Force Development Profile of the Lower Limbs in the Grab and Track Start in Swimming

Left and right ground reaction forces of 9 male and 7 female national and international level swimmers were measured during grab and track starts. Analysis of temporal, kinetic and velocity measures indicated that while swimmers left the block faster in the track start, grab starts enabled swimmers to generate greater vertical impulses and take-off velocities. No significant differences were found in the horizontal impulses and the time to 6m between the two starts. Left and right force profiles were examined qualitatively to identify key points related to force development strategies and lateral asymmetry INTRODUCTION: The grab and the track starts are the two most commonly used starting techniques in competitive swimming. The major difference is how the lower limbs are placed on the block and used to propel the body forward. While the grab start is characterised by a two-legged take-off technique similar to a two-legged jump, the track start simulates the sprint running bunch start which requires an initial rear leg drive, followed by a front leg drive. While a swimmer may prefer one technique, selection tends to be based on anecdotal beliefs, experience and comfort rather than conclusive scientific evidence. However, studies that have examined biomechanical characteristics of the two starts produced equivocal results (Arellano et al., 2000; Blanksby et aI., 2001; Juergens et al., 1999; Miller et al., 2002). Previous investigations of the kinetics of the two starts (Arellano et al., 2000; Juergens et al., 1999; Miller et al., 2002) generally focussed on the total vertical and horizontal impulses, and average vertical and horizontal forces. However, this information provides limited benefits as it is impossible to determine the force development characteristics of the left and right side independently. This is especially a problem for kinetic analyses of the track start, which is asymmetric in nature. The purpose of this study was to investigate the force profile from the left and right limbs independently during the grab and track start. Information on how the individual limbs are used to generate impulse may provide insights into how swimming starts can be optimised for individual swimmers. METHODS: Nine male and seven female national and international level swimmers participated in the study. The swimmers were proficient at both starting techniques and were given extra dive start training in the lead-up to the study. They performed 3 trials of grab starts and 3 trials of weight forward track start with the preferred foot in front on a custom-built instrumented starting block, consisting of two force plates mounted side by side. This set-up allowed measurements of ground reaction force from each foot independently. The dimensions of the block were in accordance with regulations set by the world swimming governing body, FINA. Force data were collected at 500Hz and low pass filtered (16 Hz) prior to further analysis. Temporal and kinetic information, including peak forces (FPEAK), average forces (FAVE) and impulse (I) in the vertical (z) and horizontal (y) directions, reaction time (RT), movement time (MT), block time (BT) and velocity of the centre of gravity at take-off (V), were derived from the force plate data. Time to 6 m (T6m) was used as the criterion measure and synchronised to the kinetic measures via an underwater LED. Two levels of analysis were conducted to investigate the profiles of the two starting techniques. Firstly, a series of repeated measures Multivariate Analysis of Variance (MANOVA's) were used to examine differences in temporal and kinetic measures between the two starting techniques and to ascertain whether there were any gender effects. Force and impulse variables used in