Computer simulation of crawl arm stroke
A four segment model of the crawl arm stroke is developed and validated. The construction of a theoretical model is discussed in a number of progressive stages. First, discussion is focussed on a basic two segment model; second, on a linearly configured simple three segment model; third, on the same model extended by introducing hand angle changes: then fourth, on modification of the forearm-hand configuration and on the effects of progressive lateral displacements changing as a functionofnormalised cycle time. Validation is sought from the start in a step-by-step process in which a physical model of the arm, first as a one and then a two segment system, is designed. built and tested in parallel. Thus, during the early stages of development, the behaviour of the physical model can be used to check the computer simulation of the theoretical model. Subsequently, as the theoretical model is extended to a four segment system, its validation is achieved by comparison with a series of experimental observations of five highly skilled competitive crawl-stroke swimmers. Film analysis procedures are developed to establish the velocities, lift, drag and total propulsive forces acting on, or generated from the arm and body segments. These parameters are obtained from an analysis of the digitized coordinates of eight body landmarks from two camera views. The two cameras are used with the aid of specially designed open plan periscopes for underwater filming. The initial values of the measured parameters from film are then incorporated into the model and the output values of the simulation are compared with the swimmers actual performances to establish the accuracy of the theoretical model. The comparison use of the theoretical model and the physical model results in a range of error between 6% and 30% for the total force and 6% and 11% for the body velocity. The comparison of the theoretical model and film analysis results in maximum error of 12% for the body velocity and 18% for the total propulsive force. Simulations of the crawl arm stroke during the underwater phase provide examples of the procedure. The examples illustrate the value of such simulations in that quantitative results are obtained which can be used to alter and improve performance. Considerable changes in the total force occur when the force acting on the hand is related to the angle of attack and to the rate at which the angle is altered. A maximum increase of 18% in the total force is obtained when the hand isorientated at 20 degrees to the horizontal on entry compared with an increase of only 14% at 40 degrees. During mid-stroke an entty angle of 10 degrees inaeases total force by 16%. The difference between maximum and minimum peak velocity is 19% when angles increase from 10 to 40 degrees on entry. The body velocity is found to increase by 25% if the input power is increased by 30%. The hand is able to deliver about 46% to 63% of the total propulsive force, the forearm is found to contribute by 28%. and the upper arm by about 2O%. Within limits a 5% change in hand size results in about a 1.6% change in velocity. A maximumin increase of 0.15% in body velocity is obtained as a result of a gradual increase followed by a gradual decreases of the hand angle of pitch, 0.72% as a result of modifying the forearm lateral movement and 2.6% as a result of combined modification.