Muscle Stimulation via Whole Body Vibration for Postural Control Applications

Although the ability to balance might feel effortless to the most, it should not be given for granted as even the normal process of ageing can compromise it, jeopardising people physical independence. It is therefore important to implement safe training routines that ultimately improve postural control strategies. We evaluated the suitability of whole body vibration (WBV) training –which induces muscle contraction via the stimulation of muscle spindles - for postural control applications. First, we tested the efficacy of different combinations of stimulation frequency and subjects’ posture in eliciting a response from those muscles that play a key role for the implementation of postural responses. Each combination was evaluated by jointly measuring the resulting muscular activation and soft-tissue displacement. Then, we investigated how the selected WBV stimulation affected the balance of healthy subjects. We evaluated the latter by analysing centre of pressure trajectories, muscle and cortex activation and their respective interplay. We found that high frequency vibrations, delivered to participants standing on their forefeet, evoked the greatest contraction of the plantarflexors. Undisturbed balance recorded after such stimulation was characterised by an increased sensitivity of muscle spindles. In line with the latter, the communication between the periphery and the central nervous system (CNS) increased after the stimulation and different muscle recruitment patterns were employed to maintain balance. On the posturography side, stability was found to be compromised in the acute term but seemed to have recovered over a longer term. Together, these findings suggest that, if appropriately delivered, WBV has the potential to stimulate the spindles of the plantarflexors. By doing so, vibration training seems to be able to augment the communication between the proprioceptive organs and the CNS, on which the system relies to detect and react to perturbations, leading to sensorimotor recalibration

Towards Natural Control of Artificial Limbs

The use of implantable electrodes has been long thought as the solution for a more natural control of artificial limbs, as these offer access to long-term stable and physiologically appropriate sources of control, as well as the possibility to elicit appropriate sensory feedback via neurostimulation. Although these ideas have been explored since the 1960’s, the lack of a long-term stable human-machine interface has prevented the utilization of even the simplest implanted electrodes in clinically viable limb prostheses.In this thesis, a novel human-machine interface for bidirectional communication between implanted electrodes and the artificial limb was developed and clinically implemented. The long-term stability was achieved via osseointegration, which has been shown to provide stable skeletal attachment. By enhancing this technology as a communication gateway, the longest clinical implementation of prosthetic control sourced by implanted electrodes has been achieved, as well as the first in modern times. The first recipient has used it uninterruptedly in daily and professional activities for over one year. Prosthetic control was found to improve in resolution while requiring less muscular effort, as well as to be resilient to motion artifacts, limb position, and environmental conditions.In order to support this work, the literature was reviewed in search of reliable and safe neuromuscular electrodes that could be immediately used in humans. Additional work was conducted to improve the signal-to-noise ratio and increase the amount of information retrievable from extraneural recordings. Different signal processing and pattern recognition algorithms were investigated and further developed towards real-time and simultaneous prediction of limb movements. These algorithms were used to demonstrate that higher functionality could be restored by intuitive control of distal joints, and that such control remains viable over time when using epimysial electrodes. Lastly, the long-term viability of direct nerve stimulation to produce intuitive sensory feedback was also demonstrated.The possibility to permanently and reliably access implanted electrodes, thus making them viable for prosthetic control, is potentially the main contribution of this work. Furthermore, the opportunity to chronically record and stimulate the neuromuscular system offers new venues for the prediction of complex limb motions and increased understanding of somatosensory perception. Therefore, the technology developed here, combining stable attachment with permanent and reliable human-machine communication, is considered by the author as a critical step towards more functional artificial limbs

The Role of the Central Nervous System in the Integration of Proprioceptive Information

The proprioceptive system provides feedback on human performance that makes it possible to learn and perform novel tasks. Proprioception predominately arises in the peripheral nervous system at the muscle spindle organ. Mechanical stimulus such as vibration has been implicated in altering muscle spindle afferent signals used as feedback. Researchers have utilized this understanding to document gross performance changes resulting from a muscle spindle disruption paradigm. Findings of this work have demonstrated that the altered proprioceptive feedback alters performance both during and after vibration exposure. This has also led many to postulate that altered proprioceptive feedback due to environmental working conditions may be responsible for many incidences of musculoskeletal injury, including low back pain. In order to more fully understand how proprioceptive feedback is integrated into a motor response it was required to investigate activity within the central nervous system, itself the target of the spindle afferent, both before and after receiving a modulate afferent. We developed a protocol based on measures of average velocity to test for this activity. Our investigation began we examining whether or not average velocity, in the form of seated sway velocity, would be sensitive to applied vibration. We found that while vibration was applied; mean sway speed increased significantly above pre vibration levels, regardless of feedback and task difficulty. A computer based pursuit task was then implemented in order to investigate performance relative to timing of vibration exposure. Our results revealed a significant decrease in pursuit velocity during vibration from pre-vibration velocity. Additionally, subjects demonstrated an equal magnitude but opposite increase in pursuit speed after vibration was removed. This protocol was then replicated in a functional-MRI to compare the gross motor pursuit task performance with the corresponding BOLD imaging data. We observed a similar decrease/increase pattern of joystick pursuit velocity. The corresponding cortical activity revealed patterns of inhibition consistent with cognitive inhibition. The current findings support proprioception as a central inhibitory control mechanism that shifts cortical networks dependent on available sensory stimulus

Firing properties of muscle spindles supplying the intrinsic muscles of the foot in unloaded and free-standing humans

Human posture and locomotion are dependent on the sensory apparatus – involving muscle spindles, cutaneous afferents and the vestibular system – that provides proprioception. In my previous work with my Bachelor of Medical Research, I investigated the relationship between galvanic vestibular stimulation and the sensitivity of muscle spindles of the long muscles of the leg. While that study showed no correlation between these systems it was limited by the lack of subject postural threat. In order to record from muscle spindles directly during unsupported free-standing, a new methodology for microneurographic recording from the posterior tibial nerve at the ankle was developed. For the first time, we have been able to identify the firing properties of muscle spindle endings in the small (intrinsic) muscles of the foot, as well as mechanoreceptors in the skin of the sole, while the participant is standing unsupported. This thesis presents this methodology along with the recordings made. In Study 1, the firing properties of 26 muscle spindles supplying the intrinsic muscles of the foot are described in unloaded conditions. Their responsiveness to stretch and related joint movements is shown to be similar to those in the short muscles in the hand and the long leg muscles. Only 27% were spontaneously active, of which there was no consistent resting firing rate or discharge variability. In Study 2, activity from 12 muscle spindles supplying the intrinsic foot muscles in unsupported free-standing conditions is described. In this group 50% were spontaneously firing and 67% had activity correlated with changes of centre of pressure recorded by a force plate, primarily (88%) along the anteroposterior axis. In Study 3, the activity of 28 multiunit cutaneous afferent recordings, as well as of 15 single-unit cutaneous afferents, supplying the sole of the foot in unsupported free standing is described. Activity of cutaneous afferents was found to be dependent on receptor type and location of receptive field. The data presented in this report is proof of this novel methodology’s suitability for detailed study into the sensory sources in the foot contributing to maintaining the upright posture

Models and Analysis of Vocal Emissions for Biomedical Applications

The International Workshop on Models and Analysis of Vocal Emissions for Biomedical Applications (MAVEBA) came into being in 1999 from the particularly felt need of sharing know-how, objectives and results between areas that until then seemed quite distinct such as bioengineering, medicine and singing. MAVEBA deals with all aspects concerning the study of the human voice with applications ranging from the neonate to the adult and elderly. Over the years the initial issues have grown and spread also in other aspects of research such as occupational voice disorders, neurology, rehabilitation, image and video analysis. MAVEBA takes place every two years always in Firenze, Italy. This edition celebrates twenty years of uninterrupted and succesfully research in the field of voice analysis

Novel Bidirectional Body - Machine Interface to Control Upper Limb Prosthesis

Objective. The journey of a bionic prosthetic user is characterized by the opportunities and limitations involved in adopting a device (the prosthesis) that should enable activities of daily living (ADL). Within this context, experiencing a bionic hand as a functional (and, possibly, embodied) limb constitutes the premise for mitigating the risk of its abandonment through the continuous use of the device. To achieve such a result, different aspects must be considered for making the artificial limb an effective support for carrying out ADLs. Among them, intuitive and robust control is fundamental to improving amputees’ quality of life using upper limb prostheses. Still, as artificial proprioception is essential to perceive the prosthesis movement without constant visual attention, a good control framework may not be enough to restore practical functionality to the limb. To overcome this, bidirectional communication between the user and the prosthesis has been recently introduced and is a requirement of utmost importance in developing prosthetic hands. Indeed, closing the control loop between the user and a prosthesis by providing artificial sensory feedback is a fundamental step towards the complete restoration of the lost sensory-motor functions. Within my PhD work, I proposed the development of a more controllable and sensitive human-like hand prosthesis, i.e., the Hannes prosthetic hand, to improve its usability and effectiveness. Approach. To achieve the objectives of this thesis work, I developed a modular and scalable software and firmware architecture to control the Hannes prosthetic multi-Degree of Freedom (DoF) system and to fit all users’ needs (hand aperture, wrist rotation, and wrist flexion in different combinations). On top of this, I developed several Pattern Recognition (PR) algorithms to translate electromyographic (EMG) activity into complex movements. However, stability and repeatability were still unmet requirements in multi-DoF upper limb systems; hence, I started by investigating different strategies to produce a more robust control. To do this, EMG signals were collected from trans-radial amputees using an array of up to six sensors placed over the skin. Secondly, I developed a vibrotactile system to implement haptic feedback to restore proprioception and create a bidirectional connection between the user and the prosthesis. Similarly, I implemented an object stiffness detection to restore tactile sensation able to connect the user with the external word. This closed-loop control between EMG and vibration feedback is essential to implementing a Bidirectional Body - Machine Interface to impact amputees’ daily life strongly. For each of these three activities: (i) implementation of robust pattern recognition control algorithms, (ii) restoration of proprioception, and (iii) restoration of the feeling of the grasped object's stiffness, I performed a study where data from healthy subjects and amputees was collected, in order to demonstrate the efficacy and usability of my implementations. In each study, I evaluated both the algorithms and the subjects’ ability to use the prosthesis by means of the F1Score parameter (offline) and the Target Achievement Control test-TAC (online). With this test, I analyzed the error rate, path efficiency, and time efficiency in completing different tasks. Main results. Among the several tested methods for Pattern Recognition, the Non-Linear Logistic Regression (NLR) resulted to be the best algorithm in terms of F1Score (99%, robustness), whereas the minimum number of electrodes needed for its functioning was determined to be 4 in the conducted offline analyses. Further, I demonstrated that its low computational burden allowed its implementation and integration on a microcontroller running at a sampling frequency of 300Hz (efficiency). Finally, the online implementation allowed the subject to simultaneously control the Hannes prosthesis DoFs, in a bioinspired and human-like way. In addition, I performed further tests with the same NLR-based control by endowing it with closed-loop proprioceptive feedback. In this scenario, the results achieved during the TAC test obtained an error rate of 15% and a path efficiency of 60% in experiments where no sources of information were available (no visual and no audio feedback). Such results demonstrated an improvement in the controllability of the system with an impact on user experience. Significance. The obtained results confirmed the hypothesis of improving robustness and efficiency of a prosthetic control thanks to of the implemented closed-loop approach. The bidirectional communication between the user and the prosthesis is capable to restore the loss of sensory functionality, with promising implications on direct translation in the clinical practice

Primate Motor Cortex: Individual and Ensemble Neuron-Muscle Output Relationships

The specific aims of this study were to: 1) investigate the encoding of forelimb muscle activity timing and magnitude by corticomotoneuronal (CM) cells, 2) test the stability of primary motor cortex (M1) output to forelimb muscles under different task conditions, and 3) characterize input/output relationships associated with different intracortical microstimulation (ICMS) methods. Neuronal recording and stimulating methods were used in combination with electromyographic (EMG) recording of 24 forelimb muscles to investigate questions related to M1 control of forelimb muscles. Target muscles of CM neurons were identified by the presence of post-spike facilitation (PSpF) in spike-triggered averages (SpTA) of EMG activity. Post-stimulus output effects were obtained with three different ICMS methods; stimulus-triggered averaging (StTA) of EMG activity, repetitive short duration ICMS (RS-ICMS) and repetitive long duration ICMS (RL-ICMS). Our results demonstrate that CM cells exhibit strong and consistent coactivation with their target muscles. Further, the summed activity of populations of identified CM cells was a better predictor of the common muscle's EMG activity than individual neurons. Our data support the view that M1 output encodes muscle activation related parameters. Regarding stability, we found that output effects in StTAs of EMG activity are remarkably stable and largely independent of changes in joint angle, or limb posture. This further validates the use of StTA for mapping and other studies of cortical motor output. RL-ICMS evoked EMG activity was also stable in sign, strength and distribution independent of starting position of the hand. Our data support a model in which RL-ICMS produces sustained co-activation of multiple agonist and antagonist muscles which then generates joint movements according to the length-tension properties of the muscles until an equilibrium position is achieved. Further, RL-ICMS evoked EMG activity did not sum with the existing level of activity; rather the stimulus forced a new EMG level that was independent of existing voluntary background. Our results further show that post-stimulus output effects on muscle activity obtained with StTA and RS-ICMS closely resemble one another. However, RL-ICMS produces effects that can deviate substantially from those observed with StTA

Models and Analysis of Vocal Emissions for Biomedical Applications

The International Workshop on Models and Analysis of Vocal Emissions for Biomedical Applications (MAVEBA) came into being in 1999 from the particularly felt need of sharing know-how, objectives and results between areas that until then seemed quite distinct such as bioengineering, medicine and singing. MAVEBA deals with all aspects concerning the study of the human voice with applications ranging from the neonate to the adult and elderly. Over the years the initial issues have grown and spread also in other aspects of research such as occupational voice disorders, neurology, rehabilitation, image and video analysis. MAVEBA takes place every two years always in Firenze, Italy

Multimodal assessment of neonatal pain

Pain assessment is critical to prevent suffering and harm in infants admitted to the neonatal care unit. As pain is a subjective experience, its assessment in nonverbal infants relies on surrogate measures. Current infant pain assessment tools that are based on behaviour and autonomic nervous system measurements lack face validity — they are unlikely to reflect pain in all its dimensions. In recent years, EEG-derived measures of pain have been developed in late preterm and term infants. Multimodal tools which include these cerebral measurements are conceptually more appropriate to measure pain. Yet, their use is still limited to specific research applications. This thesis focuses on outstanding questions that need to be addressed in order to advance the development of multimodal pain assessment tools that incorporate cerebral measurements. In the first part of this thesis, I focus on the characterisation of preterm infants’ noxious-evoked responses and their development. Across several modalities, premature infants have dampened or altered responsiveness compared to term infants, and it is uncertain if these responses can be reliably discriminated from tactile-evoked responses. In particular, a discriminative pattern of noxious-evoked EEG activity that is present in term infants, is unlikely to be present in preterm infants. In addition, it is unclear how noxious-evoked responses, especially brainderived responses, change with age. In this thesis, I use a classification model to show that infants aged 28–40 weeks postmenstrual age display discriminable multimodal responses to a noxious clinical procedure and a tactile control procedure, and I provide examples of how a such a model could be used in clinical trials of analgesics. I show that noxious-evoked responses change magnitude and morphology across this age range, and that discriminative brain activity emerges in early prematurity. In the second part of this thesis, I focus on improving the neuroscientific validity of a noxious-evoked EEG response measured at the cot-side, as the spatial neural correlates of these responses are still poorly understood. I present an EEG-fMRI pilot study to investigate the spatial neural correlates of inter-individual differences in noxious-evoked EEG responses and provide recommendations for a larger follow-up study. Overall, this thesis provides a characterisation of infants’ noxious-evoked responses and their development across multiple modalities, a crucial next step in improving multimodal neonatal pain assessment