Speeding up gait initiation and gait-pattern with a startling stimulus

Biomechanical Analysis of the Unaffected Limb While Using a Hands-Free Crutch

Basic human ambulation relies on a bipedal gait, which has been reported to be directly related to quality of life. However, injuries to the lower limb can cause an inability to walk and require non-weightbearing periods to heal. Among the many ambulatory aids, standard axillary crutches are prescribed. However, due to the disadvantages of having to use both hands, a slow gait, pain, nerve damage, and gait patterns that differ from that of healthy subjects, currently, a new generation of ambulatory aids has emerged. Among such aids, hands-free crutches (HFCs) are of particular interest due to their form factor, which does not require the use of the hands and facilitates a bipedal gait. In this study, we present an assessment of whether any different gait patterns, compared to overground gait, appeared on the unaffected limb during walking with an HFC. The spatiotemporal parameters, plantar force, lower-limb joint angles, and EMG patterns were evaluated. In conclusion, the results from 10 healthy subjects suggest that wearing an HFC causes only slight changes in the biomechanical gait patterns examined in the unaffected limb compared with overground walking without an HFC.

Startle-induced rapid release of a gait initiation sequence in Parkinson's disease with freezing of gait

OBJECTIVE

Freezing of gait (FOG) in Parkinson's disease (PD) is characterized by the inability to initiate stepping, despite the intention to do so. This study used a startling acoustic stimulus paradigm to examine if the capacity to select, prepare and initiate gait under simple and choice reaction time conditions are impaired in people with PD and FOG.

METHODS

Thirty individuals (10 PD with FOG, 10 PD without FOG, and 10 controls) performed an instructed-delay gait initiation task under simple and choice reaction time conditions. In a subset of trials, a startle stimulus (124 dB) was presented 500 ms before the time of the imperative go-cue. Anticipatory postural adjustments preceding and accompanying gait initiation were quantified.

RESULTS

The presentation of a startling acoustic stimulus resulted in the rapid initiation of an anticipatory postural adjustment sequence during both the simple and choice reaction time tasks in all groups.

CONCLUSIONS

The neural capacity to prepare the spatial and temporal components of gait initiation remains intact in PD individuals with and without FOG.

SIGNIFICANCE

The retained capacity to prepare anticipatory postural adjustments in advance may explain why external sensory cues are effective in the facilitation of gait initiation in people with PD with FOG.

Does height-induced threat modulate shortening of reaction times induced by a loud stimulus in a lateral stepping and a wrist extension task?

INTRODUCTION

The StartReact (SR) effect is the accelerated release of a prepared movement when a startling acoustic stimulus is presented at the time of the imperative stimulus (IS). SR paradigms have been used to study defective control of balance and gait in people with neurological conditions, but differences in emotional state (e.g. fear of failure) may be a potential confounder when comparing patients to healthy subjects. In this study, we aimed to gain insight in the effects of postural threat on the SR effect by manipulating surface height during a postural (lateral step) task and a non-postural (wrist extension) task.

METHODS

Eleven healthy participants performed a lateral step perpendicular to the platform edge, and 19 participants performed a wrist extension task while standing at the platform edge. Participants initiated the movement as fast as possible in response to an IS that varied in intensity across trials (80 dB to 121 dB) at both low and high platform height (3.2 m). For the lateral step task, we determined anticipatory postural adjustments (APA) and step onset latencies. For the wrist extension task, muscle onset latencies were determined. We used Wilcoxon signed-rank tests on the relative onset latencies between both heights, to identify whether the effect of height was different for IS intensities between 103 and 118 dB compared to 121 dB.

RESULTS

For both tasks, onset latencies were significantly shortened at 121 dB compared to 80 dB, regardless of height. In the lateral step task, the effect of height was larger at 112 dB compared to 121 dB. The absolute onset latencies showed that at 112 dB there was no such stimulus intensity effect at high as seen at low surface height. In the wrist extension task, no differential effects of height could be demonstrated across IS intensities.

CONCLUSIONS

Postural threat had a significant, yet modest effect on shortening of RTs induced by a loud IS, with a mere 3 dB difference between standing on high versus low surface height. Interestingly, this effect of height was specific to the postural (i.e. lateral stepping) task, as no such differences could be demonstrated in the wrist extension task. This presumably reflects more cautious execution of the lateral step task when standing on height. The present findings suggest that applying stimuli of sufficiently high intensity (≥115 dB) appears to neutralize potential differences in emotional state when studying SR effects.

When reflex reactions oppose voluntary commands: The StartReact effect on eye opening

A startling auditory stimulus (SAS) induces a reflex response involving, among other reactions, a strong contraction of the orbicularis oculi muscle (OOc) and subsequent eye closure. A SAS also induces the StartReact effect, a significant shortening of reaction time in subjects ready for task execution. We examined the obvious conflict appearing when a StartReact paradigm requires participants with eyes closed to open their eyes to look for a visual target. We recorded OOc EMG activity and eyelid movements in healthy volunteers who were instructed to open their eyes at perception of a somatosensory imperative stimulus (IS) and locate the position of a Libet's clock's hand shown on a computer screen at 80 cm distance. In 6 out of 20 trials, we delivered a SAS simultaneously with the IS. The main outcome measures were reaction time at onset of eyelid movement and the time gap (TG) separating subjective assessment of the clock's hand position from real IS issuing. Control experiments included reaction time to eye closing and target location with eyes open to the same IS. Reaction time was significantly faster in SAS than in noSAS trials and slower for eye opening than for eye closing in both conditions. In the eye-opening task, TG was significantly shorter in SAS with respect to noSAS trials, despite the presence of the SAS-related burst in the OOc before EMG cessation. Our results indicate that the StartReact effect speeds up eye opening and location of a target in the visual field despite the startle reaction opposing the task.

Cortical Oscillations during Gait: Wouldn't Walking be so Automatic?

Gait is often considered as an automatic movement but cortical control seems necessary to adapt gait pattern with environmental constraints. In order to study cortical activity during real locomotion, electroencephalography (EEG) appears to be particularly appropriate. It is now possible to record changes in cortical neural synchronization/desynchronization during gait. Studying gait initiation is also of particular interest because it implies motor and cognitive cortical control to adequately perform a step. Time-frequency analysis enables to study induced changes in EEG activity in different frequency bands. Such analysis reflects cortical activity implied in stabilized gait control but also in more challenging tasks (obstacle crossing, changes in speed, dual tasks…). These spectral patterns are directly influenced by the walking context but, when analyzing gait with a more demanding attentional task, cortical areas other than the sensorimotor cortex (prefrontal, posterior parietal cortex, etc.) seem specifically implied. While the muscular activity of legs and cortical activity are coupled, the precise role of the motor cortex to control the level of muscular contraction according to the gait task remains debated. The decoding of this brain activity is a necessary step to build valid brain-computer interfaces able to generate gait artificially.

Walking With a Robotic Exoskeleton Does Not Mimic Natural Gait: A Within-Subjects Study

Background

Robotic exoskeleton devices enable individuals with lower extremity weakness to stand up and walk over ground with full weight-bearing and reciprocal gait. Limited information is available on how a robotic exoskeleton affects gait characteristics.

Objective

The purpose of this study was to examine whether wearing a robotic exoskeleton affects temporospatial parameters, kinematics, and muscle activity during gait.

Methods

The study was completed by 15 healthy adults (mean age 26.2 [SD 8.3] years; 6 males, 9 females). Each participant performed walking under 2 conditions: with and without wearing a robotic exoskeleton (EKSO). A 10-camera motion analysis system synchronized with 6 force plates and a surface electromyography (EMG) system captured temporospatial and kinematic gait parameters and lower extremity muscle activity. For each condition, data for 5 walking trials were collected and included for analysis.

Results

Differences were observed between the 2 conditions in temporospatial gait parameters of speed, stride length, and double-limb support time. When wearing EKSO, hip and ankle range of motion (ROM) were reduced and knee ROM increased during the stance phase. However, during the swing phase, knee and ankle ROM were reduced when wearing the exoskeleton bionic suit. When wearing EKSO, EMG activity decreased bilaterally in the stance phase for all muscle groups of the lower extremities and in the swing phase for the distal muscle groups (tibialis anterior and soleus) as well as the left medial hamstrings.

Conclusions

Wearing EKSO altered temporospatial gait parameters, lower extremity kinematics, and muscle activity during gait in healthy adults. EKSO appears to promote a type of gait that is disparate from normal gait in first-time users. More research is needed to determine the impact on gait training with EKSO in people with gait impairments.

StartReact during gait initiation reveals differential control of muscle activation and inhibition in patients with corticospinal degeneration

Corticospinal lesions cause impairments in voluntary motor control. Recent findings suggest that some degree of voluntary control may be taken over by a compensatory pathway involving the reticulospinal tract. In humans, evidence for this notion mainly comes from StartReact studies. StartReact is the acceleration of reaction times by a startling acoustic stimulus (SAS) simultaneously presented with the imperative stimulus. As previous StartReact studies mainly focused on isolated single-joint movements, the question remains whether the reticulospinal tract can also be utilized for controlling whole-body movements. To investigate reticulospinal control, we applied the StartReact paradigm during gait initiation in 12 healthy controls and 12 patients with ‘pure’ hereditary spastic paraplegia (HSP; i.e., retrograde axonal degeneration of corticospinal tract). Participants performed three consecutive steps in response to an imperative visual stimulus. In 25% of 16 trials a SAS was applied. We determined reaction times of muscle (de)activation, anticipatory postural adjustments (APA) and steps. Without SAS, we observed an overall delay in HSP patients compared to controls. Administration of the SAS accelerated tibialis anterior and rectus femoris onsets in both groups, but more so in HSP patients, resulting in (near-)normal latencies. Soleus offsets were accelerated in controls, but not in HSP patients. The SAS also accelerated APA and step reaction times in both groups, yet these did not normalize in the HSP patients. The reticulospinal tract is able to play a compensatory role in voluntary control of whole-body movements, but seems to lack the capacity to inhibit task-inappropriate muscle activity in patients with corticospinal lesions.

StartReact effects in first dorsal interosseous muscle are absent in a pinch task, but present when combined with elbow flexion

Objective

To provide a neurophysiological tool for assessing sensorimotor pathways, which may differ for those involving distal muscles in simple tasks from those involving distal muscles in a kinetic chain task, or proximal muscles in both.

Methods

We compared latencies and magnitudes of motor responses in a reaction time paradigm in a proximal (biceps brachii, BB) and a distal (first dorsal interosseous, FDI) muscle following electrical stimuli used as imperative signal (IS) delivered to the index finger. These stimuli were applied during different motor tasks: simple tasks involving either one muscle, e.g. flexing the elbow for BB (FLEX), or pinching a pen for FDI (PINCH); combined tasks engaging both muscles by pinching and flexing simultaneously (PINCH-FLEX). Stimuli were of varying intensity and occasionally elicited a startle response, and a StartReact effect.

Results

In BB, response latencies decreased gradually and response amplitudes increased progressively with increasing IS intensities for non-startling trials, while for trials containing startle responses, latencies were uniformly shortened and response amplitudes similarly augmented across all IS intensities in both FLEX and PINCH-FLEX. In FDI, response latencies decreased gradually and response amplitudes increased progressively with increasing IS intensities in both PINCH and PINCH-FLEX for non-startling trials, but, unlike in BB for the simple task, in PINCH for trials containing startle responses as well. In PINCH-FLEX, FDI latencies were uniformly shortened and amplitudes similarly increased across all stimulus intensities whenever startle signs were present.

Conclusions

Our results suggest the presence of different sensorimotor pathways supporting a dissociation between simple tasks that involve distal upper limb muscles (FDI in PINCH) from simple tasks involving proximal muscles (BB in FLEX), and combined tasks that engage both muscles (FDI and BB in PINCH-FLEX), all in accordance with differential importance in the control of movements by cortical and subcortical structures.

Significance

Simple assessment tools may provide useful information regarding the differential involvement of sensorimotor pathways in the control of both simple and combined tasks that engage proximal and distal muscles.

Neural Constraints Affect the Ability to Generate Hip Abduction Torques When Combined With Hip Extension or Ankle Plantarflexion in Chronic Hemiparetic Stroke

Stroke lesions interrupt descending corticofugal fibers that provide the volitional control of the upper and lower extremities. Despite the evident manifestation of movement impairments post-stroke during standing and gait, neural constraints in the ability to generate joint torque combinations in the lower extremities are not yet well determined. Twelve chronic hemiparetic participants and 8 age-matched control individuals participated in the present study. In an isometric setup, participants were instructed to combine submaximal hip extension or ankle plantarflexion torques with maximal hip abduction torques. Statistical analyses were run using linear mixed effects models. Results for the protocol combining hip extension and abduction indicate that participants post-stroke have severe limitations in the amount of hip abduction torque they can generate, dependent upon hip extension torque magnitude. These effects are manifested in the paretic extremity by the appearance of hip adduction torques instead of hip abduction at higher levels of hip extension. In the non-paretic extremity, significant reductions of hip abduction were also observed. In contrast, healthy control individuals were capable of combining varied levels of hip extension with maximal hip abduction. When combining ankle plantarflexion and hip abduction, only the paretic extremity showed reductions in the ability to generate hip abduction torques at increased levels of ankle plantarflexion. Our results provide insight into the neural mechanisms controlling the lower extremity post-stroke, supporting previously hypothesized increased reliance on postural brainstem motor pathways. These pathways have a greater dominance in the control of proximal joints (hip) compared to distal joints (ankle) and lead to synergistic activation of musculature due to their diffuse, bilateral connections at multiple spinal cord levels. We measured, for the first time, bilateral constraints in hip extension/abduction coupling in hemiparetic stroke, again in agreement with the expected increased reliance on bilateral brainstem motor pathways. Understanding of these neural constraints in the post-stroke lower extremities is key in the development of more effective rehabilitation interventions that target abnormal joint torque coupling patterns.

Contribution of the supplementary motor area and the cerebellum to the anticipatory postural adjustments and execution phases of human gait initiation

Several brain structures including the brainstem, the cerebellum and the frontal cortico-basal ganglia network, with the primary and premotor areas have been shown to participate in the functional organization of gait initiation and postural control in humans, but their respective roles remain poorly understood. The aim of this study was to better understand the role of the supplementary motor area (SMA) and posterior cerebellum in the gait initiation process. Gait initiation parameters were recorded in 22 controls both before and after continuous theta burst transcranial stimulation (cTBS) of the SMA and cerebellum, and were compared to sham stimulation, using a randomized double-blind design study. The two phases of gait initiation process were analyzed: anticipatory postural adjustments (APAs) and execution, with recordings of soleus and tibialis anterior muscles. Functional inhibition of the SMA led to a shortened APA phase duration with advanced and increased muscle activity; during execution, it also advanced muscle co-activation and decreased the duration of stance soleus activity. Cerebellar functional inhibition did not influence the APA phase duration and amplitude but increased muscle co-activation, it decreased execution duration and showed a trend to increase velocity, with increased swing soleus muscle duration and activity. The results suggest that the SMA contributes to both the timing and amplitude of the APAs with no influence on step execution and the posterior cerebellum in the coupling between the APAs and execution phases and leg muscle activity pattern during gait initiation.

Evidence for Startle Effects due to Externally Induced Lower Limb Movements: Implications in Neurorehabilitation

Passive limb displacement is routinely used to assess muscle tone. If we attempt to quantify muscle stiffness using mechanical devices, it is important to know whether kinematic stimuli are able to trigger startle reactions. Whether kinematic stimuli are able to elicit a startle reflex and to accelerate prepared voluntary movements (StartReact effect) has not been studied extensively to date. Eleven healthy subjects were suspended in an exoskeleton and were exposed to passive left knee flexion (KF) at three intensities, occasionally replaced by fast right KF. Upon perceiving the movement subjects were asked to perform right wrist extension (WE), assessed by extensor carpi radialis (ECR) electromyographic activity. ECR latencies were shortest in fast trials. Startle responses were present in most fast trials, yet being significantly accelerated and larger with right versus left KF, since the former occurred less frequently and thus less expectedly. Startle responses were associated with earlier and larger ECR responses (StartReact effect), with the largest effect again upon right KF. The results provide evidence that kinematic stimuli are able to elicit both startle reflexes and a StartReact effect, which depend on stimulus intensity and anticipation, as well as on the subjects' preparedness to respond.

The Neuro-Mechanical Processes That Underlie Goal-Directed Medio-Lateral APA during Gait Initiation

Gait initiation (GI) involves passing from bipedal to unipedal stance. It requires a rapid movement of the center of foot pressure (CoP) towards the future swing foot and of the center of mass (CoM) in the direction of the stance foot prior to the incoming step. This anticipatory postural adjustment (APA) allows disengaging the swing leg from the ground and establishing favorable conditions for stepping. This study aimed to describe the neuro-mechanical process that underlies the goal-directed medio-lateral (ML) APA. We hypothesized that controlled knee flexion of the stance leg contributes to the initial ML displacement of the CoP and to the calibration of the first step. Fourteen subjects initiated gait starting from three different initial stance widths of 15 cm (Small), 30 cm (Medium), and 45 cm (Large). Optoelectronic, force platform and electromyogram (EMG) measurements were performed. During APA, soleus activity diminished bilaterally, while tibialis anterior (TA) activity increased, more so in the stance leg than in the swing leg, and to a larger extent with increasing initial stance width. Knee flexion of the stance leg was observed during APA and correlated with the ML CoP displacement towards the swing leg. ML CoP and CoM displacements during APA increased with increasing stance width. The activity of stance-leg TA was correlated with the degree of knee flexion. Swing-leg tensor fasciae latae (TFL) was also active during APA. Across subjects, when stance-leg tibialis activity was low, TFL activity was large and vice versa. The modulation of the ML CoP position during APA allowed the gravity-driven torque to place the CoM just lateral to the stance foot during step execution. Accordingly, the gravity-driven torque, the ML CoM velocity during step execution, and the step width at foot contact (FC) were lower in the Small and greater in the Large condition. Consequently, the position of the stepping foot at FC remained close to the sagittal plane in all three conditions. Conclusively, coordinated activation of hip abductors and ankle dorsiflexors during APA displaces the CoP towards the swing leg, and sets the contact position for the swing foot.

Characteristic muscle activity patterns during gait initiation in the healthy younger and older adults

It is thought that gait initiation (GI) might be an optimal task for identifying postural control deficiencies. Thus, the aim of this study was to clarify the strategies adopted by older subjects during this task. 16 healthy younger and 15 healthy older adults participated in the study. Subjects were instructed to begin forward stepping with their dominant limb in response to an auditory stimulus. The mean muscle activity, co-contraction index, and intra-subject coefficients of variation (intra-subject CVs) of dominant limb muscles in different phases of GI were measured. The level of association between the co-contraction index and intra-subject CV of muscles was also explored. This study showed that in the anticipatory phase, the younger group had larger amplitudes and more intra-subject CVs than older the group, particularly for the tibialis anterior muscle. However, the co-contraction index was greater in older subjects relative to younger subjects. During the weight transition phase, tibialis anterior, semitendinosus and vastus lateralis muscles of older adults had a lower amplitude as compared to younger adults. However, during the locomotor phase, the activity of tibialis anterior was greater in comparison to younger adults. Also, during this phase, similar to the anticipatory phase, the co-contraction index between tibialis anterior and gastrocnemius muscles was greater in older subjects relative to younger subjects. Additionally, the larger co-contraction index of some muscles was associated with smaller intra-subject CV. These findings suggest that muscle behaviors are altered with aging and older adults employ different strategies in the different phases of GI as compared to younger adults.

Accessory stimulus modulates executive function during stepping task

When multiple sensory modalities are simultaneously presented, reaction time can be reduced while interference enlarges. The purpose of this research was to examine the effects of task-irrelevant acoustic accessory stimuli simultaneously presented with visual imperative stimuli on executive function during stepping. Executive functions were assessed by analyzing temporal events and errors in the initial weight transfer of the postural responses prior to a step (anticipatory postural adjustment errors). Eleven healthy young adults stepped forward in response to a visual stimulus. We applied a choice reaction time task and the Simon task, which consisted of congruent and incongruent conditions. Accessory stimuli were randomly presented with the visual stimuli. Compared with trials without accessory stimuli, the anticipatory postural adjustment error rates were higher in trials with accessory stimuli in the incongruent condition and the reaction times were shorter in trials with accessory stimuli in all the task conditions. Analyses after division of trials according to whether anticipatory postural adjustment error occurred or not revealed that the reaction times of trials with anticipatory postural adjustment errors were reduced more than those of trials without anticipatory postural adjustment errors in the incongruent condition. These results suggest that accessory stimuli modulate the initial motor programming of stepping by lowering decision threshold and exclusively under spatial incompatibility facilitate automatic response activation. The present findings advance the knowledge of intersensory judgment processes during stepping and may aid in the development of intervention and evaluation tools for individuals at risk of falls.

The StartReact effect in tasks requiring end-point accuracy

OBJECTIVE

Fast and accurate movements are often performed in response to a sensory signal. In reaction time tasks, execution of open loop movements is speeded up when a startling auditory stimulus (SAS) is applied together with the imperative signal (IS). In this study, we examined the effects of a SAS on the performance of a task that demands accuracy.

METHODS

Nine subjects were asked to move a monitored pen to a target point located in a table at a fixed angular distance of 30 degrees from a start point. The target was a spot of three possible diameters: 5, 10, and 20mm. Finger force for pen holding, pen tip pressure against the table and kinematic variables of the forearm movement were measured for three conditions: control, SAS delivered at IS (SAS-IS trials) and SAS delivered during movement execution (SAS-MOV trials).

RESULTS

Two movement phases could be identified in the movement trajectory and force profile. The first phase, ballistic, was significantly shortened in SAS-MOV trials, with earlier and larger peak velocity and peak force with respect to control trials. The second phase, slow approach to target, was longer in SAS-IS trials but not in SAS-MOV trials. Accuracy was maintained throughout all conditions and stimulation modes.

CONCLUSIONS

A SAS speeds up only the first (ballistic) part of the movement in an accuracy task. Slower target approach compensates for the accelerated initial movement. No changes in the last part of the movement are seen when a SAS is delivered after movement onset.

SIGNIFICANCE

The StartReact effect is restricted to the onset of a complex movement, when muscles are activated in a ballistic mode, without feedback.

The StartReact effect on self-initiated movements

Preparation of the motor system for movement execution involves an increase in excitability of motor pathways. In a reaction time task paradigm, a startling auditory stimulus (SAS) delivered together with the imperative signal (IS) shortens reaction time significantly. In self-generated tasks we considered that an appropriately timed SAS would have similar effects. Eight subjects performed a ballistic wrist extension in two blocks: reaction, in which they responded to a visual IS, and action, in which they moved when they wished within a predetermined time window. In 20-25% of the trials, a SAS was applied. We recorded electromyographic activity of wrist extension and wrist movement kinematic variables. No effects of SAS were observed in action trials when movement was performed before or long after SAS application. However, a cluster of action trials was observed within 200 ms after SAS. These trials showed larger EMG bursts, shorter movement time, shorter time to peak velocity, and higher peak velocity than other action trials (P < 0.001 for all), with no difference from Reaction trials containing SAS. The results show that SAS influences the execution of self-generated human actions as it does with preprogrammed reaction time tasks during the assumed building up of preparatory activity before execution of the willed motor action.

Intention detection of gait initiation using EMG and kinematic data

Gait initiation in transfemoral amputees (TFA) is different from non-amputees. This is mainly caused by the lack of stability and push-off from the prosthetic leg. Adding control and artificial push-off to the prosthesis may therefore be beneficial to TFA. In this study the feasibility of real-time intention detection of gait initiation was determined by mimicking the TFA situation in non-amputees. EMG and inertial sensor data was measured in 10 non-amputees. Only data available in TFA was used to determine if gait initiation can be predicted in time to control a transfemoral prosthesis to generate push-off and stability. Toe-off and heel-strike of the leading limb are important parameters to be detected, to control a prosthesis and to time push-off. The results show that toe-off and heel-strike of the leading limb can be detected using EMG and kinematic data in non-amputees 130-260 ms in advance. This leaves enough time to control a prosthesis. Based on these results we hypothesize that similar results can be found in TFA, allowing for adequate control of a prosthesis during gait initiation.

Preparedness for landing after a self-initiated fall

A startling auditory stimulus (SAS) causes a faster execution of voluntary actions when applied together with the imperative signal in reaction time tasks (the StartReact effect). However, speeding up reaction time may not be the best strategy in all tasks. After a self-initiated fall, the program for landing has to be time-locked to foot contact to avoid damage, and therefore advanced execution of the program would not be convenient. We examined the effects of SAS on the landing motor program in 8 healthy subjects that were requested to let themselves fall from platforms either 50 or 80 cm high at the perception of a visual imperative signal and land on specific targets. In trials at random, SAS was applied either together with the imperative signal (SAS(IS)) or at an appropriate prelanding time (SAS(PL)). As expected, the latency of takeoff was significantly shortened in SAS(IS) trials. On the contrary, the timing of foot contact was not significantly different for SAS(PL) compared with control trials. No changes were observed in the size of the electromyograph bursts in the two experimental conditions with respect to the control condition. Our results indicate that the landing program after a self-initiated fall may in part be organized at the time of takeoff and involve precise information on timing of muscle activation. Once launched, the program is protected against interferences by external inputs.

Anticipatory postural adjustments during step initiation: elicitation by auditory stimulation of differing intensities

Step initiation is associated with anticipatory postural adjustments (APAs) that vary according to the speed of the first step. When step initiation is elicited by a "go" signal (i.e. in a reaction time task), the presentation of an unpredictable, intense, acoustic startling stimulus (engaging a subcortical mechanism) simultaneously with or just before the imperative "go" signal is able to trigger early-phase APAs. The aim of the present study was to better understand the mechanisms underlying APAs during step initiation. We hypothesized that the early release of APAs by low-intensity, non-startling stimuli delivered long before an imperative "go" signal indicates the involvement of several different mechanisms in triggering APAs (and not just acoustic reflexes triggering brainstem structures). Fifteen healthy subjects were asked to respond to an imperative visual "go" signal by initiating a step with their right leg. A brief, binaural 40, 80 or 115 dB auditory stimulus was given 1.4 s before the "go" signal. Participants were instructed not to respond to the auditory stimulus. The centre of pressure trajectory and the electromyographic activity of the orbicularis oculi, sternocleidomastoid and tibialis anterior muscles were recorded. All three intensities of the auditory stimulus were able to evoke low-amplitude, short APAs without subsequent step execution. The louder the stimulus, the more frequent the elicitation. Depending on the intensity of the stimulus, APAs prior to step initiation can be triggered without the evocation of a startle response or an acoustic blink. Greater reaction times for these APAs were observed for non-startling stimuli. This observation suggested the involvement of pathways that did not involve the brainstem as a "prime mover".

The early release of planned movement by acoustic startle can be delayed by transcranial magnetic stimulation over the motor cortex

Previous studies have shown that preplanned movements can be rapidly released when a startling acoustic stimulus (SAS) is presented immediately prior to, or coincident with, the imperative signal to initiate movement. Based on the short latency of the onset of muscle activity (typically in less than 90 ms) and the frequent co-expression of startle responses in the neck and eye muscles, it has been proposed that the release of planned movements by a SAS is mediated by subcortical, possibly brainstem, pathways. However, a role for cortical structures in mediating these responses cannot be ruled out based on timing arguments alone. We examined the role of the cortex in the mediation of these responses by testing if a suprathreshold transcranial magnetic stimulation applied over the primary motor cortex, which suppresses voluntary drive and is known to delay movement initiation, could delay the release of movement by a SAS. Eight subjects performed an instructed-delay task requiring them to make a ballistic wrist movement to a target in response to an acoustic tone (control task condition). In a subset of trials subjects received one of the following: (1) suprathreshold TMS over the contralateral primary motor cortex 70 ms prior to their mean response time on control trials (TMS(CT)), (2) SAS 200 ms prior to the go cue (SAS), (3) suprathreshold TMS 70 ms prior to the mean SAS-evoked response time (TMS(SAS)), or (4) TMS(SAS) and SAS presented concurrently (TMS+SAS). Movement kinematics and EMG from the wrist extensors and flexors and sternocleidomastoid muscles were recorded. The application of TMS(CT) prior to control voluntary movements produced a significant delay in movement onset times (P < 0.001) (average delay = 37.7 ± 12.8 ms). The presentation of a SAS alone at -200 ms resulted in the release of the planned movement an average of 71.7 ± 2.7 ms after the startling stimulus. The early release of movement by a SAS was significantly delayed (P < 0.001, average delay = 35.0 ± 12.9 ms) when TMS(SAS) and SAS were presented concurrently. This delay could not be explained by a prolonged suppression of motor unit activity at the spinal level. These findings provide evidence that the release of targeted ballistic wrist movements by SAS is mediated, in part, by a fast conducting transcortical pathway via the primary motor cortex.

Motor preparation of spatially and temporally defined movements: evidence from startle

Previous research has shown that the preparation of a spatially targeted movement performed at maximal speed is different from that of a temporally constrained movement ( Gottlieb et al. 1989b ). In the current study, we directly examined preparation differences in temporally vs. spatially defined movements through the use of a startling stimulus and manipulation of the task goals. Participants performed arm extension movements to one of three spatial targets (20°, 40°, 60°) and an arm extension movement of 20° at three movement speeds (slow, moderate, fast). All movements were performed in a blocked, simple reaction time paradigm, with trials involving a startling stimulus (124 dB) interspersed randomly with control trials. As predicted, spatial movements were modulated by agonist duration and timed movements were modulated by agonist rise time. The startling stimulus triggered all movements at short latencies with a compression of the kinematic and electromyogram (EMG) profile such that they were performed faster than control trials. However, temporally constrained movements showed a differential effect of movement compression on startle trials such that the slowest movement showed the greatest temporal compression. The startling stimulus also decreased the relative timing between EMG bursts more for the 20° movement when it was defined by a temporal rather than spatial goal, which we attributed to the disruption of an internal timekeeper for the timed movements. These results confirm that temporally defined movements were prepared in a different manner from spatially defined movements and provide new information pertaining to these preparation differences.