HEAD STABILIZATION IN FORWARD TWISTING SOMERSAULTS

In many acrobatic activities the rate of rotation is such that it is not possible to fix the gaze to determine one's orientation throughout the whole period of flight. With increasing head angular velocity the ability to use visual information deteriorates up to 350 degrees/s (Jell et al, 1982; Tomlinson et al, 1980). Stabilisation of the head at particular times during the flight allows visual information to be used during the performance of advanced skills involving rapid rotations. The purpose of this study was to investigate whether elite trampolinists stabilise the head during the performance of forward twisting somersaults. Two cine cameras operating at 100 frames per second recorded four elite trampolinists each performing five forward somersaults with one and one half twists (Rudi). Three dimensional analysis techniques were used to quantify the positions of head and trunk segments. The rate of rotation of the long axes (includes flexion/extension and lateral flexion/extension but not rotation about the long axis) of the head and trunk were compared to assess whether changes in rate of head rotation were associated with a strategy of head stabilisation or with rotation of the upper body. All subjects had two distinct periods of head stabilisation. The first occurred during the first half of flight at times which were consistent within subjects but variable among subjects. Minimum head angular velocity during this first period of stabilisation ranged from approximately 100 degrees/s to 250 degrees/s and was not significantly different from the angular velocity of the trunk. The second period of head stabilisation occurred consistently among subjects between 70% and 90% of the flight phase. Mean head angular velocity during this time ranged from 100 degrees/s to 250 degrees/s and was consistently less than the angular velocity of the trunk (p< .05). It was concluded that elite trampolinists stabilise the head to use visual information when preparing to land

LIFTING PERFORMANCE IN AQUATIC SPORTS

Performance in aquatic sports is limited by the ability to maximize propulsive forces and minimize resistive forces. Propulsive forces may be generated by moving body parts or paddles in a direction opposite the direction of desired travel, that is, by using ‘drag’ forces. ‘Lift’ forces are perpendicular to the motion. Thus, if lateral motions are used, lift forces may contribute to propulsive force. Direct quantification of drag forces and lift forces in realistic settings is problematic. Therefore, scientists have relied on indirect evidence to assess the relative importance of drag forces and lift forces to propulsion in aquatic sports. The purpose of this presentation is to review the evidence relating to the use of drag forces and lift forces in three aquatic skills. These skills are freestyle swimming, flatwater kayak paddling, and the water polo ‘eggbeater’ kick. The implications for technique are discussed. Given the lack of definitive findings, the presenter seeks to stimulate further investigation into aquatic sports rather than to draw conclusions. Since the early 1970s, the view that good technique in freestyle swimming involves sculling actions of the hands to use lift forces in preference to drag forces has predominated. Recent three-dimensional studies have contradicted this view and indicate a need for further investigation. Flatwater kayak technique has changed greatly during the last 15 years following the introduction of the ‘wing’ blade. The wing blade, shaped like an airfoil to generate lift forces by the Bernoulli Principle, has resulted in improved performances. However, based on the small amount of evidence available, it is not yet clear whether lift forces contribute to propulsion more than drag forces. Some possible reasons for the improved performances in flatwater kayaking since the introduction of the wing blade are discussed. Despite its importance in water polo, the eggbeater kick has received scant attention by sports scientists. Recent indirect evidence suggests that lift forces play an important role in the eggbeater kick. It appears that players improve by modifying their motions to use lift forces in preference to drag forces

SOME MECHANICAL PROPERTIES OF THE FIN SWIMMING FIN

The fin is a powerful propelling tool in fin swimming. The purpose of this study was to explore ways of quantifying the mechanical characteristics of fins so that future work can establish the relationships between these characteristics and performance. Measurement of natural frequency, deflection, and transfiguration under water might provide some valuable reference parameters for developing a more efficient fin

A COMPARISON OF TWO FUNCTIONS REPRESENTING VELOCITY OF A HUMAN BODY SUBJECT TO PASSIVE DRAG

The purpose of this study was to compare the goodness of fit of two functions representing horizontal velocity of a human body subject to passive drag. Hyperbolic and exponential functions were fitted to the horizontal velocity data of three glides following push-off from the wall of five swimmers. Measures of goodness of fit included root mean square errors (RMSE), and the coefficient of determination (R2). The hyperbolic function provided a better fit to the actual values of velocity, provided a closer match to the initial velocity, and predicted better the velocities beyond the fitted interval than the exponential function. It was concluded that for the swimmers and range of glide velocities tested, drag was closer to being proportional to the square of velocity than a linear function of velocity

Quantifying obesity from anthropometric measures and body volume data

Obesity has become a serious problem in several developed and developing countries. Three-dimensional photonic scanning (3DPS) is a useful tool to obtain accurate anthropometric measures and body volume data for body shape quantification. Some traditional models have been developed to estimate body fat percentages from anthropometric measures or body volume data for body composition classification and obesity quantification. However, these traditional models are very sensitive to the errors in anthropometric measures and body volume data. Small errors in anthropometric measures or body volume data reduces accuracy of body fat percentages estimated from 3DPS and may lead to misclassifications when quantifying levels of obesity. In this study, pattern recognition techniques, neural networks, were applied to develop a new model which can classify obesity levels from a combination of anthropometric measures and body volume data without estimating body fat percentages. The developed model and the traditional models were applied to determine 2209 male participants’ body composition classes for obesity quantification. The accuracy of the new and the traditional models was determined by comparing the estimated body composition classes with the real body composition classes obtained from dual energy X-ray absorptiometry scanning output. The results showed that the accuracy of the developed model was better than the traditional models. Therefore, the developed model provides more accurate results in body composition classification for obesity quantification

A NEW MATHEMATICAL SIMULATION TO STUDY FLIP TURN CHARACTERISTICS IN FRONT CRAWL SWIM

Swim turns represent an integral factor in determining the final outcome of a swimmer race. The aim of present study was to provide a comprehensive mathematical modelling for achieving kinematic parameters in freestyle flip (pike) turn. In proposed model all attempts have been applied to find out what swimmers should do in order that the turns are accomplished in shorter time. This new mathematical model has been adopted to flip turn but can also be adopted to tuck turn with minor change in calculations. Theoretical considerations suggest that faster upper limbs rotation could lead to a torso pressure gradient, which would induce significant axial flow along the upper limbs toward the torso. Our results demonstrate a better reality of the predicted rotational of body during front crawl swim flip turn. In this new model, we hypothesize that in flip turn the body can be considered and simulated as three thin hinged prisms; upper body, thigh, and shank

A BIOMECHANICAL APPROACH TO DRAG FORCE AND HYDRODYNAMIC COEFFICIENT ASSESSMENTS

A fundamental theoretical analysis for estimation of kinematic and kinetic characteristics of swimmers was developed. The purpose of the study was to present a simple method for data collection, to establish simple formulae for determining the swimmers characteristics and to evaluate the validity and accuracy of the model estimation against the other methods such as; direct measurements; Measurement of Active Drag, and added drag provided by hydrodynamic body. The active drag force was estimated during maximal swimming in front crawl but the method can also be applied for other strokes. The swimmers performed three 10-meter trials with enough rest in between and with zero initial velocity over which average velocity was calculated. By this method the maximum speed of swimmers in 10m swim could also be estimated. The swimmers began to swim from still position after whistling, and stopped swimming at the end of 10m again by whistling, and kept gliding until still position. The time of 10m swim and the glided distance were measured with reasonable precision and then used in the established formulae for determining the velocity, acceleration, and drag force. One of the elite swimmers was requested to perform swimming with different speeds in order to achieve different characteristic curves for proposed model

INITIAL BALL SPEED AND FORCE ESTIMATION AT IMPACT IN VOLLEYBALL AND FOOTBALL

This study was undertaken to show how with a simple electronic design we could estimate the initial speed of a ball at impact as well as the force exerted on it. The system has been used for top elite, top university and beginners volleyball and football players. The electronic system can provide the time of flight of ball and also the time of impact. From the time of flight, the initial ball speed can be obtained while from the time of impact the force exerted to ball can be estimated. The results showed that highly skilled players apply larger forces and produce higher ball speeds than lower- level athletes. The values of initial speeds and forces achieved by this simple arrangement are very comparable to those obtained by other researchers

KINEMATIC DIFFERENCES BETWEEN UPKICK AND DOWNKICK IN UNDULATORY UNDERWATER SWIMMING

Undulatory underwater swimming (UUS) is performed for up to 15 m of each lap in a swimming race, and is important for overall performance. This undulatory motion has two phases- the upkick (knee flexion and hip extension) and the downkick (the converse). This study assessed kinematic differences between the two phases, and determined whether these differences were related to performance in an elite sample. Each of the ten participants performed three 20 m UUS trials, and seven landmarks were manually digitised from the single camera view perpendicular to swimming direction. Differences between phases were found for vertical toe velocity, body wave velocity, hip and knee angular velocities and phase duration (p < 0.05), with differences in mean hip angular velocity and phase duration (p < 0.05) being strongly related to UUS performance