The block, flight, and underwater phase account for approximately 11%, 5%, and 84% respectively of the total start time ( Slawson et al., 2013). The flight phase is followed by the underwater phase, in which swimmers attempt to maintain a streamlined position with their arms outstretched in front of the head to minimise velocity loss while also performing multiple propulsive undulatory leg kicks (except in breaststroke) until their head resurfaces before the 15 m mark ( Formicola & Rainoldi, 2015). The subsequent flight phase is an example of projectile motion, whereby the swimmer becomes airborne and finishes when they contact the water ( Slawson et al., 2013 Tor, Pease & Ball, 2014). The block phase requires a quick reaction to the starting signal and a large take-off velocity that is primarily horizontal in direction ( Garcia-Hermoso et al., 2013). While the exact nature of starts may differ between the four swimming strokes, there are three primary phases that contribute towards the overall start performance. Swim start performance has been identified as a determining factor for success, especially in sprint distance events, as it is the part of the race that the swimmer is travelling at the fastest velocity ( Cossor & Mason, 2001 Tor, Pease & Ball, 2014). The sex-related differences in key force-time predictors suggest that male and female swimmers may require individualised strength and conditioning programs and regular monitoring of performance. Swimmers who can already generate greater levels of concentric impulse may benefit more from improving their rate of force development and/or technical aspects of the swim start performance. The results of this study highlight the importance of lower body power and strength for swim start performance, although being able to produce greater than 200 or 230 N.s concentric impulse in squat jump did not necessarily increase swim start performance over 5 m and 15 m, respectively. Variables that were statistically significant predictors of time to 15 m in females were concentric impulse, body mass, concentric rate of power development and Reactive strength index modified ( R 2 = 0.841). Concentric impulse, Reactive strength index modified and concentric mean power were identified as statistically significant predictors for female swimmers to time to 5 m ( R 2 = 0.689).
A minimum concentric impulse of 200–230 N.s appears required for faster times to 5 m and 15 m, with any additional impulse production not being associated with a reduction in swim start times for most male swimmers. With time to 15 m, concentric impulse, age and concentric impulse 2 were statistically significant predictors for males ( R 2 = 0.807). Stepwise multiple linear regression with quadratic fitting identified concentric impulse and concentric impulse 2 as statistically significant predictors for time to 5 m ( R 2 = 0.659) in males. Swim start performance was quantified via time to 5 m and 15 m using an instrumented starting block. All tests were performed on the same day, with participants performing three bodyweight squat jumps on a force platform, followed by three swim starts using their main swimming stroke. MethodsĪ total of 38 males (age 21 ± 3.1 years, height 1.83 ± 0.08 m, body mass 76.7 ± 10.2 kg) and 34 females (age 20.1 ± 3.2 years, height 1.73 ± 0.06 m, body mass 64.8 ± 8.4 kg) who had competed at either an elite ( n = 31) or national level ( n = 41) participated in this study. A secondary aim was to determine if any differences exist between males and females in jump performance predictors for swim start performance. The primary aim of this study was to develop a multiple regression model to determine key lower body force-time predictors using the squat jump for swim start performance as assessed by time to 5 m and 15 m in national and international level swimmers. However, it is still somewhat unclear what are the key physiological characteristics underpinning swim start performance. Depending on the stroke and distances of the events, swim starts have been estimated to account for 0.8% to 26.1% of the overall race time, with the latter representing the percentage in a 50 m sprint front crawl event ( Cossor & Mason, 2001).