A critical examination of compound stability predictions from machine-learned formation energies

Machine learning has emerged as a novel tool for the efficient prediction of material properties, and claims have been made that machine-learned models for the formation energy of compounds can approach the accuracy of Density Functional Theory (DFT). The models tested in this work include five recently published compositional models, a baseline model using stoichiometry alone, and a structural model. By testing seven machine learning models for formation energy on stability predictions using the Materials Project database of DFT calculations for 85,014 unique chemical compositions, we show that while formation energies can indeed be predicted well, all compositional models perform poorly on predicting the stability of compounds, making them considerably less useful than DFT for the discovery and design of new solids. Most critically, in sparse chemical spaces where few stoichiometries have stable compounds, only the structural model is capable of efficiently detecting which materials are stable. The nonincremental improvement of structural models compared with compositional models is noteworthy and encourages the use of structural models for materials discovery, with the constraint that for any new composition, the ground-state structure is not known a priori. This work demonstrates that accurate predictions of formation energy do not imply accurate predictions of stability, emphasizing the importance of assessing model performance on stability predictions, for which we provide a set of publicly available tests

KINEMATIC ASPECTS OF BLOCK PHASE TECHNIQUE IN SPRINTING

This study investigated kinematic aspects of block phase technique during the sprint start and their relationships with performance amongst a heterogeneous group of 16 sprinters. Lower limb kinematics in the ‘set’ position were not associated with block phase performance (average horizontal external power). During block exit a greater rear leg push, in particular from the hip, appeared important for performance. The front leg extended in a proximal-to-distal fashion, with more rapid hip extension again facilitating performance. Striving to achieve higher levels of block phase performance did not appear to negatively affect the first flight phase or the configuration of the sprinters at first touchdown. Sprinters should therefore be encouraged to maximise hip extensions in the blocks and use their rear leg drive to achieve a powerful block exit

DEVELOPMENT, EVALUATION AND APPLICATION OF A SIMULATION MODEL OF A SPRINTER DURING THE FIRST STANCE PHASE

This study aimed to investigate how alterations in kinematics at touchdown could improve the performance of an international-level sprinter during the first stance phase of a sprint. A seven-segment angle-driven simulation model was developed, and evaluation against empirical data revealed the model matched reality to within a mean value of 5.2%. A series of simulations altering the horizontal distance between the stance foot and the CM at touchdown were undertaken. By positioning the foot slightly further behind the CM, performance (external power) was improved due to favourable increases in horizontal force production and only small increases in stance duration. However, continuing to increase this distance between the foot and the CM led to decreased performance due to an inability to generate sufficient force despite continued increases in stance duration

A CASE STUDY OF STRIDE FREQUENCY AND SWING TIME IN ELITE ABLE-BODIED SPRINT RUNNING: IMPLICATIONS FOR AMPUTEE DEBATE

Recent research into trans-tibial double-amputee sprint performance has debated the possible inherent advantages, disadvantages and limitations to sprinting with prosthetic limbs compared to healthy limbs. Biomechanical data gathered throughout a training season from an elite able-bodied sprinter provide a new perspective on this debate. Peak stride frequency was measured at 2.62 Hz, and the corresponding swing time was estimated to be 0.287 s in the able-bodied sprinter. Published swing time and stride frequency values from the double-amputee at maximum velocity, thought to be beyond biological limits, therefore may not be so, although previously published research has provided evidence that some joint kinetic values from the double-amputee have not been shown in elite able-bodied sprinting

A CASE STUDY OF STRIDE FREQUENCY AND SWING TIME IN ELITE ABLEBODIED SPRINT RUNNING: IMPLICATIONS FOR AMPUTEE DEBATE

Recent research into trans-tibial double-amputee sprint performance has debated the possible inherent advantages, disadvantages and limitations to sprinting with prosthetic limbs compared to healthy limbs. Biomechanical data gathered throughout a training season from an elite able-bodied sprinter provide a new perspective on this debate. Peak stride frequency was measured at 2.62 Hz, and the corresponding swing time was estimated to be 0.287 s in the able-bodied sprinter. Published swing time and stride frequency values from the double-amputee at maximum velocity, thought to be beyond biological limits, therefore may not be so, although previously published research has provided evidence that some joint kinetic values from the double-amputee have not been shown in elite able-bodied sprinting

Cervical spine injuries: A whole-body musculoskeletal model for the analysis of spinal loading

This is the final version of the article. Available from Public Library of Science via the DOI in this record.Cervical spine trauma from sport or traffic collisions can have devastating consequences for individuals and a high societal cost. The precise mechanisms of such injuries are still unknown as investigation is hampered by the difficulty in experimentally replicating the conditions under which these injuries occur. We harness the benefits of computer simulation to report on the creation and validation of i) a generic musculoskeletal model (MASI) for the analyses of cervical spine loading in healthy subjects, and ii) a population-specific version of the model (Rugby Model), for investigating cervical spine injury mechanisms during rugby activities. The musculoskeletal models were created in OpenSim, and validated against in vivo data of a healthy subject and a rugby player performing neck and upper limb movements. The novel aspects of the Rugby Model comprise i) population-specific inertial properties and muscle parameters representing rugby forward players, and ii) a custom scapula-clavicular joint that allows the application of multiple external loads. We confirm the utility of the developed generic and population-specific models via verification steps and validation of kinematics, joint moments and neuromuscular activations during rugby scrummaging and neck functional movements, which achieve results comparable with in vivoand in vitrodata. The Rugby Model was validated and used for the first time to provide insight into anatomical loading and cervical spine injury mechanisms related to rugby, whilst the MASI introduces a new computational tool to allow investigation of spinal injuries arising from other sporting activities, transport, and ergonomic applications. The models used in this study are freely available at simtk.org and allow to integrate in silico analyses with experimental approaches in injury prevention.Funding: This project is funded by the Rugby Football Union (RFU) Injured Players Foundation. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

A comparison of time-motion analysis methods for field-based sports.

Purpose. To assess the validity of a digitising time-motion analysis method for field-based sports and compare this to a notational analysis method using rugby union match play. Method. Five calibrated video cameras were located around a rugby pitch and one subject completed prescribed movements within each camera view. Running speeds were measured using photocell timing gates. Two experienced operators digitised video data (operator 1 on two occasions) to allow 2D reconstruction of the prescribed movements. Results. Accuracy for total distance calculated was within 2.1% of the measured distance. For intra- and inter-operator reliability, calculated distances were within 0.5% and 0.9% respectively. Calculated speed was within 8.0% of measured photocell speed with intra- and inter-operator reliability of 3.4% and 6.0%, respectively. For the method comparison, two 20 min periods of rugby match play were analysed for five players using the digitising method and a notational time-motion method. For the 20 min periods, overall mean absolute differences between methods for percentage time spent and distances covered performing different activities were 3.5% and 198.1 ± 138.1 m, respectively. Total number of changes in activity per 20 min were 184 ± 24 vs 458 ± 48 and work-to-rest ratio’s 10.0 / 90.0% and 7.3 / 92.7% for notational and digitising methods, respectively. Conclusion. The digitising method is accurate and reliable for gaining detailed information on work profiles of field sport participants and provides the applied researcher with richer data output than the conventional notational method

Measuring impacts and informing modelling processes

Primarily using rugby union situations as case study examples for the practical demonstration, the initial part of the session will discuss both familiar and emerging techniques to measure the biomechanics of sport impact situations. We will cover some of the issues that need to be accounted for to acquire robust data in such complex environments, and we will discuss how experimental measures can be either used in their own right to develop knowledge of impact biomechanics or can provide data to input a modelling pipeline and for model validation purposes

Excessive fluctuations in knee joint moments during early stance in sprinting are caused by digital filtering procedures

Inverse dynamics analyses are commonly used to understand movement patterns in all forms of gait. The aim of this study was to determine the effect of digital filtering procedures on the knee joint moments calculated during sprinting as an example of the possible influence of data analysis processes on interpretation of movement patterns. Data were obtained from three highly trained sprinters who completed a series of 30 m sprints. Ten different combinations of cut-off frequency were applied to the two-dimensional kinematic and kinetic input data with the kinetic cut-off frequency set equal to or higher than the kinematic cut-off frequency. When using the commonly adopted practice of filtering the kinetic data with a higher cut-off frequency than the kinematic data, exaggerated fluctuations in the knee joint moment existed soon after contact. In extreme cases, the knee moved between flexor, extensor and flexor dominance in less than 33 ms and through ranges exceeding 500 Nm. During an inverse dynamics analysis of locomotion, mismatched cut-off frequencies will likely affect the calculated joint moments if the cut-off frequency applied to the kinematic data is less than the true frequency content, particularly during impact phases. In the example of sprinting, exaggerated fluctuations in the knee joint moment appear to be data processing artefact rather than genuine characteristics of the joint kinetics. When the cut-off frequencies, and thus the frequency content of all input data, are matched, the fluctuations after contact are minimal and such a procedure is suggested for inverse dynamics analyses of gait