# How Should the Transition from Underwater to Surface Swimming Be Performed by Competitive Swimmers?
## 1 Introduction  
In the last few years, the importance of the underwater segment of the start and turns in swimming competitions has increased both quantitatively and qualitatively. The contribution of underwater swimming to the total race distances has considerably increased over the last 20 years [1] and elite swimmers now spend between 15 and 25% of the 100 m race distances underwater [2]. Additionally, swimmers achieving the fastest underwater velocities (especially on 100 m events) or the longest underwater distances (on 200 m events) have a critical advantage on their performances during World Championships [3]. The lower drag resistance experienced by swimmers underwater [4] as well as the improvement on the undulatory techniques [5], allow swimming competitors to achieve the fastest race velocities and to prioritize these segments over the mid-pool swimming techniques. The benefits of the underwater swimming for competitive swimmers also include a transfer of momentum to the subsequent surface swimming during starts and turns [2]. Elite swimmers exhibited 5-10% faster swimming velocities after the start emersion compared to mid-pool swimming, with small increases of both stroke rate (SR) and length (SL) [2]. After the turn, those swimmers achieving the longest underwater distances (backstroke and butterfly) also obtained 5-6% faster velocities at emersion compared to mid-pool [2]. This positive impact of underwater on surface swimming can be achieved if a correct transition from underwater (the so-called breakout) is performed. Swimmers must restart their arm propulsion at this point and they no longer maintain the hydrodynamicposition they adopt during underwater kicking [6]. While doing so, they must optimize the stroke timing in order to finish the arm pull (and begin their aerial recovery) when the head reaches the water surface. Additionally, in this phase, swimmers change location (from fully submerged to emerged at water surface) that could have implications on active drag [7], associated with an increase in wave drag [8]. The simultaneous (butterfly and breaststroke) or alternate (front crawl and backstroke) arm stroke techniques, the supine (backstroke) or prone (front crawl, butterfly and breaststroke) body position and the beginning of the flutter kick versus the undulatory kick [5] are other constraints that swimmers must handle in this phase. In relation to the beginning of arm propulsion, previous studies have quantified the inter-limb coordination by measuring the time gap between the propulsive actions of arms and/or legs. [PERSON] et al. [9] found positive correlations between shorter inter-limb time gaps and the free-swimming velocity, although dependent on the swimmer skill [10], stroke [11], gender [12] and the use of added resistance [13]. For the swimmers' body positions, both body inclination and body depth have been examined as the different locations of the centre of mass and centre of flotation cause an active rotational torque, which influences the body projected frontal area [14]. An increased body angle has been negatively correlated with swimming underwater velocity [15] and surface velocity [16] due to increased projected area and, therefore, frontal drag [17; 18]. On the other hand, body depth has been correlated positively with underwater velocity [19] due to decreased swimming resistance [20; 21]. Finally, for the underwater kicking, [PERSON] and colleagues [22; 23] indicated that the underwater undulatory swimming (UUS) velocity is dependent on kicking kinematics (i.e., kicking rate, amplitude, and length (distance per kick)), the kicking rate being the most important parameter. When competitive swimmers begin the first UUS cycle underwater, they are located at relatively greater depth (\(-0.92\) m from water surface) [24] and display greater kicking rate and amplitude compared to the end of underwater swimming [25].