Selecting the number of trials in experimental biomechanics studies

Experimental biomechanics studies often involve the comparison of mean values from individuals across two or more experimental conditions. The purpose of this study was to evaluate two existing methods for determining the number of trials necessary to estimate these means. The sequential estimation technique (SET) was investigated in terms of the influence of input data distribution on the outcome. Paired samples t-tests were investigated in terms of the interaction between the number of subjects and number of trials necessary to achieve an acceptable level of statistical power. Simulation models were developed to perform SET and paired samples t-tests on representative synthetic input data. The SET results confirmed that the number of trials to achieve a stable estimate of the mean is independent of the input distribution provided the mean and standard deviation are fixed. For the commonly used 20 reference trials and 0.25 standard deviation threshold 9 ± 8 trials were needed to achieve stability. The paired t-test results confirmed that both number of subjects and number of trials can have a marked effect on the statistical power, e.g. a power of 0.80 can be achieved for effect size of 0.80 using 15 subjects and at least 19 trials or 20+ subjects and only 3 trials. The SET method suffers from arbitrary convergence criteria and neglecting intra-subject variance and, thus, should be applied with extreme caution. In contrast, statistical power can provide a more objective and conclusive means for determining the number of trials required for a given experimental situation

A combined muscle model and wavelet approach to interpreting the surface EMG signals from maximal dynamic knee extensions

This study aimed to identify areas of reduced surface EMG amplitude and changed frequency across the phase space of a maximal dynamic knee extension task. The hypotheses were: (1) amplitude would be lower for eccentric contractions compared to concentric contractions and unaffected by fibre length; and (2) mean frequency would also be lower for eccentric contractions and unaffected by fibre length. Joint torque and EMG signals from the vastii and rectus femoris were recorded for eight athletic subjects performing maximum knee extensions at thirteen joint velocities spanning ±250° s–1. The instantaneous amplitude and mean frequency were calculated using the continuous wavelet transform time – frequency method, and the fibre dynamics were determined using a muscle model of the knee extensions. The results indicated: (1) only for the rectus femoris were amplitudes significantly lower for eccentric contractions (p = 0.019), for the vastii amplitudes during eccentric contractions were less than maximal, but this was also the case for concentric contractions due to a significant reduction in amplitude towards knee extension (p = 0.023); and (2) mean frequency increased significantly with decreasing fibre length for all knee extensors and contraction velocities (p = 0.029). Using time – frequency processing of the EMG signals and a muscle model allowed the simultaneous assessment of fibre length, velocity and EMG

The effect of running velocity on footstrike angle - a curve-clustering approach

Despite a large number of studies that have considered footstrike pattern, relatively little is known about how runners alter their footstrike pattern with running velocity. The purpose of this study was to determine how footstrike pattern, defined by footstrike angle (FSA), is affected by running velocity in recreational athletes. One hundred and two recreational athletes ran on a treadmill at up to ten set velocities ranging from 2.2–6.1 m s−1. Footstrike angle (positive rearfoot strike, negative forefoot strike), as well as stride frequency, normalised stride length, ground contact time and duty factor, were obtained from sagittal plane high speed video captured at 240 Hz. A probabilistic curve-clustering method was applied to the FSA data of all participants. The curve-clustering analysis identified three distinct and approximately equally sized groups of behaviour: (1) small/negative FSA throughout; (2) large positive FSA at low velocities (≤4 m s−1) transitioning to a smaller FSA at higher velocities (≥5 m s−1); (3) large positive FSA throughout. As expected, stride frequency was higher, while normalised stride length, ground contact time and duty factor were all lower for Cluster 1 compared to Cluster 3 across all velocities; Cluster 2 typically displayed intermediate values. These three clusters of FSA – velocity behaviour, and in particular the two differing trends observed in runners with a large positive FSAs at lower velocities, can provide a novel and relevant means of grouping athletes for further assessment of their running biomechanics

Artificial turf research at Loughborough University

Research into artificial turf surfaces can be divided into the categories infrastructure, user safety and play performance. This paper discusses these three categories, presents current knowledge and appraises some remaining questions. A simple diagrammatic framework is proposed for describing and relating the fundamental components of sport surface related research. Infrastructure includes the design, construction, operation, and whole life costs associated with a facility. A key area for future research is to better understand maintenance and the benefits of various strategies / techniques. User safety, or injury risk, is a key concern for many stakeholders. Injury risk is a complex interaction of many factors related to the user, sport, equipment and environment. Whilst the introduction of an injury consensus in the late 1990s permitted much greater impact of studies in soccer and rugby, these have contributed little to understanding injury mechanisms. Furthermore, previous research is hampered with regard to the effect of the surface by utilizing simple mechanical tests that appear inappropriate to user activity, e.g. traction. Advancement of knowledge within this category demands better integration with play performance related measurements and research methods that support a more mechanistic approach. Play performance has been the focus of much recent research. For example, mechanical evaluation of surface systems in the laboratory / field, player testing with regard to player and surface response and perception of surface performance. There exists a real need to develop a ‘consensus’ in establishing suitable boundary conditions for both mechanical and player testing. This would help to identify the fundamental research questions related to play performance and allow improved comparison between research studies

Spatial and temporal analysis of surface hardness across a third-generation artificial turf pitch over a year

Despite the potentially negative effects on play performance and safety, little is currently known about the spatial and temporal variability in the properties of artificial turf pitches. The primary purpose of this study was to quantify the spatial and temporal variations in surface hardness across a 5-year-old third-generation artificial turf pitch over full year cycle. The secondary purpose was to investigate the key variables that contributed to these variations in surface hardness using a correlation approach. Surface hardness (2.25 kg Clegg impact hammer, average of drops 2-5), ground temperature and infill depth were measured at 91 locations across the third-generation artificial turf pitch in 13-monthly test sessions from August 2011 to August 2012 inclusive. For each month, rainfall in the 24 h prior to testing and pitch usage statistics were also obtained. Shockpad thickness was obtained from measurements taken when the carpet was replaced in 2007. Spatial and temporal variations were assessed using robust statistical measures while Spearman correlation was used to assess the contributions of the secondary variables to surface hardness variability. The results indicated that spatial variation in surface hardness exceeded temporal variation; the former demonstrated a median absolute deviation of 12 6 1 G across the pitch in any test session while the median absolute deviation for the latter was only 4 6 2 G across the 13 test sessions. Spatial variation in surface hardness was moderately correlated with shockpad thickness and weakly correlated with infill depth (both negative). These results reinforce the importance of monitoring spatial and temporal variations in play performance variables for third-generation surfaces as well as providing support for the role of maintenance in minimising the spatial variation

Predicting maximum eccentric strength from surface EMG measurements

The origin of the well documented discrepancy between maximum voluntary and in vitro tetanic eccentric strength has yet to be fully understood. This study aimed to determine whether surface EMG measurements can be used to reproduce the in vitro tetanic force – velocity relationship from maximum voluntary contractions. Five subjects performed maximal knee extensions over a range of eccentric and concentric velocities on an isovelocity dynamometer whilst EMG from the quadriceps were recorded. Maximum voluntary (MVC) force – length – velocity data were estimated from the dynamometer measurements and a muscle model. Normalised amplitude – length – velocity data were obtained from the EMG signals. Dividing the MVC forces by the normalised amplitudes generated EMG corrected force – length – velocity data. The goodness of fit of the in vitro tetanic force – velocity function to the MVC and EMG corrected forces was assessed. Based on a number of comparative scores the in vitro tetanic force – velocity function provided a significantly better fit to the EMG corrected forces compared to the MVC forces (p ≤ 0.05), Furthermore, the EMG corrected forces generated realistic in vitro tetanic force – velocity profiles. A 58 ± 19% increase in maximum eccentric strength is theoretically achievable through eliminating neural factors. In conclusion, EMG amplitude can be used to estimate in vitro tetanic forces from maximal in vivo force measurements, supporting neural factors as the major contributor to the difference between in vitro and in vivo maximal force

Advanced measurement of sports surface system behaviour under player loading

Artificial turf sport surface systems are comprised of a number of different materials. Improving the understanding of the sports surface system's response to actual player loading is important for developing enhanced products and system designs for improving play performance and durability. Previous research has tested and compared the mechanical properties of artificial turf systems with relatively simple mechanical tests intended to simulate loading from the player or ball. However, these test methods have known shortcomings in representing real in-service loading and it is often assumed a peak value of force or peak deformation is sufficient to describe the surface behaviour. Little literature exists that describes the force-deflection or stress- strain behaviour of artificial turf system under mechanical or player loading. This paper outlines methodologies developed for surface response measurement under real-time player movements including: the advanced measurement systems and data analysis methods for determining surface deflection/strain under player foot strike during a ground contact, and further evaluating the force-deflection and stress-strain relationships of the synthetic carpet-shockpad composite surface systems. The results show the ability of the surface system to accommodate the player applied loads by deforming to large strains with strong non-linearity and rate-dependent energy loss (hysteresis) in the load-unload phases. The contrast between the surface systems’ response to player loading using different shockpads is also presented and discussed. By combining these findings from the development of measurement techniques and the data analysis methods a new surface system evaluation regime is proposed for future studies into mechanical behaviour and cushioning response of artificial turf systems under player loading

Mechanical characterization and numerical modelling of rubber shockpads in 3G artificial turf

Third generation (3G) artificial turf systems use in sporting applications is increasingly prolific. These multi-component systems are comprised of a range of polymeric and elastomeric materials that exhibit non-linear and strain rate dependent behaviours under the complex loads applied from players and equipment. To further study and better understand the behaviours of these systems, the development of a numerical model to accurately predict individual layers’ behaviour as well as the overall system response under different loading conditions is necessary. The purpose of this study was to characterise and model the mechanical behaviour of a rubber shockpad found in 3G artificial surfaces for vertical shock absorption using finite element analysis. A series of uniaxial compression tests were performed to characterise the mechanical behaviour of the shockpad. Compression loading was performed at 0.9 Hz to match human walking speeds. A Microfoam material model was selected from the PolyUMod library and optimised using MCalibration software before being imported into ABAQUS for analysis. A finite element model was created for the shockpad using ABAQUS and a compressive load applied to match that of the experimental data. Friction coefficients were altered to view the effect on the loading response. The accuracy of the model was compared using a series of comparative measures including the energy loss and root mean square error

The development of a translational traction rig to investigate the mechanisms of traction in 3G turf

During football specific movements a high translational traction is desired at the shoe-surface interface to facilitate player movement. Translational traction is commonly assessed through bespoke mechanical test devices which provide a more repeatable tool for characterising the shoe-surface interaction compared to player testing. Following development, application of the rig is demonstrated through an initial investigation into the effect of the number of studs and stud orientation on translational traction. The translational rig consists of a tray attached to two trails, with surface samples of varying specification placed in the tray. A number of stud configurations were chosen and tested on a 3G artificial turf sample. The initial stiffness response of the surface as well as larger displacements were considered to help inform the mechanisms of traction. The study showed the increasing force as the number of studs increased and how the positions of the studs also relate to the forces produced in the infill and the effect on the mechanism of traction

The player surface interaction of rugby players with 3G artificial turf during rugby specific movements

A number of high profile rugby teams in the UK have installed ATS for both training and competition. However, little is known about how the player interacts with ATS during rugby specific tasks. To date the pitches are tested using mechanical testing devices with little understanding as to how these relate to the player interaction with the surface. The aim of this pilot study was to determine the viability of using 3D motion capture system to quantify the player surface interaction and surface performance characteristics during rugby specific movements. Two tasks were selected, kicking and simulated scrummaging, for players to perform on a sample ATS within a biomechanics laboratory. Using a 3D motion capture system synchronised with a force plate the movement of the players on the ATS was analysed. This analysis showed that using a 3D motion capture system with players on an ATS was a viable method to investigate and understand the interaction between the player and the surface. Baseline data for comparison between player loading and the mechanical testing devices was also obtained