets of muscle responses and knee velocities were earlier for PO, CONT, and IPS compared to KO conditions. EMG response amplitudes for CONT and IPS conditions were generally not different from the PO condition and therefore smaller than predicted. Later stabilizing responses at 400 ms had activation amplitudes generally equal to those predicted from the PO+KO conditions. Our results suggest that because EMG patterns of anticipatory postural activity of voluntary unilateral knee flexion and early balance corrections have similar profiles, the CNS is easily able to incorporate voluntary activation associated with unilateral knee flexion into automatic postural responses. Furthermore, the effect on movement strategies appears to be non-linear. These findings may have important implications for the rehabilitation of balance deficits.

Vestibular and proprioceptive influences on trunk movements during quiet standing.

We characterized upper trunk and pelvis motion in normal subjects and in subjects with vestibular or proprioceptive loss, to document upper body movement modes in the pitch and roll planes during quiet stance. Six bilateral vestibular loss (VL), six bilateral lower-leg proprioceptive loss (PL) and 28 healthy subjects performed four stance tasks: standing on firm or foam surface with eyes open or closed. Motion of the upper body was measured using two pairs of body-worn gyroscopes, one mounted at the pelvis and the other pair at the shoulders. Pitch and roll angular velocities recorded from the gyroscopes were analyzed separately for low-frequency (<0.7 Hz) and high-frequency (>3 Hz) motion. Low-frequency pitch motion was similar for all groups, consisting of in-phase pelvis and shoulder motion. High-frequency pitch motion in controls and VL subjects was dominated by pelvis motion with little shoulder motion, but vice versa in PL subjects. Low-frequency roll motion changed for all groups from mainly shoulder and little pelvis motion to in-phase pelvis and shoulder motion after moving from a firm to foam surface. In contrast, high-frequency roll motion changed from mainly shoulder motion to mainly pelvis motion with the change to a foam surface, except for PL subjects with eyes closed. Coherent low-frequency sway between pelvis and shoulder was only pronounced in VL patients. These results indicate that relative motion between the pelvis and shoulder depends on the support surface, the type of sensory loss, and whether the motion is in roll or pitch plane. Furthermore, relative motion between the pelvis and upper trunk is an integral part of movement modes used to control quiet stance. Vestibular loss patients showed very similar movement modes as controls, with larger amplitudes. Proprioceptive loss patients, however, used more shoulder motion and stabilized the pelvis for the high-frequency mode. We conclude that there is relative motion between the upper trunk and pelvis during quiet stance and suggest that it may contribute to balance control.

Control of roll and pitch motion during multi-directional balance perturbations.

Does the central nervous system (CNS) independently control roll and pitch movements of the human body during balance corrections? To help provide an answer to this question, we perturbed the balance of 16 young healthy subjects using multi-directional rotations of the support surface. All rotations had pitch and roll components, for which either the roll (DR) or the pitch (DP) component were delayed by 150 ms or not at all (ND). The outcome measures were the biomechanical responses of the body and surface EMG activity of several muscles. Across all perturbation directions, DR caused equally delayed shifts (150 ms) in peak lateral centre of mass (COM) velocity. Across directions, DP did not cause equally delayed shifts in anterior-posterior COM velocity. After 300 ms however, the vector direction of COM velocity was similar to the ND directions. Trunk, arm and knee joint rotations followed this roll compared to pitch pattern, but were different from ND rotation synergies after 300 ms, suggesting an intersegmental compensation for the delay effects. Balance correcting responses of muscles demonstrated both roll and pitch directed components regardless of axial alignment. We categorised muscles into three groups: pitch oriented, roll oriented and mixed based on their responses to DR and DP. Lower leg muscles were pitch oriented, trunk muscles were roll oriented, and knee and arm muscles were mixed. The results of this study suggest that roll, but not pitch components, of balance correcting movement strategies and muscle synergies are separately programmed by the CNS. Reliance on differentially activated arm and knee muscles to correct roll perturbations reveals a dependence of the pitch response on that of roll, possibly due to biomechanical constraints, and accounts for the failure of DP to be transmitted equally in time across all limbs segments. Thus it appears the CNS preferentially programs the roll response of the body and then adjusts the pitch response accordingly.

Postural instability in cerebellar ataxia: correlations of knee, arm and trunk movements to center of mass velocity.

The aim of this study was to investigate the correlations between body segment movements and center of mass (COM) velocity during pathological balance corrections of spinocerebellar ataxia (SCA) patients compared with controls, and to relate correlations indicating instability to EMG activity differences. Eighteen SCA patients and 21 age-matched controls were tested. Upright standing was perturbed using rotations of the support surface. We recorded body motion and surface EMG. For lateral perturbations peaks in COM lateral velocity were larger in SCA patients than controls. These peaks were correlated with increased ("hypermetric") trunk roll downhill and reduced uphill knee flexion velocity. Subsequent arm abduction partially corrected the lateral instability. Early balance correcting responses in knee and paraspinal muscles showed reduced amplitudes compared with normal responses. Later responses were consistent with compensation mechanisms for the lateral instability created by the stiffened knee and pelvis. We conclude that truncal hypermetria coupled with insufficient uphill knee flexion is the primary cause of lateral instability in SCA patients. Holding the knees and pelvis more rigid possibly permits a reduction in the controlled degrees of freedom and concentration on arm abduction to improve lateral instability. For backwards perturbations excessive posterior COM velocity coincided with marked trunk hypermetric flexion forwards. We concluded that this flexion and the ensuing backwards shift of the pelvis result from rigidity which jeopardizes posterior stability. Timing considerations and the lack of confirmatory changes in amplitudes of EMG activity suggest that lateral and posterior instability in SCA is primarily a biomechanical response to pelvis and knee rigidity resulting from increased muscle background activity rather than changed evoked responses.

Effect of subthalamic nucleus deep brain stimulation on axial motor control and protective arm responses in Parkinson's disease.

Stereotactic surgical interventions for Parkinson's disease (PD) can considerably improve appendicular motor signs, but their effect on axial motor signs--especially balance control under optimal drug therapy--remains unclear. Here, we investigated the effect of bilateral subthalamic nucleus (STN) stimulation on levodopa-resistant axial and appendicular postural impairment in PD. Fourteen patients (11 with young-onset PD) and 18 age-matched controls were included. Patients were tested after intake of a suprathreshold levodopa dose, ensuring optimal response to drug therapy, and with stimulators both turned on and off. Balance control was assessed using multidirectional dynamic posturography. Outcome measures included full body kinematics and surface electromyography of paraspinal and deltoid muscles. Patients with stimulators turned off showed early decreased trunk roll with a loss of directional dependency, followed by increased and abnormally directed--i.e. destabilizing--trunk roll. Pelvis pitch motion showed decreased directional dependency in these patients. The abnormal trunk motion was not corrected by STN stimulation, but directional dependency of both trunk and pelvis motion partially improved, along with a general decrease in muscle activity. Even with stimulators off, protective arm movements were similar in the optimally treated patients and controls, indicating that these appendicular signs respond better to dopaminergic treatment than axial motor control. Our findings indicate that instability in PD results from a reduced flexibility of the trunk and pelvis that is largely resistant to STN stimulation combined with optimal drug treatment. These postural abnormalities are therefore likely associated with non-dopaminergic pathology. In contrast, protective arm movements did appear to be levodopa-responsive. Future studies should focus on identifying subgroups of optimal responders, particularly patients with levodopa-induced dyskinesias.

Identifying deficits in balance control following vestibular or proprioceptive loss using posturographic analysis of stance tasks.

OBJECTIVE: To distinguish between normal and deficient balance control due to vestibular loss (VL) or proprioceptive loss (PL) using pelvis and shoulder sway measures. METHODS: Body-worn gyroscopes measured pelvis and shoulder sway in pitch (anterior-posterior) and roll (side-to-side) directions in 6 VL, 6 PL and 26 control subjects during 4 stance tasks. Sway amplitudes were compared between groups, and were used to select optimal measures that could distinguish between these groups. RESULTS: VL and PL patients had greater sway amplitudes than controls when standing on foam with eyes closed. PL patients also swayed more when standing with eyes closed on firm support and eyes open on foam. Standard sensory analysis techniques only differentiated VL patients from controls. Stepwise discriminate analysis showed that differentiation required pitch measures for VL patients, roll measures for PL patients, and both measures for all three groups. Pelvis measures yielded better discrimination than shoulder measures. CONCLUSIONS: Distinguishing between normal and deficient balance control due to VL or PL required pitch and roll pelvis sway measures. SIGNIFICANCE: Accurate identification of balance deficits due to VL or PL may be useful in clinical practice as a functional diagnostic tool or to monitor balance improvements in VL or PL patients.

Spatio-temporal separation of roll and pitch balance-correcting commands in humans.

This study was designed to provide evidence for the hypothesis that human balance corrections in response to pitch perturbations are controlled by muscle action mainly about the ankle and knee joints, whereas balance corrections for roll perturbations are controlled predominantly by motion about the hip and lumbro-sacral joints. A dual-axis rotating support surface delivered unexpected random perturbations to the stance of 19 healthy young adults through eight different directions in the pitch and the roll planes and three delays between pitch and roll directions. Roll delays with respect to pitch were no delay, a short 50-ms delay of roll with respect to pitch movements, (chosen to correspond to the onset time of leg muscle stretch reflexes), and a long 150-ms delay between roll and pitch movements (chosen to shift the time when trunk roll velocity peaks to the time when trunk peak pitch velocity normally occurs). Delays of stimulus roll with respect to pitch resulted in delayed roll responses of the legs, trunk, arms, and head consistent with stimulus delay without any changes in roll velocity amplitude. Delayed roll perturbations induced only small changes in the pitch motion of the legs and trunk; however, major changes were seen in the time when roll motion of the trunk was arrested. Amplitudes and directional sensitivity of short-latency (SL) stretch reflexes in ankle muscles were not altered with increasing roll delay. Small changes to balance correcting responses in ankle muscles were observed. SL stretch reflexes in hip and trunk muscles were delayed, and balance-correcting responses in trunk muscles became split into two distinct responses with delayed roll. The first of these responses was small and had a directional responsiveness aligned more along the pitch plane. The main, larger, response occurred with an onset and time-to-peak consistent with the delay in trunk roll displacement and its directional responsiveness was roll oriented. The sum of the amplitudes of these two types of balance-correcting responses remained constant with roll delay. These results support the hypothesis that corrections of the body's pitch and roll motion are programmed separately by neural command signals and provide insights into possible triggering mechanisms. The evidence that lower leg muscle balance-correcting activity is hardly changed by delayed trunk roll also indicates that lower leg muscle activity is not predominant in correcting roll motion of the body. Lower leg and trunk muscle activity appears to have a dual action in balance corrections. In trunk muscles the main action is to correct for roll perturbations and the lesser action may be an anticipatory stabilizing reaction for pitch perturbations. Likewise, the small changes in lower leg muscle activity may result from a generalized stabilizing reaction to roll perturbations, but the main action is to correct for pitch perturbations.

The balance control of bilateral peripheral vestibular loss subjects and its improvement with auditory prosthetic feedback.

OBJECTIVES: We investigated whether long-term bilateral vestibular loss subjects could combine auditory biofeedback of trunk sway with their remaining natural sensory inputs on balance to provide an improved control of trunk sway. A successful integration of natural and artificial signals would provide a basis for a balance prosthesis. METHODS: Trunk sway of 6 bilateral peripheral vestibular loss subjects (BVL) was recorded using either angular position- or velocity-based auditory feedback or no feedback during stance and gait tasks. Roll and pitch trunk movements were recorded with angular velocity transducers mounted just above the waist and feedback without a delay to 4 loudspeakers placed at the left, right, front and rear borders of the 5 m long by 4 m wide test environment. The two types of auditory feedback or no feedback were provided to the subjects in random order. In the feedback modes, sway greater than a preset angle (ca. 0.5 deg) or velocity (ca. 3 deg/s) thresholds caused a tone to be emitted from the speaker towards which the subject moved. The tone volume increased with increasing angle or angular velocity amplitude. RESULTS: For all stance tasks BVL subjects without auditory feedback had a significantly different balance control with respect to that of normal controls. BVL sway values eyes open on a normal surface were reduced with auditory feedback with the greatest reductions in the roll plane. Specifically for the task of standing on 1 leg eyes open with position-auditory- feedback, amplitudes of pitch and roll angles and angular velocities were indistinguishable from those of normal controls. Sway during stance tasks on foam with eyes closed showed no improvement with feedback, remaining greater than normal. For some gait tasks there was a decrease in trunk sway with velocity feedback. CONCLUSION: These initial results indicate that subjects with vestibular loss could incorporate the auditory prosthetic sensory information into their balance commands, particularly in the roll plane if the balance task is performed with eyes open. Position information appears more useful than velocity information in reducing trunk sway during stance tasks. Future work will need to determine the effect of a training time on the improvement in balance control using such a prosthetic device and the ideal position and velocity auditory feedback combination.

Postural abnormalities to multidirectional stance perturbations in Parkinson's disease.

OBJECTIVE: We investigated trunk control, protective arm movements, and electromyographic responses to multidirectional support-surface rotations in patients with Parkinson's disease (PD), aiming to better understand the pathophysiology underlying postural instability in PD, on and off antiparkinson medication. METHODS: Ten patients with PD were compared with 11 age matched healthy controls. Seven patients were also tested without (OFF) antiparkinson medication. All subjects received rotational perturbations (7.5 deg amplitude) that were randomly delivered in six different directions. RESULTS: The PD patients had decreased trunk rotation and ankle torque changes, consistent with a stiffening response. Stiffness appeared to be caused by the combined action of three factors: co-contraction that interfered in particular with the normal response asymmetry in trunk muscles; increased response amplitudes in agonist and antagonist muscles at both medium (approximately 80 ms) and balance correcting (approximately 120 ms) response latencies; and increased background activity in lower leg, hip, and trunk muscles. Although the patients had significantly earlier onset of deltoid muscle responses, this gave no functional protection because the arm movements were abnormally directed. Most instability in PD occurred for backward falls, with or without a roll component. Medication provided partial improvement in arm responses and trunk roll instability. CONCLUSIONS: Our results confirm previous findings in ankle muscles, and provide new information on balance impairments in hip, trunk, and arm responses in PD.

The influence of artificially increased hip and trunk stiffness on balance control in man.

Lightweight corsets were used to produce mid-body stiffening, rendering the hip and trunk joints practically inflexible. To examine the effect of this artificially increased stiffness on balance control, we perturbed the upright stance of young subjects (20-34 years of age) while they wore one of two types of corset or no corset at all. One type, the "half-corset", only increased hip stiffness, and the other, the "full-corset", increased stiffness of the hips and trunk. The perturbations consisted of combined roll and pitch rotations of the support surface (7.5 deg, 60 deg/s) in one of six different directions. Outcome measures were biomechanical responses of the legs, trunk, arms and head, and electromyographic (EMG) responses from leg, trunk, and upper arm muscles. With the full-corset, a decrease in forward stabilising trunk pitch rotation compared to the no-corset condition occurred for backward pitch tilts of the support surface. In contrast, the half-corset condition yielded increased forward trunk motion. Trunk backward pitch motion after forwards support-surface perturbations was the same for all corset conditions. Ankle torques and lower leg angle changes in the pitch direction were decreased for both corset conditions for forward pitch tilts of the support-surface but unaltered for backward tilts. Changes in trunk roll motion with increased stiffness were profound. After onset of a roll support-surface perturbation, the trunk rolled in the opposite direction to the support-surface tilt for the no-corset and half-corset conditions, but in the same direction as the tilt for the full-corset condition. Initial head roll angular accelerations (at 100 ms) were larger for the full-corset condition but in the same direction (opposite platform tilt) for all conditions. Arm roll movements were initially in the same direction as trunk movements, and were followed by large compensatory arm movements only for the full-corset condition. Leg muscle (soleus, peroneus longus, but not tibialis anterior) balance-correcting responses were reduced for roll and pitch tilts under both corset conditions. Responses in paraspinals were also reduced. These results indicate that young healthy normals cannot rapidly modify movement strategies sufficiently to account for changes in link flexibility following increases in hip and trunk stiffness. The changes in leg and trunk muscle responses failed to achieve a normal roll or pitch trunk end position at 700 ms (except for forward tilt rotations), even though head accelerations and trunk joint proprioception seemed to provide information on changed trunk movement profiles over the first 300 ms following the perturbation. The major adaptation to stiffness involved increased use of arm movements to regain stability. The major differences in trunk motion for the no-corset, half-corset and full-corset conditions support the concept of a multi-link pendulum with different control dynamics in the pitch and roll planes as a model of human stance. Stiffening of the hip and trunk increases the likelihood of a loss of balance laterally and/or backwards. Thus, these results may have implications for the elderly and others, with and without disease states, who stiffen for a variety of reasons.

Age-dependent variations in the directional sensitivity of balance corrections and compensatory arm movements in man.

We investigated the effects of ageing on balance corrections induced by sudden stance perturbations in different directions. Effects were examined in biomechanical and electromyographic (EMG) recordings from a total of 36 healthy subjects divided equally into three age groups (20-34, 35-55 and 60-75 years old). Perturbations consisted of six combinations of support-surface roll (laterally) and pitch (forward-backward) each with 7.5 deg amplitude (2 pure pitch, and 4 roll and pitch) delivered randomly. To reduce stimulus predictability further and to investigate scaling effects, perturbations were at either 30 or 60 deg s(-1). In the legs, trunk and arms we observed age-related changes in balance corrections. The changes that appeared in the lower leg responses included smaller stretch reflexes in soleus and larger reflexes in tibialis anterior of the elderly compared with the young. For all perturbation directions, onsets of balance correcting responses in these ankle muscles were delayed by 20-30 ms and initially had smaller amplitudes (between 120-220 ms) in the elderly. This reduced early activity was compensated by increased lower leg activity after 240 ms. These EMG changes were paralleled by comparable differences in ankle torque responses, which were initially (after 160 ms) smaller in the elderly, but subsequently greater (after 280 ms). Findings in the middle-aged group were generally intermediate between the young and the elderly groups. Comparable results were obtained for the two different stimulus velocities. Stimulus-induced trunk roll, but not trunk pitch, changed dramatically with increasing age. Young subjects responded with early large roll movements of the trunk in the opposite direction to platform roll. A similarly directed but reduced amplitude of trunk roll was observed in the middle-aged. The elderly had very little initial roll modulation and also had smaller stretch reflexes in paraspinals. Balance-correcting responses (over 120-220 ms) in gluteus medius and paraspinals were equally well tuned to roll in the elderly, as in the young, but were reduced in amplitude. Onset latencies were delayed with age in gluteus medius muscles. Following the onset of trunk and hip balance corrections, trunk roll was in the same direction as support-surface motion for all age groups and resulted in overall trunk roll towards the fall side in the elderly, but not in the young. Protective arm movements also changed with age. Initial arm roll movements were largest in the young, smaller in the middle aged, and smallest in the elderly. Initial arm roll movements were in the same direction as initial trunk motion in the young and middle aged. Thus initial roll arm movements in the elderly were directed oppositely to those in the young. Initial pitch motion of the arms was similar across age groups. Subsequent arm movements were related to the amplitude of deltoid muscle responses which commenced at 100 ms in the young and 20-30 ms later in the elderly. These deltoid muscle responses preceded additional arm roll motion which left the arms directed 'downhill' (in the direction of the fall) in the elderly, but 'uphill' (to counterbalance motion of the pelvis) in the young. We conclude that increased trunk roll stiffness is a key biomechanical change with age. This interferes with early compensatory trunk movements and leads to trunk displacements in the direction of the impending fall. The reversal of protective arm movements in the elderly may reflect an adaptive strategy to cushion the fall. The uniform delay and amplitude reduction of balance-correcting responses across many segments (legs, hips and arms) suggests a neurally based alteration in processing times and response modulation with age. Interestingly, the elderly compensated for these 'early abnormalities' with enlarged later responses in the legs, but no similar adaptation was noted in the arms and trunk. These changes with age provide an insight into possible mechanisms underlying falls in the elderly.