Jean-Sebastien Blouin

When balancing upright, humans must differentiate self-motion generated by their own motor commands from that induced by external events. This dynamic process is required to generate balance-correcting responses and must take into account uncertainty in the sensorimotor control of balance induced by sensorimotor noise as well as sensing and actuation delays. In the present study, I characterized how humans adapt their control of standing balance when faced with uncertainty associated with sensorimotor delays. Twenty-two young healthy adults stood upright individually in a robotic balance simulator that allowed me to manipulate the delays between their self-generated ankle torques and resulting whole-body motion. Participants balanced in the anteroposterior direction with baseline delays (10ms) and adaption to imposed delays of 250ms for 20 minutes. I observed that the introduction of sensorimotor delays destabilized how participants balanced upright, most falling frequently within the first 5 minutes. Participants also exhibited an increase in lower leg muscles activation (4-12 times compared to pre-adaptation), and agonist-antagonist co-contractions (5-13 times compared pre-adaptation), immediately after the delay was introduced. Through exposure to the imposed delays, participants adapted their control of balance by minimizing whole-body motion variability from 10.64 ± 3.47 to 1.19± 0.64 ². The sway velocity variance at the end of the adaptation was 15 times greater than post-adaptation quiet standing. Similarly, through the adaptation, the gradual decrease in the muscle activation and muscle co-contraction was also observed. When removing the imposed delay (post-adaptation period), the sway variance in the participants whole-body motion quickly return to pre-adaptation values. Muscle activation and co-activation at post-adaptation was also lower at the post-adaptation trials compared to those observed during the late adaptations. The present findings show that humans adapt their control of balance by increasing muscle activation and co-contraction when faced with imposed sensorimotor delays and exhibit minimal (or brief) after-effects in motion variability when an imposed delay is removed. The results from this study provide initial insights to help us understand how humans adapt to the changes in sensorimotor delays they may experience throughout their lifespan or encounter a neurological disorder.

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While balancing upright, dynamic changes in body spatial representations occur as time progresses. Although space and time are often considered independent processes, I propose there is no separation of these dimensions in the brain as our sensors encode self-motion through neural action potentials changing properties (e.g. rate or timing) with respect to a constant time vector. I explored this possibility by characterizing how body dynamics and time influenced the perception and control of standing balance. First, simulations revealed that inertia- and viscosity-induced changes in balance control stability were similar to those induced by sensorimotor delays. I exposed 20 healthy participants to alterations in body inertia, viscosity and sensorimotor delays using a unique robotic system. Compared to baseline balance control, participants exhibited lower time within the virtual robotic limits (1.40-21.40% reduction), larger CoM average speed (1.22-4.09×) and sway velocity variance (2.43-38.86×) in the AP and ML directions with inertia below unity and negative viscosities, replicating the direction of balance effects induced by imposed sensorimotor delays. When asked to perceive how they balanced, participants matched their control of balance with 200ms imposed delay by choosing 0.50× (0.26-0.73) their inertia or -47.33× (-56.02 to -38.63) their viscosity. In addition, participants perceived that combining their control of balance with a 200ms imposed delay with either 3.30× (2.43-4.17) inertia or 53.05× (44.47-61.63) viscosity matched their control of balance in the control (no delay) condition. Based on these results, I exposed ten naïve participants to baseline balance control, 200ms imposed delay, 3.30× inertia with 200ms delay and 53.05× viscosity with 200ms delay conditions to determine if body dynamics could compensate for delay-induced instability in balance control. Compared to the 200ms delay condition, participants showed an increase in time in limits (29.20-32.40%) and a decrease in CoM average speed (76-81%) and sway velocity variance (90-97%) when balancing with a 200ms delay combined with body dynamics representing 3.30× their inertia or 53.05× their viscosity. The findings from this study provide novel insight into the way that the brain processes incoming sensory information to build internal representations of space and time for the perception and control of standing balance.

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Uncertainty is always present in the sensory information that we receive, worsening confidence in our self-motion estimates. This includes information from the vestibular system, which detects head motion in space and evokes whole-body balance responses during locomotion. These responses are attenuated with increasing step cadence and gait speed, but the neural mechanisms behind this modulation are not clear. One model suggests that the ratio (Vres) between the motor command variability and vestibular noise drives these changes. However, there is contradictory evidence concerning the physiological underpinnings of this model and its alignment with human behaviour. Twelve participants walked outside at step cadences 40-140% of their preferred cadence. We calculated coherence between electrical vestibular stimuli and mediolateral linear accelerations from inertial measurement units (IMUs) on the back, right ankle, and left ankle to infer the vestibular control of balance during locomotion. We also calculated Vres using the linear accelerations and angular velocities from an IMU on the head. We extracted peak coherences and mean Vres measures. To compare how these changed at faster step cadences, we performed paired t-tests between the 100, 120, and 140% cadence conditions and fitted exponential decay and 2nd degree polynomial functions to the data. Peak coherences decreased between the 100% and 140% cadence conditions (p 0.025) while most linear acceleration Vres measures increased at cadences faster than 100% (multiple p values 0.025). There were no significant changes to the angular velocity Vres measures (all p values > 0.025). Furthermore, the changes in peak coherence as a function of step cadence were best fitted to an exponential decay function (adjusted R²= 0.682-0.711) while the changes to the Vres were all best fitted to a polynomial (adjusted R² = 0.359-0.891), with minima near the preferred step cadence (91-123 steps/min). These results demonstrate that Vres does not predict vestibular control at faster step cadences and suggest that head kinematic variability may be optimized at the preferred cadence. This understanding has key implications for sensorimotor processing, which can inform computational modelling and how sensory information is impaired in clinical conditions.

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Background: The pelvic floor muscles (PFM) are important for maintaining continence and a potential therapeutic target for bladder management after spinal cord injury (SCI). Locomotor training could be beneficial for bladder outcomes in people with motor-complete SCI (mcSCI), but it remains unclear if reported improvements are related to the PFM. The PFM are co-activated with abdominal and gluteal muscles, and active during regular walking in able-bodied controls; however, we do not fully understand if exoskeletons used for gait rehabilitation that require more active engagement of the trunk muscles (e.g., the Ekso), could elicit more PFM activity compared to the Lokomat, which restricts trunk movement. Further, considering the invasiveness of the necessary procedures to record PFM activity, establishing the feasibility of the surface PFM electromyography (EMG) self-setup for dynamic tasks in controls will facilitate work in this area.Objectives: To (1) determine the feasibility of PFM EMG self-setup in controls; (2) characterize PFM activation patterns with respect to exoskeleton device (Lokomat vs. Ekso) in controls; and (3) explore the presence of PFM activity during exoskeleton-assisted walking in people with mcSCI.Methods: Eleven able-bodied adults and 3 SCI participants enrolled in this within-subject, cross-sectional study. We recorded EMG from the PFM and lower back, abdominal, gluteal, and leg muscles, as well as pelvis acceleration, during walking in the Lokomat and Ekso at different speeds. Control participants completed a survey on their experiences related to the PFM EMG self-setup. Results: In controls, all except one had a left-right difference in signal quality during walking with the PFM EMG self-setup. PFM activity was 53% and 63% higher during walking in the Ekso than Lokomat at the slow and fast speeds, respectively. Visual inspection revealed higher PFM EMG amplitude with greater pelvis acceleration and trunk and gluteal muscle activation. Both the Lokomat and Ekso elicited PFM activity in SCI participants.Conclusion: PFM EMG setup by a trained professional might be necessary to ensure signal quality during dynamic tasks. The Ekso is more effective in eliciting PFM activity in controls, and further investigation is needed to better understand this phenomenon in people with mcSCI.

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A central question to our understanding of postural control is the overall goal of standing balance. Current opinions of the topic diverge: researchers have argued that minimization of movement variability or overall exerted torque could be potential goals of balance control. The purposes of the thesis were to (1) model standing balance control using the Markov Decision Process framework and identify best parameter combinations that represent the physiological characteristics of standing and (2) probe the goal of standing using computational simulations and a custom-designed robotic balancing platform with altered standing balance dynamics. Human standing balance in the anterior-posterior direction was modeled using the Markov Decision Process framework, and the Q-learning algorithm was applied to solve the control problem. Performance of the model was evaluated by comparing the range, root mean square, mean power frequency and 99% power bandwidth of the simulated center of mass data with empirical evidence. In the experimental study, participants (n = 3) were asked to balance on the robotic balancing platform during perturbations in which torque bias terms were added to the load-stiffness relationship of standing. The exerted torque and body angle were recorded and analyzed. The simulated quiet standing behavior from the Markov Decision Process model resembled the frequency characteristics of standing with larger variability in the time series analysis. In the experimental study, two participants balanced at a more backward (forward) angle when positive (negative) torque bias terms were added, which matched the predictions from my hypothesis. However, the size of the angle shifts differed from the hypothesis and they did not maintain the same torque level as my hypothesis which predicts participants would maintain their torque. In conclusion, the Markov Decision Process model generated behavior close to human balance control given specific parameters. While the direction of body angle shifts observed in the human data and Markov Decision Process model simulated data matched the prediction from my hypothesis of torque minimization, the experimental results did not fully support the statement that people always seek to maintain their torque levels during standing.

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Whiplash injuries remain the most common injury associated with rear-end low-speed collisions despite improvements in head restraint designs and the introduction of innovative anti-whiplash seats. To address this problem, our research group developed an active car seat (RoboSeat) that controls seat hinge rotation and seatback cushion deformation. Preliminary experiments showed that the RoboSeat could reduce the kinematic and kinetic responses of an anthropometric test device. The aim of this study was to compare the performance of an actively controlled experimental anti-whiplash (RoboSeat) seat to passive control anti-whiplash seats in human volunteers: General Motor’s High Retention seat (GMHR) & Volvo’s Whiplash Protection Seat (WHIPS). Twelve healthy participants were exposed to a whiplash-like perturbation (4km/h speed change) while seated on each of the three seats. We recorded the electromyographic activity of sternocleidomastoid, neck paraspinals, splenius capitis, and multifidus along with the head/torso kinematics to quantify the participants’ responses to the whiplash-like perturbations. Given that excessive strain in cervical facet capsules is suggested as a cause of whiplash injury, we quantified cervical multifidus activation while head retraction occurred because both factors can potentially increase strains in the capsular ligaments. We hypothesized that the dynamic seatback rotation and cushion deformation of the RoboSeat prevents simultaneous activation of the neck multifidus muscle and head retraction. The RoboSeat reduced the combined multifidus activation and head retraction by 76% and 43% as well as 12/16 and 6/16 kinematic variables compared to the GMHR and WHIPS seats, respectively. Overall, our results suggest that the active seat can lower some head/torso kinematic responses compared to the current anti-whiplash seats and minimize combined activation of the multifidus muscle and head retraction. Our active RoboSeat represents a promising approach to potentially decrease the risk of whiplash injuries following low-speed rear-end collisions.

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Sensorimotor delays are inherent to the control of standing balance. Increases in sensorimotor delays observed during healthy aging and in people with Multiple Sclerosis (MS) have been theorized to reduce stability. The aim was to determine if balance stability declines with increasing artificial delays in balance control. Furthermore, I sought to determine whether there is an association between estimates of sensorimotor loop delays and the artificial delays at which people become unstable. Thirty healthy participants (19 to 75 years) and three participants with MS were recruited. A rotating platform elicited balance responses to pitch rotations. Surface electromyography recorded muscle activity from soleus (Sol), medial gastrocnemius (mGas), and tibialis anterior (TA) to determine onset latencies of balance responses, providing estimates of sensorimotor loop delays in balance control. A robotic balance simulator introduced artificial delays (50-400 ms) between the participants’ motor commands and resulting whole-body movements. Participants balanced for up to 20 seconds or until a ‘virtual fall’ occurred (exceeding 3° posterior or 6° anterior). Logistic regression analysis using the number of ‘virtual falls’ at each delay was performed to obtain a threshold of stability (5% probability of falling). Maintaining balance became more difficult as delays increased: the number of ‘virtual falls’ and standard deviation of angular backboard displacement increased, while the time to fall decreased as delays increased. The mean threshold of stability was 98 ± 38 ms. In response to the tilt perturbations, short and medium latency responses were observed in Sol (50.8 ± 3.2 ms) and TA (98 ± 10 ms), respectively. Long latency responses in TA, Sol and mGas were 147 ± 12 ms, 166 ± 22 ms, and 178 ± 27ms respectively. There were no correlations between thresholds of balance stability on the robot or the evoked-balance responses and age. A weak association between long latency responses in TA and thresholds of stability was observed. My results support the postulation that balance control becomes unstable as delays in balance control increase. The weak correlation between estimates of sensorimotor delays and thresholds of stability on the robot, however, suggests these measures target different pathways involved in standing balance.

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Adolescent Idiopathic Scoliosis (AIS) is a three-dimensional spinal deformity. One of the proposed causes of AIS is asymmetric vestibular function and the related descending drive to the spine musculature. Indeed, unilateral labyrinthectomy in tadpoles results in a spinal curvature similar to scoliosis. The objective of this study was to determine if asymmetric vestibular function is present in individuals with AIS. Ten individuals with AIS (8 females, 2 males) and ten healthy controls (8 females, 2 males) were exposed to 10s virtual rotations induced by monaural or binaural electrical vestibular stimulation (EVS), and 10s real rotations delivered by sitting atop a rotary chair. Using a forced-choice paradigm, participants indicated their perceived rotation direction (right or left). A Bayesian adaptive algorithm adjusted the stimulus intensity and direction to identify a stimulus level, which we called the vestibular recognition threshold, at which participants correctly identified the rotation direction 69% of the time. For unilateral vestibular stimuli (monaural EVS), the recognition thresholds were more asymmetric in all participants with AIS compared to control participants (1.16 vs 0.06 mA; p 0.001) but the recognition thresholds for bilateral vestibular stimuli did not differ between groups for both real and virtual rotations (multiple p > 0.05). No correlation was observed between the degree/side of the spine curvature and vestibular asymmetry in persons with AIS (p = 0.30). Our results demonstrate an asymmetry in vestibular function in individuals with AIS. Previous reports of semicircular canal orientation asymmetry in individuals with AIS could not explain this vestibular function asymmetry, suggesting a functional cause of the observed vestibular asymmetry. Vestibular function related to bilateral stimuli was well compensated, showing similar recognition thresholds across both groups of participants. The present results indicate that vestibular dysfunction is linked to AIS, potentially revealing a new path for the screening and monitoring of scoliosis in adolescents.

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Maintaining upright stance involves a time-critical process in which the central nervous system monitors postural orientation and modulates muscle activity accordingly. Visual, vestibular and somatosensory systems detect body motion that the balance controller utilizes to update standing control. The time delays between motor output and the resulting sensory feedback are expected and likely accommodated for. Consequently, we perceive whole-body movement as being a consequence of our own actions. Balance control, however, also involves processes that do not rely on conscious perception, allowing us to maintain standing balance almost effortlessly. Recent studies have demonstrated that both the perception and vestibular control of balance are modulated when sensory signals of whole-body movement do not match self-generated ankle torques. The aim of this thesis was to explore the temporal properties of the sensorimotor loops driving the perception and vestibular control of standing balance. Using a robotic balance simulator, experimentally-induced time delays were introduced between human participant’s ankle-produced torques and body movement. The first experiment used a psychophysical design to determine what delay is needed for humans to perceive a change in balance control. All participants were able to perceive a 300 ms delay with 100% success, with an average 69% correct threshold of 155 ms. In the second experiment, participants were exposed to a virtual vestibular perturbation while they balanced their body at different induced delays. Vestibular-evoked muscle responses attenuated with increasing loop delays, falling to amplitudes 84% smaller than baseline when a 500 ms delay was introduced between the produced torques and body movement. This is the first study to explore the time domain relationship between sensory and motor signals in standing, and the results reveal and describe temporal constraints of the sensorimotor control of balance. The present findings will act as springboard for studying postural control mechanisms in the future, encouraging the use of this robotic simulator to alter sensorimotor relationships during ongoing balance control. Using interventions like induced delays, we can decipher the natural processes that govern posture, and explore the adaptability and plasticity of these systems.

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A single head impact in sport can cause an acute concussion, whereas repetitive head impacts are suspected to cause chronic neurological impairment. However, the diagnostic accuracy of concussion assessment tools are not well understood and sparse research evidence exists regarding the neurological implications of repetitive head impacts. The objective of this thesis was to investigate repetitive head impacts, including impact detection technology and neurocognitive function, over the duration of a collegiate football season. Thirty-five healthy participants were recruited from a collegiate football program for a three-part study. Participants adhered an impact detection sensor (xPatch, X2 Biosystems) to their right mastoid process prior to each game and practice. As well, they completed a weekly battery of neurological testing that included the graded symptom checklist, standardized assessment of concussion, balance error scoring system and King-Devick test. In experiment 1, we investigated the accuracy of the xPatch to classify each detected event as an impact or non-impact. We matched each event to game video and assigned a true positive, false positive, true negative or false negative classification. The sensitivity of the sensor was 77.6%, specificity was 70.4% and overall accuracy was 75.1%. Additionally, we determined that impact count is strongly correlated to cumulative head kinematic load, i.e. cumulative linear acceleration (r²=0.98), cumulative rotational acceleration (r²=0.98) and cumulative rotational velocity (r²=0.99). In experiment 2, we explored the relationship between alterations in neurological status and repetitive head impact exposure using linear mixed models. The number of head impacts sustained was significantly related to the number and severity of symptoms in participants, but not to any other indicator of neurological status. In experiment 3, we investigated the diagnostic accuracy of each neurological test using receiver operating characteristic curves and corresponding area under the curve values. The diagnostic accuracy for the graded symptom checklist was high (0.76-0.93), King-Devick Test was moderate (0.64-0.80), standardized assessment of concussion and balance error scoring system were poor (0.47-0.71). In summary, this thesis identified limitations in current impact detection technology, provided evidence of a link between repetitive head impacts and symptomatology, and determined that the graded symptom checklist can accurately diagnose concussion.

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During standing balance, error signals delivered to the vestibular system through an electrical vestibular stimulus elicit compensatory muscle responses in the appendicular muscles involved in the control of standing. This response is only present in muscles that are active in balancing the whole-body but not present when muscle activity is unrelated to balancing the body. Previous work has shown that visual, vestibular, and proprioceptive cues that are congruent with efferent muscle signals through a robotic simulation of standing balance elicit this vestibular reflex response. The physiological connections between the visual, vestibular and neck somatosensory systems imply that congruency between any of the three sources of information and the efferent muscle signals could elicit a vestibular reflex response, but this has not been tested due to difficulties in isolating sensory feedback during standing balance. A newly constructed robotic balance simulator enabled an experimenter to control the congruency between afferent feedback and motor actions associated with standing balance. Here, we tested whether the vestibular reflexes rely on vestibular cues of self-motion being relevant to the control of standing balance. Eight healthy subjects maintained balance on the robotic balance simulator while vestibular cues of balance were minimized. To achieve this, the robotic balance simulator maintained the whole-body stable in space while providing visual and/or lower-limb somatosensory information that was congruent with the motor control of standing. It was shown that even without vestibular cues of self-motion, a vestibular reflex response can be elicited. These results will lead to a better understanding of the vestibular control of standing balance, and may be applicable to populations with balance deficits.

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Standing balance is an important unbiased indicator of concussion severity. However, limitedaccessibility to high-end technology and unreliability of simple balance assessment tools make it difficult to assess standing balance accurately outside of research laboratory settings. The objective of this thesis was to develop and validate a simple objective balance assessment tool that can provide an accurate, reliable, and affordable alternative to the currently available sideline methods. In Experiment 1, thirty healthy subjects were filmed performing the Balance Error Scoring System (BESS) while wearing inertial measurement units (IMUs) that measured linear accelerations and angular velocities from seven landmarks: forehead, chest, waist, right & left wrist, right & left shin. Each video was scored by four experienced BESS raters. Mean experienced rater scores were used to develop an algorithm to compute objective BESS (oBESS) scores solely from IMU data. oBESS was able to accurately fit and predict mean experiencedrater BESS scores using acceleration data from only one IMU located at the forehead. In Experiment 2, twenty healthy subjects wore the same network of IMUs and serially performed 12 BESS tests in a hypoxic altitude chamber, aimed at increasing the number of balance errors. Each video was scored by three experienced raters and two athletic trainers. Similarly to Experiment 1, experienced rater scores were used along with IMU data to develop the oBESS algorithm. However, because experienced raters displayed low inter-rater and intra-rater reliability, algorithm training and analyses were performed only using trials where the raters had marginal scoring differences. The oBESS was able to fit mean experienced rater scores with greater accuracy than the two athletic trainers, but not at a level commonly associated with high clinical reliability. In summary, this thesis shows that the oBESS can reliably predict total BESS

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The vestibular system conveys information regarding head motion to the central nervous system (CNS). Independently, this vestibular signal of head motion does not provide an absolute reference of head motion as the frequency coding of the afferent nerves is influenced by adaptation properties and nonlinearities. The optic flow signal of head rotation from the visual system however, is spatially encoded and can function as an absolute reference. The aim of this study was to determine if a visual signal of head rotation can recalibrate an altered vestibular signal of head motion during standing balance and to investigate the underlying mechanisms of this recalibration at the muscular level.Eight healthy subjects were exposed to an electrical vestibular stimulus correlated to head movement (±0.125 mA/°/s) while standing on foam with eyes closed. This velocity-coupled vestibular stimulation (VcVS) was applied in a bipolar, bilateral orientation and depending on its polarity, resulted in the vestibular nerves coding for slower or faster head movements. Initially, this alteration of natural vestibular information destabilized subjects. During the conditioning phase, subjects opened their eyes and used visual information in combination with the new vestibular information to update their representation of self-orientation. Following this, subjects showed a significant decrease (~35%) in body sway while still receiving VcVS. The mechanisms underlying vestibular recalibration were examined by observing how visuo-vestibular recalibration affected the vestibular-evoked muscular responses. Muscle activity was recorded in five subjects using surface electromyography (EMG) bilaterally on the medial gastrocnemius and tensor fascia latae muscles. Stochastic vestibular stimulation (SVS) in combination with VcVS was delivered to evoke biphasic muscular responses. Prior to the conditioning period, the peak amplitude of the response was significantly attenuated and then returned to control levels following conditioning. Overall, these observations indicate that the vestibular system can be recalibrated by a visual signal of head rotation. This process is associated with an initial decrease in vestibular-evoked muscular responses which return to control levels once recalibration occurs. These results suggest that the CNS can modulate vestibular processes by down regulation or selective gating of vestibular signals in order to achieve vestibular recalibration.

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Introduction: In British Columbia, whiplash injuries and its associated disorders are serious economical and social burdens to society. Despite affecting less than 1 percent of the population, whiplash injuries costs of over 850 million dollars annually (ICBC 2007). In recent studies, the startle response was shown to form part of the neuromuscular response to whiplash-like perturbations (Blouin et al. 2006a and b). In non-whiplash experiments, a weak or startling pre-stimulus tone presented before a subsequent startling stimulus can inhibit the startle response (Ison and Krauter 1974; Valls-Sole et al. 2005). The objective of the present study was to investigate how different pre-stimulus tones (weak and startling) affected the amplitude of muscle responses and the peak magnitude of head kinematics observed in human volunteers during whiplash-like perturbations. Methods: Twenty healthy subjects experienced five consecutive whiplash-like perturbations presented simultaneously with a loud collision sound (109 decibel (dB)). The three experimental conditions differed with the intensity of pre-stimuli tone presented 250 milliseconds prior to the onset of the perturbation: 1.) no pre-stimulus tone (Control), 2.) a weak pre-stimulus tone (85dB) and 3.) a startling pre-stimulus tone (105dB). Electromyography (EMG) of neck and distal limb muscles, and kinematics of the head and trunk were simultaneously collected. Mixed model ANOVAs and post-hoc Tukey’s honest significant difference test were used to analyze each EMG and kinematic variable (alpha=0.05). Results: Presenting a startling pre-stimulus tone before the whiplash-like perturbation decreased muscular (sternocleidomastoid: -16%, C4 paraspinal: -26%, biceps brachii: -66%, triceps brachii: -62%, first dorsal interosseous: -68%, and rectus femoris: -78%) and kinematic (peak retraction: -17%, peak horizontal acceleration of the head: -23%, and peak head angular acceleration in extension: -23%) responses from Control condition (p0.05). A weak pre-stimulus tone decreased only the muscular responses of triceps brachii (-38%), first dorsal interosseous (-48%) and rectus femoris (-57%) from Control condition (p0.01).Conclusion: A startling tone presented prior to a whiplash-like perturbation alters the head-neck responses in ways that are consistent with reducing neck tissue strains. This study is an initial step in the development of preventive devices to decrease the whiplash injury potential during low-speed, rear-end automotive collisions.

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