Sports Engineering

The aim of this study was to investigate, for typical shoes and surfaces used in tennis, the relative role of the shoe and surface in providing cushioning during running. Five test surfaces ranging from concrete to artificial turf were selected, together with two shoe models. Impact absorbing ability was assessed mechanically using drop test procedures and biomechanically using peak magnitude and rate of loading of impact force and peak in-shoe pressure data at the lateral heel. Differences in biomechanical variables between shoe-surface combinations were identified using a two-way ANOVA (p < 0.05). Mechanical test results were found to rank the surfaces in the same order regardless of the shoe model, suggesting that the surface is influential in providing cushioning. However, for all mechanical and biomechanical (p < 0.05) variables representing impact absorbing ability, it was found that the difference between shoes was markedly greater than the differences between surfaces. The peak heel pressure data were found to rank the surfaces in the same order as the mechanical tests, while impact force data were not as sensitive to the changes in surface. Correlations between mechanical and biomechanical impact absorption highlighted the importance of testing the shoe-surface combination in mechanical tests, rather than the surface alone. In conclusion, mechanical testing of the shoe-surface combination was found to provide a strong predictor of the impact absorbing ability during running if pressure data were used. In addition, for typical shoe-surface combinations in tennis, the shoe was found to have more potential than the surface to influence impact loading during running. Finally, in-shoe pressure data were found to be more sensitive than force plate data to changes in material cushioning.

We propose a thesis that minimising dirt on the running surface of skis improves the surface glide. Waxing usually improves the gliding ability of skis in the short term. But how does waxing affect pollution absorption in the long term? In this study a number of skis with a transparent base and a white background were treated by steel scraping and with different glide waxes. The gliding ability of waxed and unwaxed skis, the sliding surface whiteness and the hydrophobicity were tested and documented. Tests were performed before and after the skis had been used for different distances. It was observed that all the waxed skis (regardless of the wax used) absorbed more dirt than unwaxed, and as a result all waxed skis lose their glide ability sooner than unwaxed (freshly scraped) skis in wet snow conditions.

Our previous study has demonstrated that the swimmer’s wrist acceleration in the crawl stroke and the breaststroke had distinctive characteristics between subjects (Ohgiet al. 2000). In addition, stroke phase discrimination could be achieved for the crawl stroke by using the swimmer’s wrist acceleration. This study provides evidence that the swimmer’s tri-axial wrist acceleration could be used for stroke phase discrimination in the breaststroke. Swimmers’ triaxial wrist accelerations were measured and their underwater stroke paths calculated. The authors examined the characteristics of the wrist acceleration, such as a local maximum or minimum, with reference to the swimmers’ stroke paths. As a result, it was suggested that the three phases of the breaststroke, the recovery, the insweep and the outsweep, could be distinguished using thex-axis (ulnar-radial axis) and they-axis (distal-proximal axis) acceleration. This methodology allows us to identify which stroke phase would be changed in skill training or over training, and the acceleration sensor device has the potential to become a precise stroke pedometer or stroke monitoring tool in the near future.

The purpose of this study was to identify the stationary “wrist shot” technique (movement patterns) of the ice hockey stick that corresponds to the accuracy of puck trajectory. A total of 25 subjects participated in this study, ranging from high to low caliber players. Each performed ten successful wrist shots at four targets (two top corners, two bottom corners). Performances were evaluated by recording the movements of the stick’s shaft and blade and of the puck with a 3D motion capture system at 240Hz. Kinematics of the shaft and blade of the hockey stick were examined using a multiple regression analysis to predict accuracy scores. In general, the results indicated that accuracy corresponded to release parameters (both puck release orientation and velocity), shaft bending and change in blade orientations; though, parameter weighting differed substantially for top versus bottom targets. Future studies are warranted to identify the whole body kinematic patterns associated with the hockey stick kinematics.

This bungee jumping model improves the stretch prediction accuracy of prior models by including the effects of: rubber viscoelasticity, stiffness nonlinearities, jumper air drag, and jumper horizontal push-off. Bungee (bungy) cords are made from rubber, a viscoelastic material whose stiffness is a function of the number of cycles (cycle effect), the time interval since the last cycle (interval effect), strain rate, and temperature. Stiffness was measured, on an MTS machine, over 100 cycles at four intervals (1, 5, 60, 1440 min), three strain rates (0.17, 1.5, 5.0 s−1) and constant temperature (20.5°C). At a constant cycle interval and strain rate, rubber stiffness decreased 9.8% from the 10th to the 100th cycle. This decrease was linear with the logarithm of the cycle number. Stiffness recovered 6.5% after 24 hours of non-use. Neglecting viscoelastic effects causes significant stretch-prediction errors of -11.0% (cycle effect), -3.2% (interval effect), and +1.6% (strain rate). Less significantly, neglecting jumper air drag, jumper push-off, and stress-strain nonlinearities cause prediction errors of +1.9%, +0.7%, and +0.2%, respectively. These errors are based on a typical cord, of unstretched length 7.5 m, that elongates 250%. For accurate stretch prediction, with typical sized bungee cords, viscoelastic effects are both more important and easier to implement than air drag and stress-strain non-linearity.

A Monark cycle ergometer is a device globally used in physiological studies to measure the work and energy levels of exercising humans. In this paper a rope-braked cycle ergometer is examined to determine accurate measures of work and power. The work done is generally assumed, by physiologists, to be the load (masses suspended in a basket to apply tension to the rope) multiplied by the distance through which a flywheel braked by the load is moved (Åstrand). In this case the flywheel of the ergometer is designed such that any point on the circumference moves through 6 m for one complete revolution of the pedals. This is a simplistic approach and fails to take into account rope-brake theory and mechanical analysis of the brake mechanism. The dynamic coefficient of friction between the steel flywheel and nylon rope was determined experimentally to be 0.175. The physical dimensions of the ergometer brake system were taken and the theoretical brake torque calculated for a series of loads. It was found that this calculated brake torque was 10.8% less than the assumed brake torque. This error means that the work and power measurements obtained from the ergometer are overestimated by 10.8% for steady speed tests. The braking force was measured experimentally using specially designed load cells to determine the tensions in the brake rope. The experimental results confirmed that the brake force is the same as that predicted using rope-brake theory and that the values of power and work used by sports scientists and physiologists are overestimated by approximately 11%.

The purpose of this paper was to examine whether the ball position and wrist action (different types of torque application) could be optimised to increase the horizontal golf club head speed at impact with the ball. A two-dimensional double pendulum model of the golf downswing was used to determine to what extent the wrist action affected the club head speed in a driver, and how this affected the optimum ball position. Three different patterns of wrist actions (negative, positive, and negative-positive torque at the wrist) were investigated; and two criteria (maximum and impact criteria) were used to assess their effectiveness in terms of the maximum horizontal club head speed, and the club head speed as the shaft becomes vertical when viewed ‘face-on’. The simulation results indicated that the horizontal club head speed at impact could be increased by these patterns of wrist actions and the optimum ball position could be determined by the impact criterion. Based on the analysis of the energy flow from the input joints of shoulder and wrist to the arm and club head, the way the wrist action affects the club head speed has been discussed. The sensitivity of the results to small changes in model parameter values and initial conditions was investigated. The results were also examined under different torque patterns.

The response of heel-toe runners to changes in cushioning of the impact interface was investigated. Ground reaction force and sagittal plane kinematic data were collected for six heel-toe runners performing barefoot running trials on a conventional asphalt surface and an asphalt surface with additional cushioning. Statistical analysis indicated that similar peak impact force values were maintained when running on the two surfaces (p < 0.1). When running on the less cushioned surface, significant reductions were detected in ankle dorsi-flexion angle immediately prior to ground impact and peak ankle plantar-flexion velocity immediately following impact (p > 0.1). In addition, individual subjects demonstrated reductions in heel velocity and increases in knee flexion immediately prior to ground contact. The observed reduction in ankle dorsiflexion at impact, resulting in a flatter foot at ground contact, supports previous suggestions that this is a strategy to reduce local plantar pressure loads. The additional use of adjustments in heel velocity and initial knee flexion highlights the ability of some subjects to adopt compensatory measures to reduce peak impact loading. However, some subjects appear unable to make these adjustments, resulting in higher impact loading on the less cushioned surface for these subjects. This study provides experimental evidence to support the theoretical potential of heel impact velocity and initial knee flexion to influence impact loading in running.

The kinematics of the racket and ball near impact in tennis forehands were studied to document typical variation in successful and unsuccessful shots, in order to determine biomechanically meaningful differences in advanced players and confirm models of groundstroke trajectories. Seven tennis players (six males and one female) were videoed from the side at 180 Hz as they performed 40 forehand drives on an indoor tennis court. Vertical plane kinematics of the racket and ball near impact were analysed for sub samples of successful and unsuccessful shots for each subject. Most racket kinematic variables were very consistent (mean CV< 6.3%) for successful shots, so bio mechanically meaningful differences in angles and velocities of the racket and ball (3° and 2 m s−1) near impact could be detected between successful and unsuccessful shots. Four subjects tended to miss long and three subjects missed shots in the net that were reflected in initial ball trajectories. Mean (SD) initial trajectories for long shots were 9.8° (1.4°), while netted shots were 0.7° (1.1°) above the horizontal. The initial ball trajectories and margins for error for these subjects were smaller than those previously reported (Brody, 1987) because players tended to select mean ball trajectories close to one error than another, differing amounts of topspin, or incorrect lift and drag coefficients for tennis balls had not been published when this model was created. The present data can be used to confirm if recent models (Cookeet al., 2003; Dignallet al., 2004) more closely match actual performance by advanced players.

To determine the flight of a ski jumper it is essential to know what aerodynamic forces are acting on the ski jumper. However, few data on this are available, especially for a V-style ski jumping flight. We have measured the aerodynamic forces during the free flight phase for a V-style, as well as a parallel-style, ski jump by employing a full-size model in a wind tunnel. The aerodynamic force data, (drag, lift and pitching moment) were obtained to create an aerodynamic database. These forces are given in polynomial form as functions of the angle of attack, the body-ski (forward leaning) angle and the ski-opening (V-style) angle. Using the polynomial form database is a convenient way of obtaining the aerodynamic forces. Moreover, the wind tunnel was equipped with a ground effect plate to measure the aerodynamic forces during the landing phase. It was found that the difference between the lift with and without the ground effect plate increases with the ski-opening angle. The longitudinal stability in the pitching motion of a body-ski combination is also discussed on the basis of the pitching moment data. This indicates that a stable pitching oscillation of the body-ski combination may arise around an equilibrium point in the angle of attack, the trim angle of attack, during flight.

Wind tunnel testing has been carried out on nine-knitted single jersey fabrics (100% polyester) using cylinder and leg models to determine its aerodynamic behaviour over a range of speeds (20–80km/h) representative of sports activities. Strong correlation between fabric manufacturing (cover factor) and fabric roughness and aerodynamic parameters has been established. Similar aerodynamic behaviour of fabrics was observed when tested on the cylinder model and on the leg model.

In elite cycling the resistive force is dominated by aerodynamics. Be it on the roads or in the velodrome, the sport has many examples where aerodynamics has won and lost races. Since the invention of the bicycle, engineers have strived to improve performance, often by reducing aerodynamic drag. Over the last 50 years a number of authors have presented their efforts in journals, books and magazines. This review summarises the publications that show the continued development in the aerodynamics of cycling. The review concludes by examining the shortcomings of the current understanding and making suggestions for future research and development.

When the boundary layer of a sports ball undergoes the transition from laminar to turbulent flow, a drag crisis occurs whereby the drag coefficient (C d) rapidly decreases. However, the aerodynamic properties and boundary-layer dynamics of a soccer ball are not yet well understood. In this study we showed that the critical Reynolds number (Re crit) of soccer balls ranged from 2.2 × 105 to 3.0 × 105. Wind-tunnel testing, along with visualisation of the dynamics of the boundary layer and the trailing vortex of a ball in flight, demonstrated that both non-spinning and spinning (curved) balls had lowC d values in the super-critical region. In addition, theRe crit values of the soccer balls were lower than those of smooth spheres, ranging from ∼ 3.5 × 105 to 4.0 × 105, due to the effects of their panels. This indicated that the aerodynamic properties of a soccer ball were intermediate between those of a smooth ball and a golf ball. In a flow visualisation experiment, the separation point retreated and theC d decreased in a super-critical regime compared with those in a sub-critical regime, suggesting a phenomenon similar to that observed in other sports balls. With some non-spinning and spinning soccer balls, the wake varied over time. In general, the high-frequency component of an eddy dissipated, while the low-frequency component increased as the downstream vortex increased. The causes of the large-scale fluctuations in the vortex observed in the present study were unclear; however, it is possible that a ‘knuckle-ball effect’ of the non-rotating ball played a role in this phenomenon.

The present paper describes the activity carried out to investigate the aerodynamic effects of cycling shoes for time trial competitions. This subject has not been widely studied but can be important for an accurate aerodynamic optimisation of a time trial cyclist. The study was carried out by means of wind tunnel testing: an appropriate test setup and an appropriate test procedure (based on “effective angle of attack approach”) were developed in order to produce realistic test conditions. The developed testing procedure was applied to two different shoe models, differently fastened. Furthermore, an important point was the investigation of the overshoe effect. The results showed that the power required to overcome the shoe’s drag is almost a tenth of the total power and that differences between the shoes can affect the cyclist’s performance. KeywordsCycling-Shoe-Wind tunnel

This study was carried out to design, construct and assess a training aid to assist in the learning of a backward handspring. A backward handspring is often the first backward dynamic skill gymnasts will learn and so its performance can be accompanied by anxiety. International high performance coaches were surveyed to establish the key coaching requirements of a backward handspring training aid. A video analysis of the skill was used to determine characteristic dimensions of the skill, and these were used in the design of the training aid. The aid was designed and manufactured in accordance with European Standards for safety. The device’s safety for use in supporting the backward handspring was confirmed through testing. The assessment of the training aid using 11 gymnasts showed that it permitted a safe dynamic performance; provided support; did not obstruct technically good performances; allowed progressive use by novices without additional coach support; and was adjustable for gymnasts of various size and ability. When assessed against other training aids, it was the only aid that fulfilled all of the key coaching requirements.

Computer-aided design (CAD) of sporting equipment requires knowledge of the mechanical properties of proposed materials. The mechanical properties of composite materials are often not as simple to obtain as those of conventional materials, in which case micro-mechanical modelling could be used in conjunction with CAD software. A micro-mechanical model was used to predict the flexural modulus of composite materials, based on the assumption of partial interfacial adhesion between composite components. It was found that the partial adhesion model was both practical and consistently accurate. The partial adhesion model accounted for adhesion between components by considering an ‘effective shear value’ at the interface. The model was compared to experimental data for glass, wood and carbon-fibre reinforced polyethylene. It was shown that the adhesion coefficient ranged between 0.1 for carbon fibre, 0.5 for glass fibre and 0.8 for the wood fibre composites. It was shown that using micro-mechanical modelling to predict composite mechanical properties, as opposed to simulating the comprehensive composites structure, computer processing time and file size can be reduced with little compromise in simulation accuracy. Keywords: Flexural modulus–Computer-aided design–Micro-mechanical modelling–Composite

In rock climbing, karabiners are used to arrest falls and consequently must be able to withstand dynamic loads. The current international standard for rating karabiner strength prescribes a quasi-static tensile test, which poorly simulates the dynamic nature of an actual climbing fall. In this study, a new method was developed to measure the dynamic failure loads of climbing karabiners. Both new and heavily worn karabiners were tested open and closed, and results from static and dynamic tests were compared. We found that the dynamic failure loads of closed karabiners were up to 50% lower than the failure loads in static tests, while for open karabiners, the static and dynamic values were similar. The reason for this behaviour is unclear; it is most likely due to the combined effects of different stress concentrations and loading regimes of the two tests. Irrespective of test type, karabiner strength decreased with wear level. Based on our results, we advise frequent inspection of permanently placed karabiners for signs of excessive wear. In addition, testing of climbing karabiners in a dynamic test in addition to the standard static test might be considered when developing new karabiner models.

The aim of this work is to develop a methodology for the glide testing of waxed skis at an indoor ski centre, using artificial snow. Two skiers, one acting as a control with unwaxed skis, undertook the test runs over a 50-m course. Timing gates allowed split times to be measured for a selection of waxes, as well as unwaxed skis. Five runs were conducted for each ski preparation, interlaced with control runs. Unwaxed skis produced similar run times to waxed skis for the complete course, with measurements showing their probable superiority over the first 10m. A tentative interpretation involves the delay in forming a lubricating meltwater film under waxed skis by comparison with unwaxed skis. Recommendations are made for future measurements, including a longer course, close matching of skiers’ masses and ski sizes, all timing systems accurate to 0.001s, a start 3m above the first timing gate and more effective methods of wax removal.

Hollow aluminium bats were introduced over 30 years ago to provide improved durability over wooden bats. Since their introduction, however, interest in hollow bats has focused almost exclusively around their hitting performance. The aim of this study was to take advantage of the progress that has been made in predicting bat performance using finite elements and apply it to describe bat durability. Accordingly, the plastic deformation from a ball impact of a single-wall aluminum bat was numerically modelled. The bat deformation from the finite-element analysis was then compared with experiment using a high-speed bat test machine. The ball was modelled as an isotropic, homogeneous, viscoelastic sphere. The viscoelastic parameters of the ball model were found from instrumented, high-speed, rigid-wall ball impacts. The rigid-wall ball impacts were modelled numerically and showed good agreement with the experimentally obtained response. The strain response of the combined bat-ball model was verified with a strain-gauged bat at intermediate ball impact speeds in the elastic range. The strain response of the bat-ball model exhibited positive correlation with the experimental measurements. High-speed bat-ball impacts were performed experimentally and simulated numerically at increasing impact speeds which induced correspondingly increased dent sizes in the bat. The plastic deformation from the numerical model found good agreement with experiment provided the aluminium work hardening and strain rate effects were appropriately described. The inclusion of strain rate effects was shown to have a significant effect on the bat deformations produced in the finite-element simulations. They also helped explain the existence of high bat stresses found in many performance models.

The motion of an American football in a forward pass is discussed in this paper. It was investigated by using wind-tunnel aerodynamic data in a numerical integration of the full nonlinear equations of motion, and by application of the small-angle theory for the atmospheric motion of an axisymmetric spinning body. The numerical simulations agree with certain observed features such as the range, flight duration, and ratio of the spin and precession frequencies. The analytic results reveal functional dependences of the flight parameters on the aerodynamics and give insights into the physical effects that occur. Both approaches show a feature that is known as ‘drift’ in the science of ballistics, namely that a long, highly arched trajectory of a spinning body does not lie in a vertical plane, but rather turns slightly to the left or right, depending on the direction of the spin vector and on the magnitudes of the aerodynamic loads.

A method aimed at the development of an America’s Cup racing yacht bulb is developed and validated against experimental data. The method relies heavily on Reynolds Averaged Navier Stokes computational fluid dynamics (CFD) and also factors in gravitational and aerodynamics effects through the use of a Velocity Prediction Program. Initial extensive use of towing tank results is made to be able to validate the CFD solution and thus develop a solid ground for predictive work. Then, in order to narrow down the focus on bulb developments, decoupled hypotheses are made on the influence of the hull and free surface wave. The development of a detailed performance analysis chart of a known base bulb allows the use of a relatively small series of CFD simulations together with physical hypotheses to assess any given bulb performance relative to the known base. The discussion of the results on a specific development highlights the promises and limitations of the method; final full scale results are discussed and analysed as well.

To properly assess sports helmet performance, it is important to select impact conditions that yield high peak linear or angular accelerations. This was done by measuring the kinematic response of a Hybrid III headform when impacted with a modified Wayne State University linear impactor with special consideration for impact locations and angles. The 20 impact conditions (five locations and four angles) were then compared to published thresholds to identify the conditions, which were linked to an increased risk of head injury. These conditions were the following: 1A (linear 121.3g; angular 3.84krads−2), 2A (linear 102.1g; angular 9.28krads−2), 2C (linear 94.4g; angular 8.67krads−2), 3A (linear 132.8g; angular 9.38krads−2), 4A (linear 92.8g; angular 11.49krads−2), 4D (linear 113.3g; angular 12.86krads−2), 5A (linear 116.9g; angular 9.01krads−2) and 5D (linear 87.5g; angular 8.81krads−2). The results presented in this study were specific to the test rig used as well as the tested conditions; however, it is believed that a test protocol using the above impact conditions could identify the ability of sports helmets to reduce risk of head injuries. KeywordsNeurotrauma–Biomechanics–mTBI–Angular acceleration–Sports helmets

A new sensor for detecting ski bending and torsional deflection during an actual ski turn on the snow has been developed. It consisted of bending and torsion sensors connected by light rigid beams. This structure was fixed to the upper surface of a ski and passed through a tunnel in the central binding plate. The bending and torsion sensors were strain cells, designed to reject strain orthogonal to the desired measurement direction. The calibration factor for each sensor was determined in a jig, then the calibration of the overall sensor assembly was checked by static bending experiments and a free vibration test. A data logger recorded the strain signals synchronously with other data such as the components of the earth’s magnetic field measured by a sensor on the ski. The data set allowed reconstruction in software of the instantaneous shape, direction and edge angle of the ski. The purpose of this paper is mainly to introduce the equipment used and methods developed. Tests of the sensor performance are described. Results from a ski run on snow are presented to show how the various types of data can be combined. A skilled ski athlete performed long turns with the ski at about 60km/h on a groomed snow surface at Shiga Kogen in Japan. The experiment on snow showed that the deformation of the ski was predominantly bending; torsional deflection, although measurable, had only a small effect on the shape of the running edge. The ski edge adopted a symmetrical circular bent shape with an unexpectedly small radius when on the outside, but was unconstrained, lightly loaded and nearly straight when on the inside.

Camera techniques are typically used in the study of human movement. However, as the number of joints and markers involved in a study increases, data extraction and calculation become increasingly tedious and complicated. To overcome this challenge, we propose a method of study that simplifies data extraction and calculation by using an electrogoniometer and dual Euler angles. The contribution of the rotation of each arm segment to produce a racket head’s speed was identified in the context of a badminton smash. The contribution of each segment rotation was computed using dual velocity analysis. A set of orthogonal Cartesian frames was established for computing the anatomical rotational velocities for each of the three segments of the upper arm. Electrogoniometers were attached to the subjects during the execution of the smash to obtain measurements of joint angles throughout the motion. To test the algorithm, the calculated velocity of the racket head was compared to the measured velocity. The calculated velocity was derived from an algorithm, while the measured velocity was obtained from a video image. The results are similar, indicating that the dual velocity method is suitable for determining segmental velocities in such kinematic situations.

There has been significant technological advancement in the game of tennis over the past two decades. In particular, tennis rackets have changed in size, shape and material composition. The effects of these changes on ball rebound speed have been well documented, but few studies have considered the effects on ball angular velocity. The purpose of this study was to investigate the effects of three factors on post-impact ball spin. Tennis balls were projected at three velocities toward a clamped racket simulating three levels of stiffness and strung at three string tensions. The angular velocity of each tennis ball was measured from stroboscopic images during an oblique impact with the racket. A three-way factorial ANOVA revealed significant (P < 0.01) differences in the post-impact angular velocity for string tension, racket stiffness and impact velocity, as well as two-way interactions between string tension and impact velocity, and between racket stiffness and impact velocity. The possibility of tangential elastic strain energy being stored in the racket and ball was evident in low impact velocity trials. These displayed a post-impact angular velocity where the circumference of the ball was translating faster than the relative velocity between the ball’s centre of mass and the string surface. It was concluded that increasing the relative impact velocity between the racket and ball was the best means of increasing the post-impact angular velocity of the tennis ball.

Football ankle protectors were evaluated against a kick from a studded boot. An anatomically correct test rig was used to evaluate materials and designs. Sensors were used to determine peak pressures on the bony protuberances, and load spreading. Finite element analysis (FEA), varying the material parameters and component dimensions, were used to explore pressure distributions and shell buckling. Current designs, with thin ethylene vinyl acetate (EVA) foam and low-density polyethylene (LDPE) shells, cannot prevent some football kicks causing bruising of the ankle. The protection level could be improved by using thicker foams of higher modulus, and domed shells of higher stiffness.

In time trial cycling stage, aerodynamic properties of cyclists are one of the main factors that determine performances. Such aerodynamic properties are strongly dependent on the cyclist ability to get into the most suitable posture to have minimal projected frontal area facing the air. The accurate knowledge of the projected frontal area (A) is thus of interest to understand the performance better. This study aims for the first time at a model estimating accurately A as a function of anthropometric properties, postural variations of the cyclist and the helmet characteristics. From experiments carried out in a wind tunnel test-section, drag force measurements, 3D motion analysis and frontal view of the cyclists are performed. Computerized planimetry measurements of A are then matched with factors related to the cyclist posture and the helmet inclination and length. Data show that A can be fully represented by a rate of the cyclist body height, his body mass, inclination and length of his helmet. All the above-mentioned factors are thus taken into account in the present modelling and the prediction accuracy is then determined by comparisons between planimetry measurements and A values estimated using the model. KeywordsAerodynamic-Drag coefficient-Performance-Planimetry measurement

Measuring the performance of a sports surface is typically derived from a series of field and laboratory tests that assess the playing properties under simulated game conditions. However, from a player’s perspective their own comfort and confidence in the surface and its playing characteristics are equally if not more important. To date no comparative study to measure playing preference tests has been made. The aim of this research was to develop a suitable method for eliciting player perceptions of field hockey pitches and determine the key themes that players consider when assessing field hockey pitches. To elicit meaningful unbiased human perceptions of a playing surface, an individual subjective analysis was carried out, using interviews and inductive analysis of the recorded player statements. A qualitative analysis of elite hockey players (n = 22) was performed to obtain their perceptions immediately after a competitive match. The significant surface characteristics that emerged as part of an inductive analysis of their responses were grouped together and formed five general themes or dimensions: player performance, playing environment, pitch properties, ball interaction and player interaction. Each dimension was formed from a hierarchy of sub-themes. During the analysis, relationships between the dimensions were identified and a structured relationship model was produced to highlight each relationship. Players’ responses suggested that they perceived differences between pitches and that the majority of players considered a ‘hard’ pitch with a ‘low’ ball bounce facilitating a ‘fast’ game speed was desirable. However, further research is required to understand the relative importance of each theme and to develop appropriate measurement strategies to quantify the relevant engineering properties of pitch materials.

In contrast to the situation with early artificial turf pitches, little information has previously been published on the characterisation of third-generation artificial surfaces. The spatial variation of ball rebound resilience and rotational resistance were measured here under dry conditions, late in the season, for two natural turf football pitches and a recently laid third-generation artificial turf pitch. Data for the natural turf pitches show a wider variation with position on the pitch than for the artificial pitch. The latter surface showed remarkable consistency in both quantities measured. Surprisingly, all ball rebound resilience data and some of the rotational resistance values were found to lie outside current FIFA specifications, possibly due to the level of wear in natural turf at this stage of the season. For the artificial turf, the deterioration in properties over a period of 6 months is significant and suggests more frequent testing is needed. Taking data from various pitch positions, the two measured quantities were shown for the first time, as far as we are aware, to be inversely related for both natural turf pitches. This correlation may be largely attributed to differences in the extent of grass cover and/or soil compaction. For artificial turf, the lack of variation in measured quantities with pitch position precluded the determination of any correlation.

The purpose of this work was to develop a new 2D gymnast-high bar model with horizontal bar endpoint dynamics included. To this end, a three-spring high bar model was extended with a five segment gymnast model followed by evaluation and sensitivity analysis. Evaluation over more than a complete giant swing (420°) was favourable (bar position rms errors<0.017m, center of mass angular position rms error <11°). Single parameter perturbations (10%) caused little deterioration in model performance (lower 180°of the giant swing-bar position rms errors <0.006m, arms’ angle rms error <0.9°). Combinations of parameter perturbations gave bar position rms errors<0.008m and arms’ angle rms error <1.8°. Model performance was most sensitive to errors in high bar stiffness values. KeywordsGymnastics–Horizontal bar–Dynamics–Modelling–Simulation–Sensitivity analysis

Research into knots, splices and other methods of forming an eye termination has been limited, despite the fact that they are essential and strongly affect the performance of a rope. The aim of this study was to carry out a comprehensive initial assessment of the breaking strength of eye terminations commonly used in a sailing environment, thereby providing direction for further work in the field. Supports for use in a regular tensile testing machine were specially developed to allow individual testing of each sample and a realistic spread of statistical data to be obtained. Over 180 break tests were carried out on four knots (the bowline, double bowline, figure-of-eight loop and perfection loop) and two splices (three-strand eye splice and braid-on-braid splice). The factors affecting their strength were investigated. A statistical approach to the analysis of the results was adopted. The type of knot was found to have a significant effect on the strength. This same effect was seen in both types of rope construction (three-strand and braid-on-braid). Conclusions were also drawn as to the effect of splice length, eye size, manufacturer and rope diameter on the breaking strength of splices. Areas of development and further investigation were identified.

Athletes and their support team utilise technology to measure and evaluate technique and athletic performance. Existing techniques for motion and propulsion measurement and analysis include a combination of indirect methods (high-speed video) and direct methods (force plates and pressure systems). These methods are predominantly limited to controlled laboratory environments (in a small area relative to the competition environment), require expert advice and support, and can take significant time to evaluate the data. Consequently, the more advanced measurement techniques are considered to be restricted to specific coaching sessions, or periods in the year leading up to competition, when the time and expertise of further support staff are available. The more widely used, and simple, devices for monitoring ‘performance’ during running include stopwatches, GPS tracking and accelerometer-based systems to count strides. These provide useful information on running duration, distance and velocity but lack detailed information on many key aspects of running technique. In order to begin the process of development of more innovative technologies for routine use by athletes and coaches, a study was required to improve the understanding of athletes’ and coaches’ perception of their requirements from measurement technology. This study outlines a systematic approach to elicit and evaluate their perceptions, and presents the findings from interviews and a questionnaire. The qualitative data are presented as a hierarchical graphical plot (structured relationship model) showing six general dimensions (technique, footwear and surface, environment, performance, injury and cardiovascular) and shows the development of these general dimensions from the interviewee quotations. The questionnaire quantitative data enhances the study by further ranking characteristics that arise from the interviews. A contrast is shown between short and longer distance runner groups, as might be expected. The current technology available to elite runners is briefly reviewed in relation to the 22 characteristics identified as important to measure. The conclusions highlight the need for newer technologies to measure aspects of running style and performance in a portable and integrated manner, with suggestions as to size and weight likely to be acceptable to users for emerging devices. KeywordsPerceptions-Instrumentation-Gait analysis-Running performance

In order to determine the usefulness of neural models in optimisation of recruitment processes, statistical analyses were carried out on measured results of javelin throwers using a full take off. A group of 140 Polish junior javelin throwers took part in the research. In order to choose the optimum combination of model parameters the Hellwig method was used. Linear and multilayer perceptron neural models were constructed and used to calculate combinations of variables. Statistical analysis of the results showed that the linear model was not able to describe precisely the relationship between the dependent variable and independent variables for the investigated group of young javelin throwers. For the investigated group, the perceptron network with a 4-3-2-1 structure gave the best predictive relationship for sports results of the javelin throwers. KeywordsThe method of Hellwig–Non-linear neural models–Linear neural models–Multilayer perceptron (MLP)–Artificial neural networks–Sports-selection

Modern ski-treadmills allow cross-country skiers, biathletes and ski-orienteers to test their physical fitness in a laboratory environment whilst performing classical and freestyle (skating) techniques on roller skis. For elite athletes, the differences in performance between test occasions are quite small, thus emphasising the importance of knowing the roller skis’ rolling resistance in order to allow the correct comparison between the results of different test occasions. In this study, the roller skis’ rolling resistance was measured on the ski-treadmill’s surface using a roller ski rolling resistance measurement system specially produced for this purpose. The study investigated the influence of significant changes in rolling resistance on physiological variables. The results showed that during submaximal exercise, power, oxygen uptake, heart rate and blood lactate were significantly changed by different rolling resistances, while there were no significant or only small changes to cycle rate, cycle length and ratings of perceived exertion. Incremental maximal tests showed that time to exhaustion was significantly changed by different rolling resistances and this occurred without significant changes in maximal power, maximal oxygen uptake, maximal heart rate and blood lactate, and that the influence on ratings of perceived exertion were insignificant or small.

The purpose of this study was to investigate the extent to which ice hockey facial protectors can decrease overall head acceleration during blunt impacts, as well as to identify whether attenuation differences exist between visors and cages. Commercial models of three cages and three visors were assessed. Blunt impacts were simulated, permitting the measurement of peak accelerations (PA) within the surrogate headform. Results indicated that face protectors, in combination with helmets, substantially reduced PA during blunt impacts within threshold safety limits (below 300g). In general, cages showed lower PA than visors. Differences between models were also observed during repeated impacts and impact site. In conclusion, this study demonstrates that facial protectors function beyond their role in solely preventing facial injuries, complementing the role of the helmet in attenuating head deceleration during impact. Consequently, the utilisation of facial protectors may reduce the severity and incidence of head injuries.

Recent studies have measured body segment rotation to study the kinematic sequence of the downswing. However, this sequence has yet to be determined relative to an instantaneous screw axis (ISA), free to change position and orientation during motion to reflect shifts in a segment’s dominant axis of rotation. In Part 2 of this two-part study, the objectives were to compute the amplitude of segment angular velocity relative to the corresponding ISA of that segment and verify if the magnitude of segment angular velocity followed the proximal to distal sequence of the summation of speeds principle. Results indicate that the kinematic sequence of 2 of the 5 subjects analyzed supports the summation of speeds principle, where the sequence in which the maximum angular velocity about the pelvis, shoulders and left arm occurred, for one subject, at 68.2±3.2, 72.8±1.7 and 100±0.0% of the downswing. KeywordsGolf–Biomechanics–Kinematics–Instantaneous screw axis

A number of recent studies have measured the extent and timing of segment rotation during the golf swing. A promising technique, instantaneous screw axis (ISA) theory, could provide a better expression of segment rotation. In Part 1 of this two-part study, the objectives are to identify the ISA of the pelvis, shoulders and left arm during the downswing, compute segment angular velocity relative to that segment’s ISA and verify that ISA theory is a valid tool to analyse segment rotation during the golf swing. Results indicate that for all subjects, at least 71% of marker velocity is a result of rotation about their respective ISA, when averaging results over the duration of the downswing, confirming that motion is primarily rotational. Furthermore, ISA position and orientation of each segment approaches, on average, the expected gross axis of rotation, confirming that motion about the ISA is representative of joint motion. KeywordsGolf–Biomechanics–Kinematics–Instantaneous screw axis

The compliance of a badminton racket is an important design consideration, which can be better understood by studying the deflection behaviour of the racket during a stroke. Deflection can be measured using direct methods, such as motion capture or high speed video, or by indirect methods, which then require a mathematical model in order to calculate the deflections from indirect measures. Indirect methods include strain gauges and accelerometers. Here, racket deflection is measured directly using motion capture and compared with deflections calculated from strain gauge data using a beam model. While the elastic behaviour is better calculated from strains than measured by motion capture, it is not possible to extract the whole motion of the racket from strain data. Motion capture is therefore also necessary to determine the rigid body velocity, in order to put the elastic velocity (as calculated from strains) in perspective. KeywordsBadminton-Racket-Deformation-Stiffness-Compliance

The kinematics of a badminton racket during a smash stroke was observed in this study with the purpose of investigating stroke dynamics and racket behaviour. Motion capture measurements of the racket during several smash strokes performed by three players of different skill levels indicated a clear increase in racket velocity at impact with increasing skill level. Variations between translational and rotational contributions to the impact speed could also be seen between the players. The advanced player produced a much higher peak angular velocity and also relied much less on translation, with a translational velocity of only 8% of the total velocity versus the 20% for the recreational player. It is proposed that, as an alternative to shuttlecock speeds, racket head speed measurements can be used as an indicator of performance, and can also provide some insight into the interaction between the racket and player. KeywordsBadminton smash–Racket speed–Player skill–Racket elasticity–Impact speed

Two different measurement techniques are used to examine the effect of surface geometry on soccer ball trajectories. Five professional players are observed using high-speed video when taking curling free kicks with four different soccer balls. The input conditions are measured and the average launch velocity and spin are found to be approximately 24m/s and 106rad/s. It is found that the players can apply more spin (~50%) on average to one ball, which has a slightly rougher surface than the other balls. The trajectories for the same four balls fired at various velocities and spin rates across a sports hall using a bespoke firing device are captured using high-speed video cameras, and their drag and lift coefficients estimated. Balls with more panels are found to experience a higher lift coefficient. The drag coefficient results show a large amount of scatter, and it is difficult to distinguish between the balls. Using the results in a trajectory prediction programme it is found that increasing the number of panels from 14 to 32 can significantly alter the final position of a 20m-curling free kick by up to 1m. KeywordsSoccer-Sports balls-Aerodynamics-Trajectory-Surface roughness

This paper presents a novel sensor technology to deduce the dynamics of a bowling ball. The sensor, a miniature wireless inertial measurement unit (IMU), incorporates MEMS accelerometers and angular rate gyros, a microcontroller, a low power RF transceiver, and a rechargeable battery. When embedded in a bowling ball, the IMU transmits the acceleration and angular velocity data that define the dynamics of the ball starting with the bowler’s delivery and its motion in the lane. Example results from professional bowlers illustrate how this technology can be used to assess bowler skill and ball performance. For instance, the IMU accurately measures the spin dynamics of the ball which are crucial to develop the ball “hook.” An analysis of ball dynamics in the lane is distilled to a measurable “hook potential” metric for further assessing bowler skill. Finally, the sensor presented herein is believed to be the world’s smallest, wireless IMU. This highly miniaturized and wireless design will enable parallel training systems for many sports, including basketball, baseball, crew, cricket, golf, fly fishing, soccer, softball, tennis, rowing, among others. KeywordsTenpin bowling–Sports training–Dynamics–Inertial Sensors

Eighteen elite male tennis players were tested to determine their ability to identify string tension differences between rackets strung from 210N (47lb) to 285N (64lb). Each player impacted four tennis balls projected from a ball machine before changing rackets and repeating the test. Eleven participants (61%) could not correctly detect a 75N (17lb) difference between rackets. Only two participants (11%) could correctly detect a 25N (6lb) difference. To establish whether varying string tensions affected ball rebound dynamics, the ball’s rebound speed and landing position were analysed. The mean rebound ball speed was 117kmh−1, with only the trials from the 210N racket producing significantly lower (P<0.05) rebound speeds than the 235N and 260N rackets. This is contrary to previous laboratory-based tests where higher rebound speeds are typically associated with low-string tensions. The anomaly may be attributable to lower swing speeds from participants as they were not familiar with such a low string tension. Ball placement did not appear related to string tension, with the exception of more long errors for the 235N racket and fewer long errors for the 285N racket. It was concluded that elite male tennis players display limited ability to detect changes in string tension, impact the ball approximately 6% faster than advanced recreational tennis players during a typical rallying stroke, and that ball placement is predominantly unrelated to string tension for elite performers.

The aerodynamic properties of a cricket ball have intrigued cricket players and spectators for years, arguably since the advent of the game itself. The main interest is in the fact that the ball can follow a curved flight path that may not always be under the control of the bowler. The basic aerodynamic principles responsible for the nonlinear flight or ‘swing’ of a cricket ball were identified decades ago and many papers have been published on the subject. Over the last 25 years or so, several empirical investigations have also been conducted on cricket ball swing, which revealed the amount of attainable swing and identified the parameters that affect it. Those findings are reviewed here with emphasis on phenomena such as late swing and the effects of humidity on swing. The relatively new concept of ‘reverse swing’, how it can be achieved in practice, and the role in it of ‘ball tampering’, are also discussed in detail. In particular, the ability of some bowlers to effectively swing an old ball in the conventional, reverse and the newly termed ‘contrast’ swing mode is addressed. A discussion of the ‘white” cricket ball used in the 1999 and 2003 World Cup tournaments, which supposedly possesses different swing properties compared to a conventional red ball, is also included. This is a current overview of cricket ball swing rather than a detailed review of all research work performed on the topic. The emphasis is on presenting scientific explanations for the various aerodynamic phenomena that affect cricket ball swing on a cricket ground.

Soccer equipment manufacturers invest significant amounts of time and money researching and developing soccer balls, using advanced materials and constructions in an attempt to create a ball that has better flight and impact characteristics. An important consideration in any structure subject to dynamic or impact loading is its mechanical response. The recent development of non-contact optical vibration measurement tools such as the SLDV have made the accurate measurement of such responses possible. The technique of vibrometry utilises the Doppler principle to provide a measure of the surface velocity at the point at which a laser beam is incident. The SLDV benefits from its non-contact and non-marking method, and the speed and ease with which measurements can be recorded. This paper reports the method and results from a study aimed at determining the dynamic responses of two different soccer balls. The balls were excited using an acoustic source and the velocity of each ball’s surface at a series of points was recorded. The natural frequencies and vibration mode shapes were identified and a comparison made between the responses of each ball. Significant mode shapes were observed between 150 Hz and 1500 Hz. At the lower frequencies, the mode shapes were observed to be independent of the outer panels, based more on the structure of the ball as a whole. At higher frequencies, one of the balls tested showed mode shapes centred on individual panel oscillations. The soccer balls tested show some noticeable differences in mode shapes.

Measurements are presented of the friction force acting on a tennis ball incident obliquely on the strings of a tennis racket. This information, when combined with measurements of ball speed and spin, reveals details of the bounce process that have not previously been observed and also provides the first measurements of the coefficient of sliding friction between a tennis ball and the strings of a tennis racket. At angles of incidence less than about 40° to the string plane, the ball slides across the strings during the whole bounce period. More commonly, the ball is incident at larger angles in which case the ball slides across the string plane for a short distance before gripping the strings. While the bottom of the ball remains at rest on the strings, the remainder of the ball continues to rotate for a short period, after which the ball suddenly releases its grip and the bottom of the ball slides backwards on the string plane. The bounce angle depends mainly on the angle of incidence and the rotation speed of the incident ball. Differences in bounce angle and spin off head-clamped and hand-held rackets are also described.

There has been little three-dimensional (3D) analysis of the interaction of a tennis ball and racket during realistic play conditions. This paper is a descriptive study of elite players in practice conditions. The method used records racket and ball movement in 3D, intrudes minimally into the player’s environment and has a high level of portability. Testing was performed using two Phantom V4.2 high speed video cameras operating at 1,000 frames per second. Racket movement was tracked using five reflective markers attached to the player’s racket and the ball was tracked as a single point. The method allowed accurate measurement of ball and racket speeds, impact positions, and angular velocities of the racket in three-dimensions. It was used at the 2006 Wimbledon qualifying tournament in practice conditions to record 106 shots from 16 internationally ranked players. The results obtained showed that all players aim to hit the node point on the racket face in a standard forehand drive. The average post-impact ball velocity of male players was 9.4% greater than that of female players at 33.9ms−1, post-impact ball spin was 22.3% higher at 1,480rpm. These results could be used to confirm previous research into player movement and impact, or as a basis for future investigation into the interaction between the ball, racket and player. KeywordsTennis–3D stereogrammetry–High speed video–Velocity–Impact angle–Spin–Sweet spot

An explicit finite-element (FE) model of a pressurised tennis ball is presented. The FE model was used to model an oblique impact between a tennis ball and a rigid tennis surface, to further the understanding of this impact. Impacts were also conducted in the laboratory and the results from the FE model were in good agreement with this experimental data. The FE model was used to illustrate why a tennis ball rebounds with a higher vertical coefficient of restitution in an oblique impact compared to an equivalent impact perpendicular to the surface; this equivalent perpendicular impact has the same inbound velocity as the vertical component of the oblique impact. The FE model was also used to illustrate that the structural compliance of the felt covering on a tennis ball was a contributing factor to the ball attaining more spin in the impact than would have been calculated using a conventional analytical model. Also, the spin values calculated in the FE simulation were in good agreement with experimental data.

Handling errors are often seen in professional rugby games and even more so in amateur rugby. This paper analyses the problem of ball mishandling using high-speed video footage of passes and a bespoke finger friction rig. The high-speed video analysis showed that when the ball is caught, often there is a fluctuating movement of the fingers over the surface of the ball. It also showed that the fingers move over the surface of the ball when the ball is thrown, confirming that the dynamic friction is a good measure of how easily a ball can be handled. Rugby ball surface samples were used, on a finger friction rig, to assess the coefficient of friction between the finger and the balls. The currently manufactured balls displaying the highest coefficients of friction in clean, dry conditions were the design with square, ‘sharp’ pimples and also the design with a mixture of small and large pimples. The most consistent ball across wet and dry conditions was the ball with round, large, densely populated pimples. It was also shown that when water is added to the surface of the ball or finger, there was little variation in performance between the ball varieties.

In recent years, variability in behaviour of the sliotar, a small leather-bound ball used in the Irish sport of hurling, has become evident in championship matches. The inconsistency in performance was attributed to the range of constructions and material compositions of currently approved ball types. With a view to adopting a standard core, a new methodology has been commissioned to assess the dynamic impact behaviour of approved sliotar cores. In this paper, the relationship between the dynamic stiffness and the coefficient of restitution is presented with regard to material properties, ball construction and viscoelastic strain and strain-rate dependencies. The modern polymer ball types were shown to exhibit strain-rate sensitivity, while the performance of the traditional multi-compositional ball types exhibited lesser strain-rate dependence. The traditional balls types were shown to be up to 2.5 times stiffer than the modern ball types, with this finding having implications for ball energy dissipation.

Because youth athletes are smaller and weaker than their adult counterparts, smaller equipment and fields are often used in youth sports. Previous research has shown that youth baseball pitchers use similar motions to older pitchers, but generate lower kinetics and angular velocities at the shoulder and elbow. The purpose of this study was to determine potential biomechanical benefits for youth pitchers to use lighter baseballs. Thirty-four youth (11.1 ± 0.7 years) pitchers pitched both standard [5 ounce (142 g)] and lightweight [4 ounce(113 g)] baseballs in a laboratory setting. Kinematic and kinetic parameters were measured with a six-camera high-speed motion analysis system. Three repeated measures MANOVAs were used to compare (p > 0.05) position, velocity, and kinetic parameters between the standard and lightweight baseballs. Subjective data were also collected. Pitching the lightweight ball produced no difference in arm position, but greater shoulder, elbow, and ball velocities. With the lightweight ball, pitchers produced decreased kinetics.Post-hoc analysis of the kinetic data revealed significant decreases in elbow varus torque and shoulder internal rotation torque. The data suggest that playing with lightweight baseballs may reduce the risk of overuse injury in the youth pitcher and also help develop arm speed. However, before introducing lightweight baseballs into the youth game, the effect of lighter, faster pitched balls for the batters and fielders should also be considered.