Tennis Racket Research Articles

Image-based visual hull of a tennis racket

Researchers are often interested in tracking object movement in sport. This could be useful to identify equipment designs that match the technique of players. Previously, stereo camera systems have been used to track markers attached to striking implements to measure their movement in three dimensions. However, manual selection of markers on the image plane can be time consuming and inaccurate. There is potential however to reduce these drawbacks associated with marker-based analysis by tracking a striking implement using its visual hull. The closest geometric approximation of an object that can be reconstructed using only its silhouette images is its visual hull. Early applications of visual hulls include size and shape estimation of objects such as stones. Recently, subject specific visual hulls constructed from multiple camera views combined with anatomical tracking algorithms have measured human motion through markerless motion capture. However, multiple camera systems are not practical for real play conditions in most sports. The application of visual hulls to measure movements of striking implements used in sport has not yet been explored. A set of calibrated views of a tennis racket were captured and segmented into binary images to obtain silhouettes. The visual hull of a tennis racket was constructed by intersection of the volume of space formed by back-projecting the silhouettes from all input views. This research is the first stage in the development of a system that measures movement of a striking implement in real play conditions by combining its visual hull with footage from a single camera.

MOSAIC for Multiple-Reward Environments

Reinforcement learning (RL) can provide a basic framework for autonomous robots to learn to control and maximize future cumulative rewards in complex environments. To achieve high performance, RL controllers must consider the complex external dynamics for movements and task (reward function) and optimize control commands. For example, a robot playing tennis and squash needs to cope with the different dynamics of a tennis or squash racket and such dynamic environmental factors as the wind. In addition, this robot has to tailor its tactics simultaneously under the rules of either game. This double complexity of the external dynamics and reward function sometimes becomes more complex when both the multiple dynamics and multiple reward functions switch implicitly, as in the situation of a real (multi-agent) game of tennis where one player cannot observe the intention of her opponents or her partner. The robot must consider its opponent's and its partner's unobservable behavioral goals (reward function). In this article, we address how an RL agent should be designed to handle such double complexity of dynamics and reward. We have previously proposed modular selection and identification for control (MOSAIC) to cope with nonstationary dynamics where appropriate controllers are selected and learned among many candidates based on the error of its paired dynamics predictor: the forward model. Here we extend this framework for RL and propose MOSAIC-MR architecture. It resembles MOSAIC in spirit and selects and learns an appropriate RL controller based on the RL controller's TD error using the errors of the dynamics (the forward model) and the reward predictors. Furthermore, unlike other MOSAIC variants for RL, RL controllers are not a priori paired with the fixed predictors of dynamics and rewards. The simulation results demonstrate that MOSAIC-MR outperforms other counterparts because of this flexible association ability among RL controllers, forward models, and reward predictors.

The Kinematics of Trunk and Upper Extremities in One-Handed and Two-Handed Backhand Stroke

The aim of this study was to present kinematics of trunk and upper extremities in tennis players who perform one-handed and two-handed backhand strokes. The study aimed to address the question of whether one of those techniques has some important advantage over the other. If so, what makes it superior?The study included 10 tennis coaches with average coaching experience of 9 years. The coaches were asked to hit 15 one-handed and two-handed backhands. The tests were carried out in a laboratory. A sponge ball was used in order to protect the measurement equipment. Video motion analysis was carried out using BTS SMART system; images were recorded with 6 cameras with a rate of 120 frames per second. The analysis of both backhand strokes focused on the second phase of the stroke (acceleration).The use of an eight-element model of human body for description of upper body motion in both techniques revealed kinematic differences in how both backhands are performed. The two-handed backhand was performed in closed kinetic chain with 8 degrees of freedom, whereas the one-handed backhand involved an open kinetic chain with 7 degrees of freedom. Higher rigidity of upper extremities which are connected with trunk in the two-handed backhand, contributes to an elevated trunk effect in this stroke. This is confirmed by higher component velocities for racket handle, which result from trunk rotation in the two-handed backhand and a negative separation angle in the two-handed backhand at the moment of contact of the racket with the ball.The study does not provide a clear-cut answer to the question of advantages of one technique over the other; however, it reveals dissimilar patterns of driving the racket in both techniques, which suggests the need for extending the analysis of techniques of both backhands with additional kinematics of tennis racket in consideration of measurements of ball velocities.

The Usefulness of Oncoplastic Volume Displacement Techniques in the Superiorly Located Breast Cancers for Korean Patients With Small to Moderate-Sized Breasts

Because the average breast size of Korean women is not as large as that of western women, it is not so easy to apply the oncoplastic volume displacement technique. However, we have tried to use volume displacement techniques that work well with small-breasted women. In this study, we report the usefulness of displacement techniques in the superiorly located cancers with small- to moderate-sized breasts. Between September 2006 and August 2009, 58 women were treated for superiorly located cancers. All patients who were selected had small- to moderate-sized breasts and small to moderate breast defects. They were divided into 3 groups. The groups were split based on the distance from the nipple-areolar complex, which are as follows: near, intermediate, and far. Their mean age was 46 and the average follow-up interval was 21 months. Median weight of the specimen on the tumor side was 84 g (range, 29-140 g). The various surgical techniques used were the "round block" technique (n = 8) and "batwing mastopexy" (n = 4) for near lesions; "tennis racket" method (n = 32) and "rotational flap" (n = 8) for intermediate lesions; and "parallelogram mastopexy lumpectomy" (n = 6) for far lesions. There were 4 complications (nipple-areolar complex partial necrosis, radiation burn, and 2 wound dehiscence). No local recurrences have been observed and a majority of the patients were satisfied with cosmetic results. Oncoplastic volume displacement techniques in superiorly located breast cancers for Korean patients with small- to moderate-sized breast are reliable and should be considered in the case of breast conserving surgery (BCS).

Analytic study on structural behavior of the string bed in a tennis racket

The structural behavior of string bed of tennis rackets was investigated subjected to transverse force perpendicular to the string bed. The mathematical model developed for the string bed was implemented into a computer programming code. This code was used to conduct extensive parametric studies on the structural behavior of the string bed for various parameters, including string tension, axial rigidity of the string, string spacing and head size. The analysis results showed that while the transverse stiffness of the string bed is proportional to the string tension, the transverse stiffness of the string bed is inversely proportional to string spacing and head size. In addition, the axial rigidity of the string significantly amplifies the transverse stiffness of the string bed for relatively large transverse deflection of the string bed.

Increased beam attenuation and surface dose by different couch inserts of treatment tables used in megavoltage radiotherapy

The use of solid carbon fiber table materials in radiotherapy has become more common with the implementation of image‐guided radiotherapy (IGRT), since the solid materials give less imaging artifacts than the so‐called tennis racket couchtops. The downside of the solid carbon fiber couch inserts is that they increase the beam attenuation, resulting in increased surface doses and inaccuracies in determine the dose in the patient. The purpose of this study was to evaluate the interaction of 6 and 15 MV photons with eight different couch inserts. The presented results enable direct comparison of the attenuation properties of the studied couchtops. With a direct posterior beam the maximum attenuations reach 3.6% and 2.4% with 6 and 15 M V, respectively. The measured maximum attenuation by a couchtop with an oblique gantry angle was 10.8% and 7.4% at 6 and 15 MV energies, respectively. The skin‐sparing effect was decreased substantially with every couchtop. The highest increases in surface doses were recorded to be four‐ and threefold, as compared to the direct posterior open field surface doses of 6 and 15 MV, respectively. In conclusion, the carbon fiber tabletops decrease the skin‐sparing effect of megavoltage photon energies. The increased beam attenuation and skin doses should be taken into account in the process of treatment planning.PACS number: 07.90.+c

Dynamic Analysis of Elbow Joint in the Stroke of Tennis

The paper presents a multi-rigid model of arm for an impact between a tennis ball and racket, and makes dynamic analysis of it. The input to the model is the impact force caused by a tennis ball and racket. Its value comes from the deflection of the racket stringbed detected by a high speed camera and the stiffness of the racket stringbed is measured by a force gage. The effect of the impact force on the elbow is analyzed in order to research the pathogenesis of tennis elbow.

Impact characteristics of the ball and racket during play at the Wimbledon qualifying tournament

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 33.9 m s−1, post-impact ball spin was 22.3% higher at 1,480 rpm. 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.

Tennis Star Says MH Advocacy Helped Him Defeat Depression

Tennis Star Says MH Advocacy Helped Him Defeat Depression

Effect of tennis racket parameters on a simulated groundstroke

Composite materials have given manufacturers the freedom to develop a broad range of tennis rackets, allowing them to change key parameters such as the structural stiffness, mass, and position of the balance point. The aim of this research was to determine how changing these parameters could affect ball resultant rebound velocity and spin for a simulated groundstroke. A finite element model of a freely suspended racket and strings was used to determine the effect of racket parameters for oblique spinning impacts at a range of locations on the stringbed. The finite element simulations were conducted in the laboratory frame of reference, where the ball is projected onto an initially stationary racket. The mean rebound velocity of the ball was 9% higher for a structurally stiff racket, 37% higher for a heavy racket, and 32% higher for a head-heavy racket. In addition, the mean rebound topspin of the ball was 23% higher for a heavy racket and 21% higher for a head-heavy racket. Therefore, in relation to a groundstroke with an impact location away from the node, the rebound velocity of the ball is likely to increase with the structural stiffness of a racket. The effect of changing the mass and position of the balance point is more complex, as it is dependent on the relationship between the transverse moment of inertia and maximum pre-impact swing velocity.

Infantile Sinonasal Myxoma: A Unique Variant of Maxillofacial Myxoma

Myxomas are benign, slow-growing, locally invasive neoplasms derived from mesenchymal elements. Maxillofacial myxomas are rare and most frequently involve the jawbones. The origin of most maxillofacial myxomas is believed to be odontogenic, although in rare cases, tumors have occurred in the sinonasal area, outside the area of the odontogenic apparatus, and, thus, could not have originated in the odontogenic mesenchyme. In traditional radiographs, maxillofacial myxomas have been described as radiolucent, usually multilocular, with features resembling honeycomb, soap bubbles, or a tennis racket.

A computer simulation model of tennis racket/ball impacts

A forward dynamics computer simulation for replicating tennis racket/ball impacts is described consisting of two rigid segments coupled with two degrees of rotational freedom for the racket frame, nine equally spaced point masses connected by 24 visco-elastic springs for the string-bed and a point mass visco-elastic ball model. The first and second modal responses both in and perpendicular to the racket string-bed plane have been reproduced for two contrasting racket frames, each strung at a high and a low tension. Ball/string-bed normal impact simulations of real impacts at nine locations on each string-bed and six different initial ball velocities resulted in <3% RMS error in rebound velocity (over the 16–27 m/s range observed). The RMS difference between simulated and measured oblique impact rebound angles across nine impact locations was 1°. Thus, careful measurement of ball and racket characteristics to configure the model parameters enables researchers to accurately introduce ball impact at different locations and subsequent modal response of the tennis racket to rigid body simulations of tennis strokes without punitive computational cost.

Investigations into the effect of grip tightness on off-centre forehand strikes in tennis

Previous experimental studies into the dynamics of a ball–racket impact have recorded ball velocities and trajectories in one or two dimensions. These studies were limited in terms of possible impact positions and the ability to represent realistic impact conditions. As the number of dimensions in an experiment increases, it becomes difficult to isolate single variables and to monitor their effect on the output. This paper describes a method of accurately assessing the effect of specific independent variables in impact experiments with a large number of variables. A total of 900 impacts on a Head Ti-S6 tennis racket were recorded within bespoke impact apparatus. The impact velocity, the impact angle, the impact position, and a restrictive torque about the handle were varied between each impact. Each test was recorded with two Phantom high-speed video cameras at 600 frames/s; the use of two cameras allowed the ball trajectory to be tracked in three dimensions. A multi-variate fit based on a second-order polynomial was used to establish trends within the data. It was found that applying a torque about the handle reduced the angular deviation for all impact positions along the racket's transverse axis. In a standard forehand shot, a firm grip reduces the flight time and angle of trajectory of the ball after impact, suggesting that a firm grip is beneficial during competitive play.

Anaplastic large cell lymphoma: a critical evaluation of cytomorphological features in seven cases

Anaplastic large cell lymphoma (ALCL) is a rare subtype of non-Hodgkin's lymphoma that is difficult to diagnose on fine needle aspiration cytology (FNAC). Detailed descriptions about the cytomorphological features of ALCL are few and do not illustrate the various categories of cells, which we have done in this study. Eight FNAC specimens from seven patients with biopsy-proven ALCL over a period of 6 years were reviewed. Cytomorphological details were analysed and the various cell types identified were enumerated on Papanicolaou and May-Grünwald-Giemsa (MGG)-stained smears. Of the seven cases, five were of conventional type, one of the lymphohistiocytic variant and one of the small cell variant. Aspiration from the conventional type showed a mixed population of lymphoid cells with many plasma cells. The tumour cells were intricately admixed within and distributed as single cells without grouping. Distinct hallmark cells, doughnut cells, embryo cells, tennis racket cells, cells with wreath-like nuclei, Reed-Sternberg-like cells and nondescript cells were seen. Nucleoli were rounded, occasionally tubular in shape. The tumour cells showed abundant blue cytoplasm on MGG stain with a perinuclear hoff. Aspiration from the lymphohistiocytic and small cell variants showed a small number of similar characteristic giant cells. All cases were confirmed as ALK-1 positive. The morphological spectrum of ALCL is wide and it may be misdiagnosed as a metastatic poorly differentiated malignant tumour on FNAC. Identification of unusual giant cells on FNAC helps in establishing a diagnosis on an early lymph node biopsy.

Properties of friction during the impact between tennis racket surface and ball

The “feel” of hitting a ball with a tennis racket has been quantified by vibrations to the hand and by the coefficient of restitution. As play styles are shifting toward hitting with topspin to the ball, the friction between the ball and racket surface, in other words tangential forces in the racket surface, is becoming another index to quantify the feel. In this work, we detect friction by attaching two strain gauges close to the center of right and left frames supporting the cross strings at the middle, and aim to understand the mechanism behind friction from the viewpoint of difference in the angle of incidence of the ball when impacting the racket surface. Results suggest that there is a critical angle of incidence between 45 and 30 degrees, and that this angle is related to the feel.

Dubai International Airport expansion A self‐adjusting prestressed tension rod façade system with parametric design

AbstractDubai International Airport experienced an increase in passenger numbers from 4.3 million in 1990 to some 40 million in 2008, and this figure is expected to rise further. To meet the needs of this influx of travellers and airlines, the government of Dubai launched a major expansion of the airport in 2002, involving the construction of the new terminal 3 plus its concourses 2 and 3 — a total investment of US$ 4.1 billion.The envelope of the double‐curvature concourses, 950 m and 600 m long respectively, is intersected by 142 triangular façades, the “teardrop‐windows”. The prestressed tension rod system has been designed to adjust automatically to the different geometries of the individual façades with up to 475 m2 of surface area. Besides the filigree appearance, easy and reliable installation and pretensioning are the major advantages of the system. The structure of the teardrop‐windows consists of two orthogonally arranged tension rods with opposing curvature. The tension rod system is connected to a perimeter edge beam and is therefore self‐retaining, similar to the well‐known “tennis racket” system. Thus, no reaction forces caused by pretensioning are transferred into the primary steel construction.

Poster Session II, July 14th 2010 — Abstracts: Characterization of grip force during badminton strokes

Badminton is one of the most popular racket sports for which the grip remains crucial for performance. For ergonomic purposes, the hand–handle interface of tennis racket has been studied quite extensively. However, the literature shows a very limited knowledge on badminton grip force intensity. Particularly, the relationship between grip force and different badminton strokes is still poorly understood. Therefore, the purpose of this study was to measure the differences of grip force intensity between four basic badminton strokes, forehand (FH), backhand (BH), smash (SH) and net roll (NR). Ten asymptomatic adult badminton players took part in this study. Grip force was recorded using an embedded capacitive pressure-sensing mat (TekScan 9811) wrapped around a badminton racket handle, from which total grip force could be determined. The sensor was loaded using a force rating of 0-1000N (R2=0,93) and the sampling rate was 500 Hz. The subjects were instructed to hit 20 shuttlecocks, for each stroke, into a target on the court. Only successful trials were analyzed. A one-way repeated measure ANOVA (stroke) was used to compare the effect of grip force. Tukey post-hoc test was used when significant level (p<.05) was reached. Average peak force magnitude was 117,0N±45, 7N, 166, 1N±58, 3N, 209, 0N±56, 1N and 83, 8N±59, 2N for FH, BH, SH and NR conditions respectively. The statistical analysis revealed that grip force applied on the handle was strongly depending upon the stroke, NR grip force was significantly greater than BH (p<.01) and SH (p<.01) grip force. Moreover, the grip force applied in the SH condition was higher than BH condition (p<.01). Data analysis showed that each player had an individual grip force pattern repeatable, but it was observed a strong intersubject variability. The strokes in badminton are a mix of power, precision and manoeuvrability. These characteristics appear to be contradictory: e.g. the literature shows that an increase grip force (power) led to a decrease in wrist motion (manoeuvrability). The main result of the present study is that the subjects took into account these constraints and the type of the strokes by adapting their grip force. Particularly, the precision (NR) and power stroke (SH) were inversely related: a lower grip force led to a greater precision probably allowed by a greater wrist range of motion.

Motor learning

Although learning a motor skill, such as a tennis stroke, feels like a unitary experience, researchers who study motor control and learning break the processes involved into a number of interacting components. These components can be organized into four main groups. First, skilled performance requires the effective and efficient gathering of sensory information, such as deciding where and when to direct one's gaze around the court, and thus an important component of skill acquisition involves learning how best to extract task-relevant information. Second, the performer must learn key features of the task such as the geometry and mechanics of the tennis racket and ball, the properties of the court surface, and how the wind affects the ball's flight. Third, the player needs to set up different classes of control that include predictive and reactive control mechanisms that generate appropriate motor commands to achieve the task goals, as well as compliance control that specifies, for example, the stiffness with which the arm holds the racket. Finally, the successful performer can learn higher-level skills such as anticipating and countering the opponent's strategy and making effective decisions about shot selection. In this Primer we shall consider these components of motor learning using as an example how we learn to play tennis.

Prediction of Impact Shock Vibrations at Tennis Player's Wrist Joint: Comparison between Conventional Weight Racket and Light Weight Racket with Super Large Head Size

The lightweight racket with handle-light configuration and large head size is recent tendency of high-tech tennis rackets, increasing power or post-impact ball velocity with an increasing racket swing speed. This paper investigated the performance of lightweight tennis racket with super-large head size in terms of feel or comfort. It predicted the effect of the mass and mass distribution of super-large sized rackets on the impact shock vibrations of the racket handle and the player's wrist joint when a player hits a flat forehand drive. The prediction is based on the identification of the racket characteristics, the damping of the racket-arm system, equivalent mass of the player's arm system and the approximate nonlinear impact analysis in tennis. A super-light weight balanced racket (mass: 292 g, the center of gravity LG: 363 mm from the butt end) and a conventional weight and weight balanced racket (349 g, LG: 323 mm) are selected as representatives. They are the super-large sized rackets made of carbon graphite with a head size of 120 square inches and the same geometry. The result showed that the shock vibration of the super-light weight balanced racket with super-large sized head is much larger than that of the conventional weight balanced type racket. It also showed that the sweet area of the former in terms of the shock vibration shifts from the center to the topside on the racket face compared to the latter. This is because the location of the grip on the racket handle is further from the location of the node on the handle of the first mode of super-light racket than that of the conventional weight racket.

テニスラケットのスピン性能におよぼすガット・ノッチの影響 : スピン量・接触時間・打球速度の超高速ビデオ画像解析(機械力学,計測,自動制御)

テニスラケットのスピン性能におよぼすガット・ノッチの影響 : スピン量・接触時間・打球速度の超高速ビデオ画像解析(機械力学,計測,自動制御)