# How Should the Transition from Underwater to Surface Swimming Be Performed by Competitive Swimmers?
## 4 Discussion
### Transition Phase  
During transition, the swimmers' SR in all strokes showed greater correlations (medium to large) with average velocity than the SL (small to medium) (Figure 2). This was in line with the increased SR values of elite swimmers after start and turn emersion compared to mid-pool swimming [2], due to a shorter relative duration of the propulsive stroke phases. In a situation of an increased overall drag due to wave drag emergence [8], unfatigued swimmers [38], probably prioritized to increase the SR to overcome resistance. Compared to mid-pool swimming, the stroking parameters during transition exhibited lower relationships with forward velocity (moderate to large) than previously reported (for instance, \(r\) values were from 0.87 to 0.92 for the SR [39; 40; 41]). This highlights the specific characteristic of the transition phase, where other kinematic factors (depth, inclination... ) besides stroking parameters seem to have an important role for building velocity. Indeed, the regression models proposed in the present research, based on the body position and inter-limb coordination of transition, explained up to the 30% of the average velocity for the butterfly stroke, which represents a meaningful finding for swimmers and coaches. When beginning the transition phase, swimmers in the present research were located between 0.20 and 0.52 m below the water surface and, interestingly, the body inclinations averaged 9\({}^{\circ}\) degrees regardless of stroke. These body inclinations were similar (10.68\({}^{\circ}\)) to those reported by [PERSON] et al. [16] in surface swimming. Probably, maximum velocity at which swimming trials were performed in the present research and the swimmer's skill level helped them to counteract the active rotational torque and maintaining a close body alignment to horizontal [14; 42]. This could be the reason why the correlations between body inclinations and average velocity were low, especially in alternate strokes where no undulations were present. For the body depth, values close to 0.4 m below the water surface were in line to the minimum suggested by [PERSON] et al. [20] to avoid wave drag. Interestingly, at this point, backstroke swimmers presented the deepest body positions compared to the ventral strokes that could be influenced by the more incongruent body side (compared to prone position) approaching water surface that would cause higher friction and form drag [43]. However, additionally, different arm pull position in backstroke compared to the ventral strokes (hand trajectory below or lateral to the body position) could explain depth differences. For the coordinative parameters, a shorter time gap between the arm propulsion of both backstroke arms was related to faster transition velocities, which is in line with positive correlations between inter-limb coordination and free swimming velocity [9; 40]. However, surprisingly, the second discrete relative phase in front crawl was negatively correlated with transition velocity (more time gap, faster velocity). This could be related to the role of breathing on the inter-limb coordination [44] after the second arm pull, as swimmers are usually encouraged to avoid breathing on the first arm propulsion after underwater. In the simultaneous strokes, shorter time gaps between the arm and leg butterfly propulsion were correlated with faster velocities during transition, indicating a preferable propulsion continuity to overcome drag forces. In breaststroke, on the contrary, longer times gaps between the arm and leg propulsion (DRP1) were related to faster velocities. Although motor continuity is generally recommended [45], breaststroke swimmers in transition from underwater must recover arms from the underwater pullout (hands close to thighs) instead of a regular arm pull. Therefore, the time gap between the propulsive phases of arms and legs could be expected to be longer. This represents an interesting point linked to the correct coordination of the underwater pullout [46].