Browsing by Author "John, Lester"

Now showing 1 - 10 of 10
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    Open Access
    Design and implementation of an apparatus for hydrodynamic and fatigue testing of prosthetic aortic valves
    (2008) Krynauw, Hugo; Marais, Stephan; John, Lester
    Aortic valve replacement in humans may be needed due to pathology leading to valve stenosis and regurgitation. Replacement is by either mechanical or soft tissue prosthetic valves. Before new valves are medically approved and introduced into the market they are required to undergo rigorous testing to verify performance and product life expectancy. Performance testing is done in a hydrodynamic test apparatus and life expectancy verified in an accelerated test apparatus. The Cardiology Department at Tygerberg Hospital has proposed a project for the design and implementation of a prosthetic aortic valve test apparatus. This device is to be used primarily for fatigue, but also limited hydrodynamic, testing of prosthetic heart valves. The design of the test apparatus was based on the four-element Windkessel model of the arterial system. This simple lumped parameter electrical analogy of the arterial system takes aortic and arterial resistance, arterial compliance, and blood inertance into account to simulate total arterial impedance. This model was developed with physiological reference and thus the element parameters only hold for physiological simulation as the equation governing impedance is speed sensitive. The model was adapted to provide theoretidal, physiological loads from physiological speeds of 60BPM through to accelerated speeds up to 1OOOBPM through mathematical optimisation of the Windkessel.The test apparatus was designed and built taking into account the varying Windkessel parameters where possible. Both compliance and resistance could be varied within an acceptable range, inertance however, could not be varied due to the limitations of the project. The apparatus was controlled and pressures on either side of the valve monitored with a LabView® graphical user interface. The apparatus was able to mimic in vivo closely and satisfied the ISO requirements for valve testing up to speeds of 230BPM. Various modifications are proposed to both the Windkessel model and the physical apparatus to compensate for hydrodynamic effects at high testing speeds in improve performance, as well as increase the maximum testing speed.
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    Open Access
    Detection of mental task related EEG for brain computer interface implementation (using SVM classification approach)
    (2007) Lin, Tsu-Hui Angel; John, Lester; Tapson, J
    Brain computer interface (BCI) technology provides a method of communication and control for people with severe motor disabilities. This thesis explores the application of a Fast Fourier transform and support vector machine (FFT-SVM) to the problem of mental task detection in EEG-based brain computer interface implementation.
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    Open Access
    The effect of shoulder pain on the neuromuscular activity of the scapular stabilizing muscles
    (2005) Oliver, Delphine; Gibson, Alan St Clair; Gray, Janine; John, Lester; Taliep, Sharhidd
    Neuromuscular activity of the scapular stabilizing muscles in subjects with and and without chronic shoulder impingement syndrome. To examine differences in neuromuscular activity of the scapular stabilizing muscles in subjects with and without chronic shoulder impingement syndrome during an abduction movement of the shoulder.
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    Open Access
    An investigation into the kinematic, cortical electrical and visual search strategies of skilled and less-skilled cricket batsmen in response to projected video footage of a medium-fast bowler
    (2008) Taliep, Mogammad Sharhidd; John, Lester
    The aim of this dissertation was to investigate various systems affecting performance in skilled and less-skilled batsmen. These systems included kinetics, visual perception, psychophysiology and visual tracking. Individual chapter abstracts have been presented because of the multifaceted nature of this dissertation.
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    Open Access
    Investigation of differences in cortical activation during wrist flexion and extension performed under real, passive and motor imagined paradigms
    (2016) Stoeckigt, Stefan; John, Lester; Franz, Thomas; Douglas, Tania S
    The neuromuscular control comparison between flexion and extension of the upper extremities has been conducted in a number of studies. It has been speculated that differences in the corticospinal pathway between flexion and extension may play a role in the cortical difference detected between flexion and extension, resulting in higher cortical activation for extension. However, it is still unclear as to what roles these pathways play, and to what degree other factors (muscle force activation, sensory feedback, frequency of movement, structural and/or functional differences) might influence the cortical activation in the brain. It has been speculated that the difference in cortical muscular pathways is due to flexion movements being used more often in day to day activities, therefore requiring less cortical activation for that movement. Through the investigation of the cortical differences present during different movement types, a deeper understanding into the differences between flexion and extension may be obtained. No previous study has compared the cortical differences between flexion and extension of the upper extremities during different movement types. In this study, an offline investigation is conducted between wrist flexion and extension, during real, passive and motor imaginary movement with the help of a servo controlled hand device. Simultaneous recording of EEG, EMG and wrist dynamics (velocity, angle, strain) were made on fifteen healthy right handed subjects performing 60 randomized repartitions of right wrist flexion and extension, for kinaesthetic motor imaginary, passively moved, and voluntary real active movements. Real movements were conducted at 10% relative subject maximum voluntary contraction (MVC). A servo controlled hand device was used to regulate dynamic force applied for real movements, and provide motion during passive movements. The use of different movement types with the aid of a servo controlled hand device, may give a deeper understanding into the effects of muscle force activation, rate of movement and corticospinal pathway on flexion and extension. In order to investigate the cortical differences between flexion and extension, subjects perceived difficulty, movement dynamics, movement related cortical potential (MRCP), event related desynchronization and synchronization (ERD/ERS), and phase locking value (PLV) were measured. Each measurement examines a different aspect of the cortical activation present in the brain, during the different movement types. Although relative muscle force activation between wrist real flexion and extension was similar, the motor cortex activation during extension was higher than during flexion, by MRCP and mu-band ERD, with subjects also perceiving real wrist extension to be more difficult to perform. Passive movements found higher motor cortex activation for flexion (MRCP, beta-band ERD), however higher somatosensory cortical activation was present during extension, by mu-band ERS and PLV. Motor imagined wrist flexion showed higher cortical activation during wrist flexion, by MRCP and beta-band ERD. Although numerous variables were tested (each in difference frequency bands), with some being significant and others being non-significant, overall it can be suggested that there was higher cortical activation for extension. The higher cortical activation during wrist extension movements may be due to corticospinal and somatosensory motor control pathways to motor neuron and from sensory neuron pools for extensor/flexor muscle and muscle spindle of the upper extremities. This investigation contributes to the current literature relating to cortical differences between flexion and extension of the upper extremities, by including the real, passive and motor imaginary differences between flexion and extension.
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    Non-invasive detection of the electromyographic activity of the deep extrinsic thumb muscles using surface electrodes
    (2015) Pitman, Jeremy David; John, Lester
    Motivation: Conventional surface electromyography (EMG) methods cannot be used to detect deep muscle activation. A new non-invasive superficial and deep muscle EMG (sdEMG) technique has recently been used to derive the EMG activity of Brachialis and Tibialis Posterior muscles in the upper and lower limb respectively. The aim of the present study was to apply a modified version of sdEMG to the forearm to detect EMG activity of the deep extrinsic thumb muscles Flexor Pollicis Longus (FPL), Extensor Pollicis Longus (EPL), Extensor Pollicis Brevis (EPB) and Abductor Pollicis Longus (APL) using surface electrodes. Methods: High density monopolar EMG was detected from 2 concentric rings, each consisting of 20 custom designed and manufactured silver electrodes, placed at the distal and proximal thirds of the right forearm of 15 healthy male participants. The EMG signals were recorded by a custom synthesised from open source components, EMG amplifier system interfacing with a custom designed LabVIEW® program. The participants performed 10 repetitions of isometric thumb flexion (TFl), thumb extension (TEx), thumb abduction (TAb), thumb adduction (TAd), index finger flexion (IFFl) and index finger extension (IFEx). Each isometric contraction was performed in a randomized order at a standardized effort level of 30% of the participant's maximum voluntary contraction (verified by a custom designed and built thumb dynamometer). The Independent Component Analysis (ICA) algorithm, fastICA, was used to un-mix the 40 monopolar EMG waveforms (containing EMG activity attributable to both superficial and deep muscles) into 40 constitutive components, known as the Independent Components (ICs). The activation envelope of the ICs was found using a 250ms RMS smoothing filter and normalized between 0 and 1. A contraction sequence specific predicted EMG waveform based on intramuscular measurements (from existing studies in the literature) was created for each deep muscle and correlated with the processed ICs using Pearson's Correlation Coefficient (r). The ICs were ranked according to the corresponding r value and the highest r ranked IC for each muscle was considered to represent the recovered EMG activity from that particular muscle. Finally, a per sample basis accuracy, sensitivity and specificity analysis was conducted between each deep muscle's predicted EMG and highest r ranked IC at different activation thresholds. A linear mixed-effects statistical model was used to find the overall accuracy, sensitivity and specificity values over all the thresholds for each deep muscle. Results: Overall correlations of 0.81 for FPL (D), 0.88 for EPL (D), 0.92 for EPB (D) and 0.83 for APL (D) (p<0.001 for all muscles) were found between the predicted EMG waveforms and ICs. Using an activation threshold of 3 standard deviations above a resting baseline level, statistically significant (p<0.001) accuracy, sensitivity and specificity measures were found between the predicted EMG waveforms and top r ranked ICs for each of the deep muscles. The values of the 3 statistical measures (accuracy, sensitivity, specificity) for each of the deep muscles were: FPL (0.76, 0.88, 0.70); EPL (0.87, 0.85, 0.91); EPB (0.94, 0.93, 0.94); APL (0.80, 0.87, 0.87). Conclusions: The results indicate that this is the first non-invasive detection of the EMG activity of FPL (D), EPL (D), EPB (D) and APL (D). The ability to detect movement intention as a result of activation from these muscles may be of use for robot based targeted rehabilitation of the hand or in the control of prosthetic hand devices.
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    Open Access
    An offline multi-class auditory P300 brain-computer interface using principal and independent component analysis
    (2011) Bentley, Alexander Simon Jeremy; John, Lester
    This thesis investigated a multi-class auditory P300 BCI as a step towards FES applicability. A multi-class P300 paradigm approach provides degrees-of-freedom in operating an FES device over the traditional P300 paradigm. Accuracy in classification of target P300s contributes to the paradigm's applicability in a 'real' environment. The computational effectiveness of the paradigm can be enhanced through signal processing prior to classification. A combination of principal component analysis (PCA) and independent component analysis (ICA), together with a method of enhancing the P300 properties through temporal and spatial manipulation are investigated as a means of improving classification accuracy. The combination of these techniques and the use of a multi-class P300 paradigm presents a different approach as a step towards FES applicability in an auditory BCI.
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    Towards an algorithm for the prediction of non-contact anterior cruciate ligament injuries
    (2015) Fickling, Shaun Dean; John, Lester; Sivarasu, Sudesh
    Background: The anterior cruciate ligament (ACL) of the knee is one of the most frequently injured ligaments in the body. 70% of ACL injuries are sustained without any direct contact to the knee, during the early stance phase of a rapid deceleration movement. Females have a significantly greater risk of injury than males participating in the same activities. In the years following injury, ACL deficient individuals are likely to experience lasting joint pain, functional instabilities and the onset of osteoarthritis. The best practice model for management of ACL injuries is a continued emphasis on prevention, which is currently limited by an incomplete understanding of how the injuries occur. Hypothesis: Body biomechanics occurring during the terminal swing phase of a dynamic deceleration movement can predict the resulting weight acceptance phase ACL loading in both ligament bundles. This will further the understanding of the sequence of events that result in non - con tact ACL injuries. Methods: For a preliminary feasibility study, a musculoskeletal model was developed in OpenSim incorporating both anteromedial (AMB) and posterolateral (PLB) bundles of the ACL. Motion capture data of female soccer players (n = 10, mean age = 19.60 ± 1.49 years) performing unanticipated side - step cutting movements were recorded. Instantaneous, three dimensional joint angles and angular velocities at the mid - swing stage of the side - step were selected as the independent variables. The dependent variables were the maximum stance - phase AMB and PLB strains. Multiple pairwise correlation analyses were used to quantify linear relationships between these variables. To evaluate the overall potential to predict ACL strain, a best subsets linear regression model was implemented using only the significantly correlated independent variables. Each ligament bundle was analysed independently. Results: Hip internal rotation at the mid - swing stage explained 79.1% (95% CI: 59.9% - 98.2%) of the variance in maximum stance - phase anteromedial bundle strain (p = 0.0006). Mid - swing knee varus position and knee valgus velocity combined explained 83.3% (95% CI: 69.2% - 97.3%) of the variance in maximum stance - phase posterolateral bundle strain (p = 0.0019). Conclusions: Swing - phase body kinematics during a side - step movement can provide meaningful predictive information as to the future strain in both bundles of the ACL. They are thus useful components in understanding and exploring elements of the inciting e vent, particularly a kinematic "sequence of no return" that directly precedes the injury. The results validate continued research in this area, where the iv relationships identified in this preliminary investigation can guide the development of a priori hypotheses for future studies to be completed at higher levels of evidence. Clinical Relevance: A more comprehensive understanding of the variables that result in non - contact ACL injuries will allow for the design and implementation of more effective preventative measures. For example, knowledge of the "sequence of no return" could be used in sophisticated statistical systems to predict ACL injury events in real - time. This could be used to trigger an active knee brace to apply external support to the knee, pre venting damage to the ligament. The long - term outcome of this project is to move towards reducing the risk and incidence of ACL injuries and the associated negative effects, preserving knee - vitality and ensuring quality of life for athletes and active individuals.
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    Towards improved visual stimulus discrimination in an SSVEP BCI
    (2010) Hodgskiss, Dean Leslie; John, Lester
    The dissertation investigated the influence of stimulus characteristics, electroencephalographic (EEG) electrode location and three signal processing methods on the spectral signal to noise ratio (SNR) of Steady State Visual Evoked Potentials (SSVEPs) with a view for use in Brain-Computer Interfaces (BCIs). It was hypothesised that the new spectral baseline processing method introduced here, termed the 'activity baseline', would result in an improved SNR.
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    Towards improving the Statscan X-ray image quality through sliding mode control of the C-arm
    (2012) Esmail, Mohammed; Tsoeu, Mohohlo Samuel; John, Lester
    This dissertation investigates methods for improving the image quality of a digital radiography system. The Lodox Statscan™ X-ray system provides a full-body scanned image for initial diagnosis. The system is driven by a permanent magnet linear motor (PMLM) controlled by a cascaded proportional-proportional integral controller (P-PI). Transient errors in the trapezoidal motion profile of the scanning C-arm may cause mismatches between the detector and the collimated beams from the X-ray source. This results in a partial degradation of image quality. The Statscan™ X-ray system was investigated and the following constraints were identified: The scanning time is limited to 13 seconds and the maximum scan length is limited to 1.8 m. Since it was not possible to obtain the Lodox Statscan™ dynamics model, because of the similarity, a characteristic model was then developed using a DC motor in order to investigate the control dynamics. It is not advisable for a designer to manipulate the controllers on commercial machines except for changing the parameters for tuning. Therefore, a P-PI controller and a proportional-integral sliding mode controller (P-ISMC) as well as a Boundary Layer variant (P-ISMC+BD) were designed for fair comparison purposes. Root locus, Bode diagrams and integral sliding mode control methods, respectively, were used to design the P-PI and P-ISMC controller groups. Each controller consists of an inner loop and an outer position loop. Proportional integral (PI) and integral sliding mode controllers (ISMC) were used for the inner loop. The two inner loop controllers were tuned, and then tested, before cascading them with the outer position loop. The simulation and experiments were conducted to compare each controller’s performance on step set-point tracking, trapezoidal motion profile tracking, the time transient’s specifications and robustness against disturbances. In order to test image quality, 27 distance profiles were generated from P-PI, P-ISMC and P-ISMC+BD. In addition, four images captured by the Statscan™ were also selected. A time delay integrator (TDI) simulator was used on the distance profiles and the four images to generate 108 distorted image profiles.