INFLUENCE OF RACKET LENGTH ON TENNIS STROKE

The current design of tennis rackets tends toward so-called long body rackets, expected to produce a higher rebound velocity because of the more distally situated hitting point. To study the influence of racket length on the tennis stroke, a standardized computer- model was used, as described by Detlefs and Glitsch (1996). This computer simulation allows a 100% reproducibility with an almost arbitrary time resolution and an independent variation of all input parameters. The experimentally determined geometrical mass distributions of several existing rackets served as input variables for this model. The interesting results are the rebound velocity of the ball and the joint forces of the grip, wrist and elbow. The investigation indicates that the shape of the longer rackets is either obtained by a simple elongation of the grip, keeping the design of the short version, or by creating a completely new design in regard to the mass geometry. As we can see from Fig 1, the gain from a 2% higher rebound velocity increases the loads in the wrist (16%), elbow (17%) and particularly the grip joint (212%), which results in no advantages at acceptable costs for the hobby player. On the other hand, more sophisticatedly designed long body rackets increase ball velocity without producing higher impact loads on the arm. Thus only rackets that are especially designed for a long version yield advantages in tennis performance. An increase in length alone is not a significant feature for the performance of a racket. References: Detlefs, C.; Glitsch, U. (1996). Kinetics of the computer simulated tennis stroke with different rackets. Proc. XIVth Intern. Symposium on Biomechanics in Sports, Funchal, 573-576

MODELING OF ELASTIC RACKET PROPERTIES IN THE DYNAMIC COMPUTER SIMULATION OF TENNIS

INTRODUCTION: Experimental difficulties in tennis research caused by the complexity of the stroke and the short contact phase demand the development of complex computer simulations models which can lead to a better understanding of the tennis stroke. Two different methods are currently in use: (1) The direct dynamics approach, which simulates the dynamic interaction between arm, hand, racket and ball, considering all inertial properties. (2) The finite element method, which analyses the elastic behavior of rackets under static conditions. The aim of this study was to evaluate a complex dynamic simulation model of the tennis stroke, including the elastic racket properties. Therefore a combination of both approaches was tested, by using the results of a finite element analysis as input for a flexible racket model in direct dynamics. METHODS: The racket model was an elastic beam (78 nodes) with parameters determined experimentally. For the dynamic analysis, the model was combined with a multiple rigid body pendulum simulating the players’ arm. By comparing the results of various stroke simulations with different boundary conditions and experimental data, the requirements of a flexible simulation model were worked out. The interesting parameters are acceleration and the vibration frequencies of the racket. RESULTS: First it must be stated that a complex dynamic tennis simulation including all important mechanical properties (inertial and elastic) of the racket is possible (Fig. 1). Second, the analysis of the vibrational parameters indicates that the tennis racket behaves as a freely vibrating body. Only the combination of this racket model with a suitable hand-racket-connection can simulate a real tennis stroke (Fig. 2). Thus, with the described model different previous results could be validated by computer simulation: (1) It is not necessary to fix the handle with large grip forces. (2) Racket tests with a clamped handle lead to incorrect results. CONCLUSIONS: The findings of this evaluation study confirm the possibilities of dynamic tennis simulation. Further investigations concerning the influences of racket properties (e.g., stiffness, node locations) on stroke characteristics are conceivable. [Figures

MECHANICAL ENERGY DIFFERENCES BETWEEN WALKING AND RUNNING AT DIFFERENT VELOCITIES ON TREADMILL

INTRODUCTION: Cavanagh (1990) described a variation from 170 to 1700 W in power output for the same movement (running at 3.6 m/s) calculated by six different authors. These differences occurred mainly due to different procedures for energy calculation and generated data that are not comparable. The purpose of this investigation was to describe, analyze, and compare the mechanical energy curves (total, internal and external energies) for six subjects while walking and running on treadmill, by using the same procedure for energy calculation. METHODS: Six male subjects were filmed with two video-cameras (Sony-50Hz) while walking at 1.5 m/s and running at 3.0 and 4.0 m/s on a treadmill. After a manual digitizing process, a 3D analysis was performed from the kinematics. The analysis was based on a 13 segment model. Positions of segmental centers of gravity, segmental weights, and moments of inertia were estimated on the basis of tables devised by Dempster (1955) as revised by Winter (1979). The components of mechanical energy were calculated at each instant of time, using the equations described by Zatsiorsky et al. (1987). RESULTS AND DISCUSSION: In relation to the differences between walking and running, the following observations were made: a) in walking the greatest contribution to the total change derived from the internal energy, while in running it derives from the external energy; b) the internal and external energy were in phase in walking, and in opposition in running. Comparing the variations in the two velocities of running, the following conclusions were drawn: a) the average value of the absolute total energy at 3.0 m/s was 1237.9 J and at 4.0 m/s 1544.2 J; b) there was a linear correlation (r = 0.84) between the change in velocity and the change in total energy; b) with the increase in velocity, the average increase in the total contribution of the change in internal energy was about 72% and of the external energy 36%; c) there was no change in the contribution of the potential energy to the change in external energy; d) the increase in the internal energy was chiefly dependent on the increase in the kinetic energy. CONCLUSION: Although the results related to the shape of the curves for mechanical energy for walking and running are already a matter of consensus in the field of biomechanics, it would appear that the numerical results are still open to broad discussion

Epidemiological Evidence for Work Load as a Risk Factor for Osteoarthritis of the Hip: A Systematic Review

Osteoarthritis of the hip (OA) is a common degenerative disorder of the joint cartilage that presents a major public health problem worldwide. While intrinsic risk factors (e.g, body mass and morphology) have been identified, external risk factors are not well understood. In this systematic review, the evidence for workload as a risk factor for hip OA is summarized and used to derive recommendations for prevention and further research.Epidemiological studies on workload or occupation and osteoarthritis of the hip were identified through database and bibliography searches. Using pre-defined quality criteria, 30 studies were selected for critical evaluation; six of these provided quantitative exposure data.Study results were too heterogeneous to develop pooled risk estimates by specific work activities. The weight of evidence favors a graded association between long-term exposure to heavy lifting and risk of hip OA. Long-term exposure to standing at work might also increase the risk of hip OA.It is not possible to estimate a quantitative dose-response relationship between workload and hip OA using existing data, but there is enough evidence available to identify job-related heavy lifting and standing as hazards, and thus to begin developing recommendations for preventing hip OA by limiting the amount and duration of these activities. Future research to identify specific risk factors for work-related hip OA should focus on implementing rigorous study methods with quantitative exposure measures and objective diagnostic criteria

Hip Osteoarthritis and Physical Workload: Influence of Study Quality on Risk Estimations—A Meta-Analysis of Epidemiological Findings

In this paper, we critically evaluate the quality of epidemiological evidence on hip osteoarthritis and workload published so far. The influence of study quality on risk estimations was analyzed in sensitivity meta-analyses and meta-regression analyses. Comprehensive searches for epidemiological studies of hip osteoarthritis and occupational workload were performed in literature databases and current reviews. All studies were assessed on the basis of study design, defined quality scores, and relevant confounders considered. In total, 34 suitable studies were identified for critical evaluation. Of these, 20 are prevalence studies and 14 incidence studies. Strong heterogeneity is observed in study design, quality level, and estimated exposure parameters. A consistent positive association between heavy physical workload and hip osteoarthritis was observed only among the male populations, not among the female populations. In general, cohort studies provided lower effect estimates than cross-sectional and population-based case-control studies. Studies with high quality scores also produced lower effect estimates than studies with low quality scores. Consideration of BMI as a confounder in published studies also yielded lower effect estimates than studies without consideration of BMI as a confounder. Our analyses indicate that high-quality studies of the association between occupational workload and hip osteoarthritis provide lower effect estimates than studies of lower quality

The Assembly Specific Force Atlas

Though working in hi-tech industries, physical workload is partly considerable and cannot be avoided due to product and process requirements. Especially in automotive, aviation and marine industries, the assembly of the product geometry requires force exertions in ergonomically unfavorable conditions. Unfortunately internationally accepted methods for the evaluation of those force exertions are widely unknown. Besides some traditional German methods and EN 1005-3 hardly any evaluation methods exist. And even those methods refer to force exertions in primarily upright symmetric working postures. In order to overcome these problems the “Assembly Specific Fore Atlas” was created during the recent years. From a sample of automotive workers (n=273) a set of 54 whole body forces (6 main force directions while standing, sitting, kneeling; in an upward, bent and overhead posture) and a set of 38 types of finger-hand forces (all at MWC value) were measured in the field. The inputs for the types of force exertions required in practice were sampled from a consortium of major European car and truck companies. In addition, evaluation methods were developed that allow calculating maximum recommended force limits from the measured maximum static forces and task and user group relevant parameters. These methods were designed in a traditional as well as screening approach with respect to existing international examples. This contribution gives a short overview on the data collected and focuses on the evaluation methods realized and the results drawn from the first field tests

U-Linien-Montagesysteme – Methoden zur ganzheitlichen Gefährdungsbeurteilung sowie zur Ableitung von Gestaltungsempfehlungen

Im Rahmen eines Kooperationsprojekts untersuchen Forschungspartner der Universität Kassel (Fachgebiet A&O), des Instituts für Arbeitswissenschaft der TU Darmstadt (IAD) und des Instituts für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung (IFA) Arbeitsbelastungen an U-Linien-Arbeitssystemen in einem ganzheitlichen Ansatz sowohl die psychischen als auch die physischen Aspekte. Das vorliegende Paper beschreibt das Projekt und seine Zielstellungen sowie die Messmethodik