Movement patterns underlying first trial responses in human balance corrections

Postural Responses to Achilles Tendon Vibration Depend on Feet Positioning

Mechanical vibration of the Achilles tendon is widely used to analyze the role of proprioception in postural control. The response to this tendon vibration (TV) has been analyzed in the upright posture, but the feet positions have varied in past research. Moreover, investigators have addressed only temporal parameters of the center of pressure (CoP). We investigated the effect of TV on both temporal and spectral characteristics of the CoP motion. Eighteen healthy young adults, stood barefoot, with one foot on each side of a dual platform, wearing glasses with opaque lenses. We applied 20 seconds of Achilles TV (bilaterally with inertial vibrators at a frequency of 80 Hz and an amplitude of .2-.5 mm). We analyzed CoP signals pre-vibration (PRE,4-seconds), during vibration (VIB,20 seconds), and after vibration cessation (REC,20 seconds). We repeated this protocol in natural and standardized positions (15° feet angular opening). For determining CoP amplitude and velocity, we divided the 20 seconds into five phases of four seconds each and calculated spectral parameters for the whole 20-second signals. There was an adaptation process in the speed of the CoP mediolateral (p < .01) and anteroposterior (p < .01) and in the displacement of the CoP anteroposterior (p < .01), with higher values in the VIB condition. Velocity and displacement decreased progressively in the REC condition. Median and peak frequencies were higher in the VIB condition when compared to the REC condition, but only in the mediolateral direction (p = .01). The standardized foot position led to increased speed in CoP mediolateral, anteroposterior, and mediolateral displacement (p < .01). CoP spectral characteristics were not affected by foot positioning. We concluded that adaptation of CoP motion in the presence of TV and after its cessation are observable both in time and frequency domains. Feet positioning influenced CoP motion in the presence of TV and after its cessation but it did not affect its spectral characteristics.

Systematic Review of the Importance of Hip Muscle Strength, Activation, and Structure in Balance and Mobility Tasks

OBJECTIVE

The aim of this systematic review was to identify the associations of the hip abductor muscle strength, structure, and neuromuscular activation on balance and mobility in younger, middle-aged, and older adults.

DATA SOURCES

We followed PRISMA guidelines and performed searches in PubMed, Embase, CINAHL, and Physiotherapy Evidence Database.

STUDY SELECTION

Study selection included: (1) studies with patients aged 18 years or older and (2) studies that measured hip abduction torque, surface electromyography, and/or muscle structure and compared these measures with balance or mobility outcomes.

DATA EXTRACTION

The extracted data included the study population, setting, sample size, sex, and measurement evaluated.

DATA SYNTHESIS

The present systematic review is composed of 59 research articles including a total of 2144 young, middle-aged, and older adults (1337 women). We found that hip abductor strength is critical for balance and mobility function, independent of age. Hip abductor neuromuscular activation is also important for balance and mobility, although it may differ across ages depending on the task. Finally, the amount of fat inside the muscle appears to be one of the important factors of muscle structure influencing balance.

CONCLUSIONS

In conclusion, a change in all investigated variables (hip abduction torque, neuromuscular activation, and intramuscular fat) appears to have an effect during balance or mobility tasks across age ranges and may elicit better performance. Future studies are necessary to confirm the effect of these variables across age ranges and the effects of interventions.

Vigor of reactive postural responses is set from feedback and feedforward processes

I explore a distinct perspective from that brought in the book by arguing that in postural control our organism selects the vigor of reactive responses guided by an optimization rule considering first the required postural response for balance recovery as indicated by afferent information from a myriad of sensory receptors, and second the history of previous responses to similar perturbations.

Compensatory control between the legs in automatic postural responses to stance perturbations under single-leg fatigue

In response to sudden perturbations of stance stability, muscles of both legs are activated for balance recovery. In conditions that one of the legs has a reduced capacity to respond, the opposite leg is predicted to compensate by responding more powerfully to restore stable upright stance. In this investigation, we aimed to evaluate between-leg compensatory control in automatic postural responses to sudden perturbations in a situation in which plantar flexor muscles of a single leg were fatigued. Young participants were evaluated in response to a series of perturbations inducing forward body sway, with a focus on activation of plantar flexor muscles: lateral and medial gastrocnemii and soleus. Muscular responses were analyzed through activation magnitude and latency of muscular activation onset. For evaluation of balance and postural stability, we also analyzed the center of pressure and upper trunk displacement and weight-bearing asymmetry between the legs. Responses were assessed in three conditions: pre-fatigue, under single-leg fatigue, and following the recovery of muscular function. Results showed (a) compensation of the non-fatigued leg through the increased magnitude of muscular activation in the first perturbation under fatigue; (b) adaptation in the non-fatigued leg over repetitive perturbations, with a progressive decrement of muscular activation over trials; and (c) maintenance of increased muscular activation of the non-fatigued leg following fatigue dissipation. These findings suggest that the central nervous system is able to modulate the descending motor drive individually for each leg's muscles apparently based on their potential contribution for the achievement of the behavioral aim of recovering stable body balance following stance perturbations.

High contextual interference in perturbation-based balance training leads to persistent and generalizable stability gains of compensatory limb movements

Reactive responses to balance perturbations have been shown to be improved by training. This investigation aimed to compare the effects of block and random training perturbation schedules on stability gains of compensatory arm and leg movements in response to unpredictable large-magnitude balance perturbations. Perturbations were produced by means of sudden displacements of the support base, associating mode (rotation, translation, combined), direction, and velocity of platform motion. Healthy young participants were assigned to one of three groups: random, block, and control. For the random group, perturbation sequence was unpredictable. For the block group, each balance perturbation was repeated over blocks of four trials. Controls were tested only, serving as reference of first trial responses in the post-test. Evaluation was made through a scale rating stability of compensatory arm and leg movements (CALM). We probed immediate and persistence gains (1-week retention), in addition to generalizability to perturbations of higher velocity and to dual-tasking (mental subtraction). In the post-test both the block and random groups achieved higher leg and global scores in comparison with controls in the most challenging perturbations. In retention and transfer tests, results for the global score indicated higher values for the random than for the block and control groups. These results support the conclusion that high but not low contextual interference in perturbation-based balance training leads to enduring and generalizable increased stability gains of compensatory limb movements in response to unpredictable balance perturbations.

Automatic postural responses are scaled from the association between online feedback and feedforward control

Generation of automatic postural responses (APRs) scaled to magnitude of unanticipated postural perturbations is required to recover upright body stability. In the current experiment, we aimed to evaluate the effect of previous postural perturbations on APR scaling under conditions in which the current perturbation is equal to or different from the previous perturbation load inducing unanticipated forward body sway. We hypothesized that the APR is scaled from the association of the current perturbation magnitude and postural responses to preceding perturbations. Evaluation was made by comparing postural responses in the contexts of progressive increasing versus decreasing magnitudes of perturbation loads. Perturbation was applied by unanticipatedly releasing a cable pulling the body backwards, with loads corresponding to 6%, 8% and 10% of body mass. We found that the increasing as compared to the decreasing load sequence led to lower values of (a) displacement and (b) velocity of center of pressure, and of activation rate of the muscle gastrocnemius medialis across loads. Muscular activation onset latency decreased as a function increasing loads, but no significant effects of load sequence were found. These results lead to the conclusion that APRs to unanticipated perturbations are scaled from the association of somatosensory feedback signaling balance instability with feedforward control from postural responses to previous perturbations.

Improvements in balance reaction impairments following reactive balance training in individuals with sub-acute stroke: A prospective cohort study with historical control

Background: Reactive balance training (RBT) has been previously found to reduce fall risk in individuals with sub-acute stroke; however, our understanding of the effects of RBT on specific balance impairments is lacking.Objective: To quantify changes in common balance reaction impairments in individuals with sub-acute stroke resulting from RBT, relative to traditional balance training, using a prospective cohort study design with a historical control group.Methods: Individuals with sub-acute stroke completed either RBT or traditional balance training as part of their routine care during physiotherapy in inpatient rehabilitation. Reactive balance control was assessed using lean-and-release perturbations pre-intervention, post-intervention, and 6-months post-intervention (follow-up). Individuals with impaired balance reactions (delayed foot-off times, slide steps, and/or a preference for stepping with the preferred limb) at the pre-intervention assessment were identified using video and force plate data. Outcome measures (foot-off times, frequency of trials with slide steps, and stepping with the preferred limb) from the RBT participants with impaired reactions were compared for each of the three assessments to the mean values for the participants with impaired reactions in the historical control group.Results: Improvements were observed in all outcome measures for the RBT participants between pre-intervention and post-intervention, and/or between post-intervention and follow-up. These improvements were generally equivalent to, if not better than, the improvements demonstrated by the historical control group.Conclusions: Findings further support the use of RBT for post-stroke inpatient rehabilitation, and provide insight into specific balance reaction impairments that are improved by RBT.

Instantaneous interjoint rescaling and adaptation to balance perturbation under muscular fatigue

Adaptation of automatic postural responses (APR) to balance perturbations might be thought to be impaired by muscle fatigue, given the associated proprioceptive and effector deficits. In this investigation, we aimed to evaluate the effect of muscular fatigue on APR adaptation over repetitive balance perturbations through support base backward translations. APR adaptation was evaluated in three epochs: (a) pre-fatigue; (b) post-fatigue, immediately following fatigue of the plantiflexor muscles through isometric contractions and (c) post-recovery, 30 min after the end of fatiguing activity. Results showed the following: (a) Decreasing amplitudes of joints' maximum excursion over repetitive perturbations in the three fatigue-related epochs. (b) Modulation of joints' excursion was observed in the first trial in the post-fatigue epoch. (c) In the post-fatigue epoch, we found interjoint rescaling, with greater amplitude of hip rotation associated with reduced amplitude of ankles' rotation. (d) Amplitudes of ankles' rotation were similar between the post-fatigue and post-recovery epochs. These findings lead to the conclusions that adaptation of automatic postural responses over repetitive trials was effective under focal muscular fatigue; modulation of the postural response took place in the first perturbation under fatigue, and generalization of response characteristics from post-fatigue to post-recovery suggests that feedforward processes in APRs generation are affected by the recent history of postural responses to stance perturbations.

Estimation of the Balance-Keeping Control Law Applied by a Human Being Upon a Sudden Sagittal Tilt Perturbation

This study aims to estimate the control law employed by the central nervous system (CNS) to keep a person in balance after a sudden disturbance. For this aim, several experiments were carried out, in which the subjects were perturbed sagittally by using a single-axis tilt-platform and their motions were recorded with appropriate sensors. The analysis of the experimental results leads to the contribution of this paper as a conjecture that the CNS commands the muscular actuators of the joints according to an adaptive proportional-derivative (PD) control law such that its gains and set points are updated continuously. This conjecture is accompanied with an assumption that the CNS is able to acquire perfect and almost instantaneous position and velocity feedback by means of a fusion of the signals coming from the proprioceptive, somatosensory, and vestibular systems. In order to verify the conjectured control law, an approximate biomechanical model was developed and several simulations were carried out to imitate the experimentally observed motions. The time variations of the set points and the control gains were estimated out of the experimental data. The simulated motions were observed to be considerably close to the experimental motions. Thus, the conjectured control law is validated. However, the experiments also indicate that the mentioned adaptation scheme is quite variable even for the same subject tested repeatedly with the same perturbation. In other words, this experimental study also leads to the implication that the way the CNS updates the control parameters is not quite predictable.

The Benefit of Combining Neuronal Feedback and Feed-Forward Control for Robustness in Step Down Perturbations of Simulated Human Walking Depends on the Muscle Function

It is often assumed that the spinal control of human locomotion combines feed-forward central pattern generation with sensory feedback via muscle reflexes. However, the actual contribution of each component to the generation and stabilization of gait is not well understood, as direct experimental evidence for either is difficult to obtain. We here investigate the relative contribution of the two components to gait stability in a simulation model of human walking. Specifically, we hypothesize that a simple linear combination of feedback and feed-forward control at the level of the spinal cord improves the reaction to unexpected step down perturbations. In previous work, we found preliminary evidence supporting this hypothesis when studying a very reduced model of rebounding behaviors. In the present work, we investigate if the evidence extends to a more realistic model of human walking. We revisit a model that has previously been published and relies on spinal feedback control to generate walking. We extend the control of this model with a feed-forward muscle activation pattern. The feed-forward pattern is recorded from the unperturbed feedback control output. We find that the improvement in the robustness of the walking model with respect to step down perturbations depends on the ratio between the two strategies and on the muscle to which they are applied. The results suggest that combining feed-forward and feedback control is not guaranteed to improve locomotion, as the beneficial effects are dependent on the muscle and its function during walking.

Young and older adults adapt automatic postural responses equivalently to repetitive perturbations but are unable to use predictive cueing to optimize recovery of balance stability

Processing of contextual cues has been proposed to modulate the generation of automatic postural responses to unanticipated balance perturbations. In this investigation, we compared young and older individuals in responses to sudden rotations of the support base inducing either planti- or dorsiflexion of the ankles. Assessment was made in conditions resulting from the combination of visual directional cueing of the forthcoming platform rotation, and block versus random sequences of platform rotation directions. Results showed that, for both rotation directions, the block sequence led to reduced magnitude of activation of distal agonist muscles and direction-specific modulation of ground reaction forces to recover body balance. Visual directional cueing, conversely, failed to modulate either muscular responses or forces applied to the support base through the feet for balance recovery. Effects were similar between ages, suggesting that aging does not increase the influence of cognition on the generation of automatic postural responses, and that adaptation to repeated postural perturbations over trials is preserved in healthy older individuals.

Disambiguating the cognitive and adaptive effects of contextual cues of an impending balance perturbation

Contextual cueing advancing the characteristics of an impending balance perturbation has been thought to induce optimized automatic postural responses. In this investigation, we aimed to disambiguate the cognitive and adaptive components of cueing a balance perturbation through the direction sequence of a series of base of support translations. We compared three experimental conditions: (a) block, with one perturbation cueing that the following one would be in the same direction; (b) serial, with one perturbation cueing that the following one would be in the opposite direction; and (c) random, representing a control uncued condition. Participants were instructed about the perturbation sequences. With this arrangement, at the cognitive level there was no directional uncertainty both in the block and serial sequences, while at the non-cognitive level only the block sequence was expected to lead to optimized responses in comparison to the random sequence. Results showed that the block sequence led to the generation of more stable automatic postural responses in comparison to the serial and random sequences, as indicated by lower amplitudes of body sway and lower velocity of center of pressure displacement. Increased balance stability in the block sequence was associated with longer delays of activation onset of leg distal muscles. Comparisons between the serial and random perturbation sequences failed to show any significant differences. These results indicate that optimized postural responses in the block sequence are due to adaptive processes underlying repetitive perturbations over trials rather than to processing of contextual cues at the cognitive level reducing uncertainty about characteristics of an impending perturbation.

Force direction patterns promote whole body stability even in hip-flexed walking, but not upper body stability in human upright walking

Directing the ground reaction forces to a focal point above the centre of mass of the whole body promotes whole body stability in human and animal gaits similar to a physical pendulum. Here we show that this is the case in human hip-flexed walking as well. For all upper body orientations (upright, 25°, 50°, maximum), the focal point was well above the centre of mass of the whole body, suggesting its general relevance for walking. Deviations of the forces' lines of action from the focal point increased with upper body inclination from 25 to 43 mm root mean square deviation (RMSD). With respect to the upper body in upright gait, the resulting force also passed near a focal point (17 mm RMSD between the net forces' lines of action and focal point), but this point was 18 cm below its centre of mass. While this behaviour mimics an unstable inverted pendulum, it leads to resulting torques of alternating sign in accordance with periodic upper body motion and probably provides for low metabolic cost of upright gait by keeping hip torques small. Stabilization of the upper body is a consequence of other mechanisms, e.g. hip reflexes or muscle preflexes.

Cognition and balance control: does processing of explicit contextual cues of impending perturbations modulate automatic postural responses?

Processing of predictive contextual cues of an impending perturbation is thought to induce adaptive postural responses. Cueing in previous research has been provided through repeated perturbations with a constant foreperiod. This experimental strategy confounds explicit predictive cueing with adaptation and non-specific properties of temporal cueing. Two experiments were performed to assess those factors separately. To perturb upright balance, the base of support was suddenly displaced backwards in three amplitudes: 5, 10 and 15 cm. In Experiment 1, we tested the effect of cueing the amplitude of the impending postural perturbation by means of visual signals, and the effect of adaptation to repeated exposures by comparing block versus random sequences of perturbation. In Experiment 2, we evaluated separately the effects of cueing the characteristics of an impending balance perturbation and cueing the timing of perturbation onset. Results from Experiment 1 showed that the block sequence of perturbations led to increased stability of automatic postural responses, and modulation of magnitude and onset latency of muscular responses. Results from Experiment 2 showed that only the condition cueing timing of platform translation onset led to increased balance stability and modulation of onset latency of muscular responses. Conversely, cueing platform displacement amplitude failed to induce any effects on automatic postural responses in both experiments. Our findings support the interpretation of improved postural responses via optimized sensorimotor processes, at the same time that cast doubt on the notion that cognitive processing of explicit contextual cues advancing the magnitude of an impending perturbation can preset adaptive postural responses.

Timing of reactive stepping among individuals with sub-acute stroke: effects of 'single-task' and 'dual-task' conditions

Performance decrements in balance tasks are often observed when a secondary cognitive task is performed simultaneously. This study aimed to determine whether increased cognitive load resulted in altered reactive stepping in individuals with sub-acute stroke, compared to a reactive stepping trial with no secondary task. The secondary purpose was to determine whether differences existed between the first usual-response trial, subsequent usual-response trials, and the dual-task condition. Individuals with sub-acute stroke were exposed to external perturbations to elicit reactive steps. Perturbations were performed under a usual-response (single-task) and dual-task condition. Measures of step timing and number of steps were based on force plate and video data, respectively; these measures were compared between the usual-response and dual-task trials, and between the first usual-response trial, later usual-response trials (trials 2-5) and a dual-task trial. A longer time of unloading onset and greater number of steps were identified for the first usual-response trial compared to later usual-response trials. No significant differences were identified between usual-response and dual-task trials. Although improvements were observed from the first to subsequent usual-response lean-and-release trials, performance then tended to decrease with the introduction of the dual-task condition. These findings suggest that when introduced after usual-response trials, the dual-task trial may represent the first trial of a new condition, which may be beneficial in reducing the potential for adaptation that may occur after multiple repetitions of a reactive stepping task. Therefore, these findings may lend support to the introduction of a new condition (i.e. a dual-task trial) in addition to usual-response trials when assessing reactive balance in individuals with stroke.

Calibration of the Leg Muscle Responses Elicited by Predictable Perturbations of Stance and the Effect of Vision

Motor adaptation due to task practice implies a gradual shift from deliberate control of behavior to automatic processing, which is less resource- and effort-demanding. This is true both for deliberate aiming movements and for more stereotyped movements such as locomotion and equilibrium maintenance. Balance control under persisting critical conditions would require large conscious and motor effort in the absence of gradual modification of the behavior. We defined time-course of kinematic and muscle features of the process of adaptation to repeated, predictable perturbations of balance eliciting both reflex and anticipatory responses. Fifty-nine sinusoidal (10 cm, 0.6 Hz) platform displacement cycles were administered to 10 subjects eyes-closed (EC) and eyes-open (EO). Head and Center of Mass (CoM) position, ankle angle and Tibialis Anterior (TA) and Soleus (Sol) EMG were assessed. EMG bursts were classified as reflex or anticipatory based on the relationship between burst amplitude and ankle angular velocity. Muscle activity decreased over time, to a much larger extent for TA than Sol. The attenuation was larger for the reflex than the anticipatory responses. Regardless of muscle activity attenuation, latency of muscle bursts and peak-to-peak CoM displacement did not change across perturbation cycles. Vision more than doubled speed and the amount of EMG adaptation particularly for TA activity, rapidly enhanced body segment coordination, and crucially reduced head displacement. The findings give new insight on the mode of amplitude- and time-modulation of motor output during adaptation in a balancing task, advocate a protocol for assessing flexibility of balance strategies, and provide a reference for addressing balance problems in patients with movement disorders.

First trial response to sudden support surface displacement: the effect of vestibular compensation

CONCLUSION

The effect of visual condition is more intense in the first trial response in normal subjects and patients and in last trial response only in patients. The first trial effect is more evident in compensated patients in the eyes open condition with any type of perturbation, and in non-compensated patients with the angular displacements in either visual condition.

OBJECTIVE

The study of body reaction to FTR can help to understand the complex mechanisms involved in the postural response and to develop new therapies to improve stability and prevent falls in unilateral vestibular deficit (UVD). This work describes the adaptation effect and the visual influence on the postural response to repetitive balance perturbation stimulus in normal subjects, compensated, and uncompensated UVD patients.

METHODS

The magnitude of displacement has been measured when the support surface is linearly or angularly displaced. The differences between results in the first and late trial, and the differences between the eyes open and eyes closed situation have been compared.

RESULTS

Compensated patients recover the adaptation ability to unexpected changes on the support surface through visual preference mechanism. Not compensated patients present hypermetric postural response with greater instability in the eyes open and eyes closed situations.

What startles tell us about control of posture and gait

Recently, there has been an increase in studies evaluating startle reflexes and StartReact, many in tasks involving postural control and gait. These studies have provided important new insights. First, several experiments indicate a superimposition of startle reflex activity on the postural response during unexpected balance perturbations. Overlap in the expression of startle reflexes and postural responses emphasizes the possibility of, at least partly, a common substrate for these two types of behavior. Second, it is recognized that the range of behaviors, susceptible to StartReact, has expanded considerably. Originally this work was concentrated on simple voluntary ballistic movements, but gait initiation, online step adjustments and postural responses can be initiated earlier by a startling stimulus as well, indicating advanced motor preparation of posture and gait. Third, recent experiments on StartReact using TMS and patients with corticospinal lesions suggest that this motor preparation involves a close interaction between cortical and subcortical structures. In this review, we provide a comprehensive overview on startle reflexes, StartReact, and their interaction with posture and gait.

Postural adjustments to support surface perturbations during reaching depend upon body-target reference frame

We investigated whether target position relative to the body modifies the postural adjustments produced when reaching movements are perturbed by unexpected displacements of the support surface. Eleven healthy participants reached to a target located at their midline, acromion height and at 130% their outstretched arm length. They stood on two force plates mounted on a moveable platform, capable of delivering horizontal forward ramp-and-hold perturbations. Three types of trial were given: reach only (R), perturbations only (P) and reaching movements during which a perturbation was given at a random delay after reach onset (RP). The target could be mounted either on a frame suspended from the ceiling such that it remained world-fixed (exocentric target, RP/X) or at an equivalent position on the moving platform so that it moved with the body (egocentric target, RP/E). Arm and body 3D kinematics and muscle activity from the right tibialis anterior (rTA) and soleus (rSOL) muscles were recorded. Normalised rTA activity was significantly lower in RP than in P trials. Furthermore, long-latency rTA muscle activity was lower in RP/E than in RP/X conditions when perturbations were given during either the arm deceleration phase of reaching. The rSOL muscle activity was lowest for the RP/E (arm deceleration) condition. When balance is perturbed during reaching, the manner in which the target moves relative to the body determines the muscle activity produced in the lower-limb muscles. Furthermore, a target that moves with the body requires a different regulation of muscle activity compared with one that moves independently of the body.

Adaptation to Coriolis perturbations of voluntary body sway transfers to preprogrammed fall-recovery behavior

In a rotating environment, goal-oriented voluntary movements are initially disrupted in trajectory and endpoint, due to movement-contingent Coriolis forces, but accuracy is regained with additional movements. We studied whether adaptation acquired in a voluntary, goal-oriented postural swaying task performed during constant-velocity counterclockwise rotation (10 RPM) carries over to recovery from falling induced using a hold and release (H&R) paradigm. In H&R, standing subjects actively resist a force applied to their chest, which when suddenly released results in a forward fall and activation of an automatic postural correction. We tested H&R postural recovery in subjects (n = 11) before and after they made voluntary fore-aft swaying movements during 20 trials of 25 s each, in a counterclockwise rotating room. Their voluntary sway about their ankles generated Coriolis forces that initially induced clockwise deviations of the intended body sway paths, but fore-aft sway was gradually restored over successive per-rotation trials, and a counterclockwise aftereffect occurred during postrotation attempts to sway fore-aft. In H&R trials, we examined the initial 10- to 150-ms periods of movement after release from the hold force, when voluntary corrections of movement path are not possible. Prerotation subjects fell directly forward, whereas postrotation their forward motion was deviated significantly counterclockwise. The postrotation deviations were in a direction consistent with an aftereffect reflecting persistence of a compensation acquired per-rotation for voluntary swaying movements. These findings show that control and adaptation mechanisms adjusting voluntary postural sway to the demands of a new force environment also influence the automatic recovery of posture.

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.