## 1 Introduction  
The analysis of swimmers' performance has traditionally examined spatiotemporal variables representing the start, turn and clean swimming distance [1]. The start and turn are fundamentally different skills than free swimming, which does not necessarily indicate a similar level of start or turning performance [1]. Generally, at the elite level, it is not only swimming speed that wins the races but rather the start and turn where most expert swimmers are travelling at their fastest velocity [2, 3]. For instance, the start phase of the 50 m men's freestyle at the 2019 FINA Champions Swim Series at Budapest has showed that 15 m after the start take-off phase, the second-placed swimmer was 0.08 s slower and the final race time difference was 0.15 s. Moreover, over a 200 m event, the turn contributes 21% to total race performance and progressively more as race distance increases [4]. External forces generated during the ventral starts using instrumented starting blocks have been measured since 1971 with a uniaxial force platform placed at the edge of the pool, which had assessed horizontal forces applied by the swimmers' feet during the conventional circular arm swing technique [5]. [PERSON] et al. [6] had optimised [PERSON]'s solution including eight strain gauges in a horizontal bar to measure the horizontal and vertical forces applied by the hands and feet during the grab ventral start technique. Three-dimensional forces were assessed in starts since 2003 by [PERSON] et al. [7] revealing that the grab start performed with hands in between feet was less stable in preventing platform twist. Horizontal forces applied by feet and hands in the backstroke start were firstly measured in 2011 ([PERSON]) using a uniaxial platform and a load cell, being fixed on the starting wall and on the handgrip, respectively. The changes in the starting block design in 2008 implied adjustments in the previously developed instrumented platforms for start (e.g., [2]) force analyses. [PERSON] and co-authors [2] have presented an instrumented block comprised of four triaxial force sensors placed in a main force plate fixed over the starting block, inclined rear plate, handles for ventral and backstroke start and underwater platform with holes over the front surface. Recently, researchers have shown a new device to measure independently three-axial forces exerted on hands and feet in the ventral kick start technique [8], but limitations have been identified on hand placement dependence. Despite the relevant shortcomings for start analysis that the previous systems have provided, none of them are able to narrow in a unique solution for all ventral, backstroke and relay start force analysis possibilities, assuming laterality effects and independency in hands and feet force assessment. The horizontal and vertical force components determine steering start strategies, being the most assessed. However, lateral responses are essentially a controlling movement in starts [7; 9; 10] and it can be better understood when assessing each swimmer's limb force contribution. The description of forces generated during swimming turns also started in the 1970s using uniaxial force plates to assess the horizontal component in tumble and open turns (e.g., [11]). Swimming turns external forces analysis using three-dimensional force plates is still scarce, with the flip technique and its variants being the most commonly investigated (e.g., [12; 13]). A double underwater tri-axial force plate solution developed for independent assessment of forces applied by each foot in a backstroke start can also provide a direct measurement of the combined forces exerted during turns and the determination of foot position. Furthermore, in some turn techniques, coaches can assess laterality data if the foot location is compatible with each force platform (e.g., [13]). [PERSON] and co-authors' underwater force platform design has a multitude of holes system to reduce wave effects [2].