Threat effects on human oculo-motor function.

Neuro-anatomical evidence supports the potential for threat-related factors, such as fear, anxiety and vigilance, to influence brainstem motor nuclei controlling eye movements, as well as the vestibular nuclei. However, little is known about how threat influences human ocular responses, such as eye saccades (ES), smooth pursuit eye tracking (SP), and optokinetic nystagmus (OKN), and whether these responses can be facilitated above normal baseline levels with a natural source of threat. This study was designed to examine the effects of height-induced postural threat on the gain of ES, SP and OKN responses in humans. Twenty participants stood at two different surface heights while performing ES (ranging from 8° to 45° from center), SP (15, 20, 30°/s) and OKN (15, 30, 60°/s) responses in the horizontal plane. Height did not significantly increase the slope of the relationship between ES peak velocity and initial amplitude, or the gain of ES amplitude. In contrast height significantly increased SP and OKN gain. Significant correlations were found between changes in physiological arousal and OKN gain. Observations of changes with height in OKN and SP support neuro-anatomical evidence of threat-related mechanisms influencing both oculo-motor nuclei and vestibular reflex pathways. Although further study is warranted, the findings suggest that potential influences of fear, anxiety and arousal/alertness should be accounted for, or controlled, during clinical vestibular and oculo-motor testing.

Attentional requirements of postural control in people with spinal cord injury: the effect of dual task.

STUDY DESIGN: Cross-sectional study. OBJECTIVES: To investigate the attentional requirements for maintaining standing balance in people with spinal cord injury (SCI) using a dual-task paradigm and to compare standing balance performance between SCI and able-bodied (AB) controls. SETTING: LaboratoryMethods:Nine adults with incomplete SCI, who were able to stand unassisted were recruited, along with eight AB controls. Subjects performed a dual task involving counting backwards by 3 s out loud while standing with eyes open or closed. The primary outcome measures were the differences between SCI and control groups for movement reinvestment and the change in performance between single task and dual task for: (i) maximum standing time (STime); (ii) error ratio and total number of words uttered; and (iii) center of pressure measures. Perceptual measures included perceived mental workload, fear and confidence. RESULTS: SCI subjects stood for shorter duration during dual task (stand and count) than single task (stand) compared with controls during eyes closed. Significant differences between groups were observed for movement reinvestment, center of pressure, perceived mental effort, fear and confidence. No significant effects were observed for math-task performance. CONCLUSIONS: Total STime during eyes closed is adversely affected by the addition of a math task for SCI subjects. Perceptual measures appear to correspond to increases in postural sway and conscious control of standing in subjects with SCI. Individuals who can stand for >60 s with eyes closed do not appear to be significantly affected by the addition of a concurrent secondary task of minimal mental workload.

Physiological arousal accompanying postural responses to external perturbations after stroke.

OBJECTIVE: The purpose of this study was to examine simultaneously the level of physiological arousal and the postural response to external perturbations in people post-stroke compared to age-matched controls to build a more comprehensive understanding of the effect of stroke on postural control and balance self-efficacy. METHODS: Participants stood with each foot on separate force platforms. Ten applications of loads of 2% body weight at the hips perturbed the participant anteriorly under two conditions: investigator-triggered or self-triggered (total 20). Electrodermal activity (EDA; measurement of physiological arousal), electromyography (EMG) of the ankle plantarflexor muscles and anterior-posterior center of pressure measurements were taken pre-perturbation (anticipatory) and post-perturbation (response) and compared between the initial (first two) and final (last two) perturbations. RESULTS: Participants post-stroke demonstrated significantly higher levels of anticipatory EDA and anticipatory paretic plantarflexor EMG during both self- and investigator-triggered conditions compared to controls. Anticipatory EDA levels were higher in the final perturbations in participants post-stroke in both conditions, but not in controls. Habituation of the EDA responses post-perturbation was exhibited in the self-triggered perturbations in controls, but not in participants post-stroke. CONCLUSIONS: Physiological arousal and postural control strategies of controls revealed habituation in response to self-triggered perturbations, whereas this was not seen in participants post-stroke. SIGNIFICANCE: Understanding the physiological arousal response to challenges to standing balance post-stroke furthers our understanding of postural control mechanisms post-stroke.

Vestibulo-spinal and vestibulo-ocular reflexes are modulated when standing with increased postural threat.

We investigated how vestibulo-spinal reflexes (VSRs) and vestibulo-ocular reflexes (VORs) measured through vestibular evoked myogenic potentials (VEMPs) and video head impulse test (vHIT) outcomes, respectively, are modulated during standing under conditions of increased postural threat. Twenty-five healthy young adults stood quietly at low (0.8 m from the ground) and high (3.2 m) surface height conditions in two experiments. For the first experiment (n = 25) VEMPs were recorded with surface EMG from inferior oblique (IO), sternocleidomastoid (SCM), trapezius (TRP), and soleus (SOL) muscles in response to 256 air-conducted short tone bursts (125 dB SPL, 500 Hz, 4 ms) delivered via headphones. A subset of subjects (n = 19) also received horizontal and vertical head thrusts (∼150°/s) at each height in a separate session, comparing eye and head velocities by using a vHIT system for calculating the functional VOR gains. VEMP amplitudes (IO, TRP, SOL) and horizontal and vertical vHIT gains all increased with high surface height conditions (P < 0.05). Changes in IO and SCM VEMP amplitudes as well as horizontal vHIT gains were correlated with changes in electrodermal activity (ρ = 0.44-0.59, P < 0.05). VEMP amplitude for the IO also positively correlated with fear (ρ = 0.43, P = 0.03). Threat-induced anxiety, fear, and arousal have significant effects on VSR and VOR gains that can be observed in both physiological and functional outcome measures. These findings provide support for a potential central modulation of the vestibular nucleus complex through excitatory inputs from neural centers involved in processing fear, anxiety, arousal, and vigilance.

Increased gain of vestibulospinal potentials evoked in neck and leg muscles when standing under height-induced postural threat.

OBJECTIVE: To measure changes in amplitudes of vestibular evoked myogenic potentials (VEMPs) elicited from neck, upper and lower limb muscles during a quiet standing task with increased postural threat achieved by manipulating surface height. METHODS: Twenty eight subjects were tested while standing on a platform raised to 0.8 m and 3.2 m from the ground. Surface electromyography was recorded from the ipsilateral sternocleidomastoid (SCM), biceps brachii (BB), flexor carpi radialis (FCR), soleus (SOL) and medial gastrocnemius (MG) muscles. Stimulation was with air-conducted short tone bursts (4 ms). After controlling for background muscle activity, VEMP amplitudes were compared between heights and correlated with changes in state anxiety, fear and arousal. RESULTS: VEMP amplitude significantly increased in SCM (9%) and SOL (12.7%) with increased surface height (p<0.05). These modest increases in SCM VEMP amplitude were significantly correlated with anxiety (Rho=0.57, p=0.004) and confidence (Rho=-0.38, p=0.047) and those for SOL were significantly correlated with anxiety (Rho=0.33, p=0.049) and fear (Rho=0.36, p=0.037). CONCLUSION: Postural threat significantly increased vestibulospinal reflex (VSR) gains. Results demonstrate that VEMPs can be used to test different VSR pathways simultaneously during stance. Since fear and anxiety are prevalent with vestibular disorders, they should be considered as potential contributing factors for clinical vestibular outcome measures.

Diagnostic accuracy of common clinical tests for assessing abdominal muscle function after motor-complete spinal cord injury above T6.

STUDY DESIGN: Diagnostic study. OBJECTIVES: The objective of this study was to compare patterns of electromyography (EMG) recordings of abdominal muscle function in persons with motor-complete spinal cord injury (SCI) above T6 and in able-bodied controls, and to determine whether manual examination or ultrasound measures of muscle activation can be accurate alternatives to EMG. SETTING: Research center focused on SCI and University laboratory, Vancouver, Canada. METHODS: Thirteen people with SCI (11 with American Spinal Injury Association Impairment Scale (AIS) A and 2 AIS B; C4-T5), and 13 matched able-bodied participants volunteered for the study. Participants completed trunk tasks during manual examination of the abdominal muscles and then performed maximal voluntary isometric contractions, while EMG activity and muscle thickness changes were recorded. The frequency of muscle responses detected by manual examination and ultrasound were compared with detection by EMG (sensitivity and specificity). RESULTS: All individuals with SCI were able to elicit EMG activity above resting levels in at least one abdominal muscle during one task. In general, the activation pattern was task specific, confirming voluntary control of the muscles. Ultrasound, when compared with EMG, showed low sensitivity but was highly specific in its ability to detect preserved abdominal muscle function in persons with SCI. Conversely, manual examination was more sensitive than ultrasound but showed lower specificity. CONCLUSION: The results from this study confirm preserved voluntary abdominal muscle function in individuals classified with motor-complete SCI above T6 and highlight the need for further research in developing more accurate clinical measures to diagnose the level of trunk muscle preservation in individuals with SCI.

First trial and StartReact effects induced by balance perturbations to upright stance.

Postural responses (PR) to a balance perturbation differ between the first and subsequent perturbations. One explanation for this first trial effect is that perturbations act as startling stimuli that initiate a generalized startle response (GSR) as well as the PR. Startling stimuli, such as startling acoustic stimuli (SAS), are known to elicit GSRs, as well as a StartReact effect, in which prepared movements are initiated earlier by a startling stimulus. In this study, a StartReact effect paradigm was used to determine if balance perturbations can also act as startle stimuli. Subjects completed two blocks of simple reaction time trials involving wrist extension to a visual imperative stimulus (IS). Each block included 15 CONTROL trials that involved a warning cue and subsequent IS, followed by 10 repeated TEST trials, where either a SAS (TESTSAS) or a toes-up support-surface rotation (TESTPERT) was presented coincident with the IS. StartReact effects were observed during the first trial in both TESTSAS and TESTPERT conditions as evidenced by significantly earlier wrist movement and muscle onsets compared with CONTROL. Likewise, StartReact effects were observed in all repeated TESTSAS and TESTPERT trials. In contrast, GSRs in sternocleidomastoid and PRs were large in the first trial, but significantly attenuated over repeated presentation of the TESTPERT trials. Results suggest that balance perturbations can act as startling stimuli. Thus first trial effects are likely PRs which are superimposed with a GSR that is initially large, but habituates over time with repeated exposure to the startling influence of the balance perturbation.

Recruitment order of the abdominal muscles varies with postural task.

Abdominal muscle recruitment strategies in response to a postural perturbation contradict the theory that the deeper abdominal muscles are always recruited in advance of the more superficial muscles. The purpose of this study was to determine whether such contrasting muscle recruitment patterns are due to the postural task or the predictability of a postural task. Participants performed an arm raise task as well as an unpredictable and a predictable balance perturbation task (i.e. support-surface translation) while intramuscular electromyographic (EMG) recordings were obtained from the deep [transversus abdominis (TrA)] and superficial [obliquus externus (OE)] abdominal muscles. The abdominal muscle recruitment order was dependent on the postural task but not on the predictability of a postural perturbation. Whereas arm raises elicited similar EMG onset latencies in TrA and OE, the OE onset latency was 48 ms earlier than the TrA following an unpredictable translation (P = 0.003). The early OE activation persisted when the translation was made predictable to the participant (P = 0.024). These results provide evidence that the abdominal muscle recruitment order varies with the trunk stability requirements specific to each task. Rehabilitation strategies focusing on an early TrA activation to improve postural stability may not be appropriate for all everyday tasks.

Contributions to enhanced activity in rectus femoris in response to Lokomat-applied resistance.

The application of resistance during the swing phase of locomotion is a viable approach to enhance activity in the rectus femoris (RF) in patients with neurological damage. Increased muscle activity is also accompanied by changes in joint angle and stride frequency, consequently influencing joint angular velocity, making it difficult to attribute neuromuscular changes in RF to resistance. Thus, the purpose of this study was to evaluate the effects of resistance on RF activity while constraining joint trajectories. Participants walked in three resistance conditions; 0 % (no resistance), 5 and 10 % of their maximum voluntary contraction (MVC). Visual and auditory biofeedback was provided to help participants maintain the same knee joint angle and stride frequency as during baseline walking. Lower limb joint trajectories and RF activity were recorded. Increasing the resistance, while keeping joint trajectories constant with biofeedback, independently enhanced swing phase RF activity. Therefore, the observed effects in RF are related to resistance, independent of any changes in joint angle. Considering resistance also affects stride frequency, a second experiment was conducted to evaluate the independent effects of resistance and stride frequency on RF activity. Participants walked in four combinations of resistance at 0 and 10 %MVC and natural and slow stride frequency conditions. We observed significant increases in RF activity with increased resistance and decreased stride frequency, confirming the independent contribution of resistance on RF activity as well as the independent effect of stride frequency. Resistance and stride frequency may be key parameters in gait rehabilitation strategies where either of these may be manipulated to enhance swing phase flexor muscle activity in order to maximize rehabilitation outcomes.

Startle induces early initiation of classically conditioned postural responses.

Startling acoustic stimuli (SAS) induce the early release of prepared motor responses. The current study used SAS, in conjunction with a classical conditioning paradigm, to examine advanced motor preparation of conditioned postural responses (PRs). After generalized startle responses were induced, standing posture was perturbed in 2 blocks of 15 Conditioning trials, where in each trial the onset of a nonstartling auditory cue [i.e., a conditioned stimulus (CS)] preceded a leftward support-surface translation. Upon completion of each block, a single trial was conducted. After block 1, a CS-Only trial was used to induce conditioned PRs in the absence of balance perturbations. After block 2, a post-Conditioning Startle trial that involved a CS subsequently followed by a SAS was used to examine motor preparation of conditioned PRs. PRs were quantified in terms of center of pressure displacements, ankle and hip kinematics, as well as surface electromyography of proximal and distal bilateral muscle pairs. Results indicated that repeated experience with cued balance perturbations led to PR conditioning and, more importantly, motor preparation of PRs. Conditioning was evidenced in biomechanical and electromyographic responses observed in CS-Only trials, as well as the progressive changes to evoked response parameters during repeated Conditioning trials. SAS presented in post-Conditioning Startle trials evoked early onsets of biomechanical and electromyographic responses, while preserving relative response parameters that were each distinct from generalized startle responses. These results provide important insight into both the consequences of using cues in dynamic postural control studies and the neural mechanisms governing PRs.

Exploratory behavior during stance persists with visual feedback.

Recent evidence showing center of pressure (COP) displacements increase following an external stabilization of the center of mass (COM) supports the theory that postural sway may be exploratory and serve as a means of acquiring sensory information. The aim of the current study was to further test this theory and rule out potential confounding effects of sensory illusions or motor drift on prior observations. Participants stood as still as possible in an apparatus which allowed movements of the COM to be stabilized ("locked") without subject awareness, and they were provided real-time visual feedback of their COM or COP throughout the trial. If there was an influence of sensory illusions or motor drift, we hypothesized that the change in COP displacement with locking would be reduced when participants were provided visual confirmation of COM stabilization (COM feedback), or when they were aware of the position of the COP throughout the trial (COP feedback). Confirming our previous results, increases in COP displacement were observed when movements of the COM were stabilized. In addition, our results showed that increases in COP displacement could not be explained by the presence of sensory illusions or motor drift, since increases in COP were observed despite being provided convincing evidence that the COM had been stabilized, and when participants were aware of their COP position throughout the trial. These results provide further support for an exploratory role of postural sway. The theoretical basis of current clinical practices designed to deal with balance control deficits due to age or disease is typically based on the opinion that increases in sway are a consequence of a failing balance control system. Our results suggest that this may not be the case, and if sway is in fact exploratory, a serious re-evaluation of current clinical practices may be warranted.

Review of first trial responses in balance control: influence of vestibular loss and Parkinson's disease.

The reaction to an unexpected balance disturbance is unpracticed, often startling and frequently associated with falls. This everyday situation can be reproduced in an experimental setting by exposing standing humans to sudden, unexpected and controlled movements of a support surface. In this review, we focus on the responses to the very first balance perturbation, the so-called first trial reactions (FTRs). Detailed analysis of FTRs may have important implications, both for clinical practice (providing new insights into the pathophysiological mechanisms underlying accidental falls in real life) and for understanding human physiology (what triggers and mediates these FTRs, and what is the relation to startle responses?). Several aspects of the FTRs have become clear. FTRs are characterized by an exaggerated postural reaction, with large EMG responses and co-contracting muscles in multiple body segments. This balance reaction is associated with marked postural instability (greater body sway to the perturbation). When the same perturbation is repeated, the size of the postural response habituates and the instability disappears. Other issues about FTRs remain largely unresolved, and these are addressed here. First, the functional role of FTRs is discussed. It appears that FTRs produce primarily increased trunk flexion during the multi-segmental response to postural perturbations, thus producing instability. Second, we consider which sensory signals trigger and modulate FTRs, placing specific emphasis on the role of vestibular signals. Surprisingly, vestibular signals appear to have no triggering role, but vestibular loss leads to excessive upper body FTRs due to loss of the normal modulatory influence. Third, we address the question whether startle-like responses are contributing to FTRs triggered by proprioceptive signals. We explain why this issue is still unresolved, mainly because of methodological difficulties involved in separating FTRs from 'pure' startle responses. Fourth, we review new work about the influence of perturbation direction on FTRs. Recent work from our group shows that the largest FTRs are obtained for toe-up support surface rotations which perturb the COM in the posterior direction. This direction corresponds to the directional preponderance for falls seen both in the balance laboratory and in daily life. Finally, we briefly touch upon clinical diagnostic issues, addressing whether FTRs (as opposed to habituated responses) could provide a more ecologically valid perspective of postural instability in patients compared to healthy subjects. We conclude that FTRs are an important source of information about human balance performance, both in health and disease. Future studies should no longer discard FTRs, but routinely include these in their analyses. Particular emphasis should be placed on the link between FTRs and everyday balance performance (including falls), and on the possible role played by startle reactions in triggering or modulating FTRs.

Balance problems with Parkinson's disease: are they anxiety-dependent?

Non-motor symptoms, such as fear of falling and anxiety, are frequently reported in Parkinson's disease (PD). Recent evidence of anxiety and fear directly influencing balance control in healthy young and older adults, raises the question whether fear of falling and anxiety also directly contribute to the balance deficits observed in PD. The goal of the current study was to examine whether PD patients and controls responded similarly or differently to experimentally induced increases in anxiety. For this purpose, 14 PD patients (tested during a subjective optimal ON state) and 16 healthy age-matched control subjects stood in three conditions of different levels of postural threat: normal threat (quiet standing at ground level); medium threat (standing at the edge of a surface elevated to 80 cm); and high threat (same, but to 160 cm). Outcome measures included mean position, mean power of frequency (MPF) and root mean square (RMS) of centre of pressure (COP) displacements in the anterior-posterior (AP) and medial-lateral (ML) directions. Physiological and psychosocial measures of fear and anxiety were also recorded. Increased threat changed postural control similarly in PD patients and controls; MPF of AP and ML COP increased and the mean COP position was shifted backward in both groups. These results indicate that during the ON state, static balance in PD patients and controls is equally susceptible to the influence of anxiety. Significant correlations observed between COP changes and measures of fear and anxiety provide evidence to support the proposed neural links between structures controlling emotion and postural control. Future studies should further address this issue by including more severely affected patients, by testing the influence of dopaminergic medication, by including more anxious patients, and by using dynamic measures of balance.

Shifting the balance: evidence of an exploratory role for postural sway.

Humans and other species are unable to stand perfectly still; their bodies continuously sway during stance even during concentrated efforts to avoid such movement. Traditionally, this phenomenon has been viewed as an inability of the central nervous system (CNS) to maintain perfect equilibrium because of its reliance on feedback from sensory signals to control corrective ground-reaction forces. Using a novel method to minimize movements of the body during stance without subject awareness, we have made the unique discovery that ground-reaction forces are generated independent of body sway, as evidenced by observations of increased centre of pressure variability when postural sway is minimized experimentally. Contrary to traditional views, our results suggest that postural sway may be used by the CNS as an exploratory mechanism to ensure that continuous dynamic inputs are provided by multiple sensory systems. This novel paradigm has the potential to significantly shift long-standing views on balance, and questions the theoretical basis behind conventional treatment strategies for balance deficits associated with age and disease.

Does increased postural threat lead to more conscious control of posture?

Although it is well established that postural threat modifies postural control, little is known regarding the underlying mechanism(s) responsible for these changes. It is possible that changes in postural control under conditions of elevated postural threat result from a shift to a more conscious control of posture. The purpose of this study was to determine the influence of elevated postural threat on conscious control of posture and to determine the relationship between conscious control and postural control measures. Forty-eight healthy young adults stood on a force plate at two different surface heights: ground level (LOW) and 3.2-m above ground level (HIGH). Centre of pressure measures calculated in the anterior-posterior (AP) direction were mean position (AP-MP), root mean square (AP-RMS) and mean power frequency (AP-MPF). A modified state-specific version of the Movement Specific Reinvestment Scale was used to measure conscious motor processing (CMP) and movement self-consciousness (MSC). Balance confidence, fear of falling, perceived stability, and perceived and actual anxiety indicators were also collected. A significant effect of postural threat was found for movement reinvestment as participants reported more conscious control and a greater concern about their posture at the HIGH height. Significant correlations between CMP and MSC with AP-MP were observed as participants who consciously controlled and were more concerned for their posture leaned further away from the platform edge. It is possible that changes in movement reinvestment can influence specific aspects of posture (leaning) but other aspects may be immune to these changes (amplitude and frequency).

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.

Postural responses explored through classical conditioning.

The purpose of the study was to determine whether the central nervous system (CNS) requires the sensory feedback generated by balance perturbations in order to trigger postural responses (PRs). In Experiment 1, twenty-one participants experienced toes-up support-surface tilts in two blocks. Control blocks involved unexpected balance perturbations whereas an auditory tone cued the onset of balance perturbations in Conditioning blocks. A single Cue-Only trial followed each block (Cue-Only(Control) and Cue-Only(Conditioning) trials) in the absence of balance perturbations. Cue-Only(Conditioning) trials were used to determine whether postural perturbations were required in order to trigger PRs. Counter-balancing the order of Control and Conditioning blocks allowed Cue-Only(Control) trials to examine both the audio-spinal/acoustic startle effects of the auditory cue and the carryover effects of the initial conditioning procedure. In Experiment 2, six participants first experienced five consecutive Tone-Only trials that were followed by twenty-five conditioning trials. After conditioning, five Tone-Only trials were again presented consecutively to first elicit and then extinguish the conditioned PRs. Surface electromyography (EMG) recorded muscle activity in soleus (SOL), tibialis anterior (TA) and rectus femoris (RF). EMG onset latencies and amplitudes were calculated together with the onset latency, peak and time-to-peak of shank angular accelerations. Results indicated that an auditory cue could be conditioned to initiate PRs in multiple muscles without balance-relevant sensory triggers generated by balance perturbations. Postural synergies involving excitation of TA and RF and inhibition of SOL were observed following the Cue-Only(Conditioning) trials that resulted in shank angular accelerations in the direction required to counter the expected toes-up tilt. Postural synergies were triggered in response to the auditory cue even 15 min post-conditioning. Furthermore, conditioned PRs were quickly extinguished as participants became unresponsive by the third trial in extinction. In conclusion, our results reveal that the CNS does not require sensory feedback from postural perturbations in order to trigger PRs.

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.

Control of the triceps surae during the postural sway of quiet standing.

AIM: The present study investigated how the triceps surae are controlled at the spinal level during the naturally occurring postural sway of quiet standing. METHODS: Subjects stood on a force platform as electrical stimuli were applied to the posterior tibial nerve when the center of pressure (COP) was either 1.6 standard deviations anterior (COP(ant)) or posterior (COP(post)) to the mean baseline COP signal. Peak-to-peak amplitudes of the H-reflex and M-wave from the soleus (SOL) and medial gastrocnemius (MG) muscles were recorded to assess the efficacy of the Ia pathway. RESULTS: A significant increase in the H(max) : M(max) ratio for both the SOL (12 +/- 6%) and MG (23 +/- 6%) was observed during the COP(ant) as compared to the COP(post) condition. The source of the modulation between COP conditions cannot be determined from this study. However, the observed changes in the synaptic efficacy of the Ia pathway are unlikely to be simply a result of an altered level of background electromyographic activity in the triceps surae. This was indicated by the lack of differences observed in the H(max) : M(max) ratio when subjects stood without postural sway (via the use of a tilt table) at two levels of background activity. CONCLUSIONS: It is suggested that the phase-dependent modulation of the triceps surae H-reflexes during the postural sway of quiet standing functions to maintain upright stance and may explain the results from previous studies, which, until now, had not taken the influence of postural sway on the H-reflex into consideration.