The development of methods for improving accuracy and validity of musculoskeletal modelling of the lower limb

The aim of present study was to address limitations in model design, anatomical data, and implementation of muscle force-length and force velocity relationships for the purpose of improving accuracy and validity of musculoskeletal modelling. To achieve these a comprehensive three dimensional musculoskeletal model of the leg was developed and implemented in the analysis of gait. The following were achieved: i) Anatomical data for 48 muscle elements of the lower limb, including skeletal coordinate data to define muscle-tendon paths and moment arms throughout joint movements. ii) Optimisation of muscle model parameters to changes in muscle belly lengths to define an optimal fibre length and force-length relationship. iii) Equations describing muscle model geometry, force-length and force-velocity relationships to describe muscle model contraction dynamics. iv) A cost function which minimised the sum of squared normalised muscle forces with soft constraints on maximum and minimum muscle force, which allowed for the unconstrained minimisation of the cost function. v) An optimisation procedure that combined the equipollence equations, muscle excitation and contraction dynamics and muscle force limits in the minimisation of the cost function. The limitations identified include: i) Accuracy of velocity data achieved was not sufficient to determine muscle contraction dynamics which relied on muscle contractile element velocity to determine maximum muscle forces; ii) Calculation of moment arms from muscle-tendon co-ordinate data was extremely sensitive to errors including : • accurate location of body-fixed axes from external markers; • relative motion of muscle point co-ordinates and joint centres; • the use of straight as opposed to curved-line tendon paths; • accuracy of three dimension segment location during movement. iii) The muscle model predicted contractile element velocities larger than those modelled by the present force-velocity equations, indicating a need to scale force-velocity relationships to the maximum contractile element velocities. iv) The optimisation approach predicted consistent forces for the 48 muscle elements of the lower limb, however limitations were: • determining initial muscle force estimates in accordance with the equipollence equations; • minimisation of the cost function did not change muscle forces significantly from their initial estimated values. Improvements suggested include: i) Improvements in moment arm prediction by: • use of pre-trial functional evaluations of predicted centres of rotation to improve location of body fixed axes; • improved location of muscle point co-ordinates to define muscle lines of action at varying joint angles. ii) Optimise the muscle model parameters to the range of muscle lengths determined by the end-range of motion instead of the motion within a trial to achieve a more realistic optimal fibre length and force-length relationship by considering the change in fibre lengths as an approximation of the maximum changes in fibre lengths. iii) Improve the validity of the joint models to: • determine moments to be balanced by muscular forces. • improve the optimisation procedure to obtain muscle forces. • increase accuracy of muscle force prediction. iv) Review the force-velocity relationship, the dynamic response of the muscle model, and assess the validity of the muscle model in predicting maximum muscle forces; v) Improved initial muscle force estimates which meet the equipollence equations and achieve convergence to a global minimum. vi) Apply the optimisation procedures to movements involving higher muscle forces, where accurate prediction of maximum muscle forces, modelling of joint passive forces, and excitation dynamics become more critical. While the present study succeeds in many respects, its succeeds most in identifying the complexity of the process and proposing methods to achieve greater success

Exploring factors influencing low back pain in people with non-dysvascular lower limb amputation: a national survey

Background: Chronic low back pain (LBP) is a common musculoskeletal impairment in people with lower limb amputation. Given the multifactorial nature of LBP, exploring the factors influencing the presence and intensity of LBP is warranted. Objective: To investigate which physical, personal, and amputee-specific factors predicted presence and intensity of low back pain (LBP) in persons with non-dysvascular transfemoral (TFA) and transtibial amputation (TTA). Design: A retrospective cross-sectional survey. Setting: A national random sample of people with non-dysvascular TFA and TTA. Participants: Participants (N = 526) with unilateral TFA and TTA due to non-dysvascular aetiology (i.e. trauma, tumours, and congenital causes) and a minimum prosthesis usage of one year since amputation were invited to participate in the survey. The data from 208 participants (43.4% response rate) were used for multivariate regression analysis Methods (Independent variables): Personal (i.e. age, body mass, gender, work status, and presence of comorbid conditions), amputee-specific (i.e. level of amputation, years of prosthesis use, presence of phantom limb pain, residual limb problems, and non-amputated limb pain), and physical factors (i.e. pain provoking postures including standing, bending, lifting, walking,sitting, sit-to stand, and climbing stairs). Main outcome measures (Dependent variables): LBP presence and intensity. Results: A multivariate logistic regression model showed that the presence of two or more comorbid conditions (prevalence odds ratio (POR) = 4.34, p = .01), residual limb problems (POR 22 = 3.76, p<.01), and phantom limb pain (POR = 2.46, p = .01) influenced the presence of LBP. Given the high LBP prevalence (63%) in the study, there is a tendency for overestimation of PORand the results must be interpreted with caution. In those with LBP, the presence of residual limb problems (beta = 0.21, p = .01), and experiencing LBP symptoms during sit-to-stand task (beta = 0.22, p = .03) were positively associated with LBP intensity, while being employed demonstrated a negative association (beta = - 0.18, p = .03) in the multivariate linear regression model. Conclusions: Rehabilitation professionals should be cognisant of the influence that comorbid conditions, residual limb problems, and phantom pain have on the presence of LBP in people with non-dysvascular lower limb amputation. Further prospective studies could investigate the underlying causal mechanisms of LBP

ISBS 2018 AUCKLAND CONFERENCE SPRINZ-HPSNZ-AUT MILLENNIUM APPLIED PROGRAMME

An interactive afternoon of sessions delivered by High Performance Sport New Zealand (HPSNZ) and AUT SPRINZ Biomechanists, Performance Analysts and other biomechanics relevant sport facing practitioners. The 11 sessions are at AUT Millennium (AUTM), which is a satellite site of AUT University and the Auckland training hub for many HPSNZ supported sports such as athletics, sailing, and swimming. These sports and others (cycling, rowing, snow sports etc.) will be represented in the line-up. The applied sessions involve practical demonstrations of aspects of analysis and/or tools used to deliver in the field to directly positively impact athletes performances on the world stage. Following these engaging sessions there will be tasting of New Zealand wine, allowing for further discussion and networking. Sir Graeme Avery will be acknowledged for his contribution to sport science. Mike Stanley is AUT Millennium Chief Executive & NZ Olympic Committee President will explain the partners in the facility. AUT Millennium is a charitable trust established to help New Zealanders live longer and healthier lives, and to enjoy and excel in sport through the provision of world-class facilities, services, research and education. Founded in 2002 as Millennium Institute of Sport and Health (MISH) by Sir Stephen Tindall and Sir Graeme Avery as a premium health and fitness facility for both athletes and the public alike. Partnered with AUT University in 2009, forming AUT Millennium, to expand research and education in the sporting sector. Professor Barry Wilson is an Adjunct Professor with SPRINZ at Auckland University of Technology and will be outlining the research and student opportunities. Martin Dowson is the General Manager Athlete Performance Support at High Performance Sport New Zealand and has overall responsibility for the programme. Simon Briscoe, AUT Millennium Applied Session Coordinator, is the head of the Performance and Technique Analysis discipline within HPSNZ. Simon is coordinating the applied sessions along with technical support from Dr Allan Carman, Research Fellow, AUT SPRINZ. Jodi Cossor and Matt Ingram will provide a demonstration of a multidisciplinary approach driven by biomechanical analysis for Paralympic swimmers. Justin Evans and Sarah-Kate Millar will provide a practical session assessing the athletes rowing stroke to assist the coach on technical changes. This session will demonstrate various rowing traits and how the biomechanist and coach can work together to optimise boat speed. Mike Schofield and Kim Hébert-Losier will provide a session looking at shotput and the evidence based approach to coaching. Dr Craig Harrison and Professor John Cronin will provide examples from the AUTM Athlete Development programme. Kim Simperingham and Jamie Douglas who work with high performance rugby athletes will outline sprinting mechanics in practice. Dr Bruce Hamilton, Fiona Mather, Justin Ralph and Rone Thompson will demonstrate the approach of HPSNZ and Cycling NZ performance health teams in the use of some specific tools for prevention of injury and optimisation of performance. Kelly Sheerin, Denny Wells and Associate Professor Thor Besier will provide examples of using IMU and motion capture methods for running and basketball biomechanics research, education and service. Dr Rodrigo Bini and Associate Professor Andrew Kilding will show how linking of biomechanics and physiology improves injury prevention and performance enhancement. Robert Tang, Andre de Jong and Farhan Tinwala discuss select projects developed by Goldmine, HPSNZ’s in-house engineering team, and how these innovations have enabled unprecedented levels of biomechanics feedback. Cameron Ross and Paul McAlpine demonstrate the technology being used at the Snow Sports NZ training centre in Cadrona to enhance load monitoring of athletes. This application allows greater insight into training performances and biomechanical loads than has been previously possible in the training environment. AUT Millennium tour guides are coordinated by Josh McGeown and include Enora Le Flao, Dustin Oranchuk, Erika Ikeda, Jono Neville, Aaron Uthoff, Andrew Pichardo, Farhan Tinwala, Shelley Diewald, Renata Bastos Gottgtroy, Jessica Yeoman, Casey Watkins, Eric Harbour, Anja Zoellner, Alyssa Joy Spence, Victor Lopez Jr, and Albert Chang

Model for screening of resonant magnetic perturbations by plasma in a realistic tokamak geometry and its impact on divertor strike points

This work addresses the question of the relation between strike-point splitting and magnetic stochasticity at the edge of a poloidally diverted tokamak in the presence of externally imposed magnetic perturbations. More specifically, ad-hoc helical current sheets are introduced in order to mimic a hypothetical screening of the external resonant magnetic perturbations by the plasma. These current sheets, which suppress magnetic islands, are found to reduce the amount of splitting expected at the target, which suggests that screening effects should be observable experimentally. Multiple screening current sheets reinforce each other, i.e. less current relative to the case of only one current sheet is required to screen the perturbation.Comment: Accepted in the Proceedings of the 19th International Conference on Plasma Surface Interactions, to be published in Journal of Nuclear Materials. Version 2: minor formatting and text improvements, more results mentioned in the conclusion and abstrac

On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection

A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR

Relationship of edge localized mode burst times with divertor flux loop signal phase in JET

A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM