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Breath-by-breath energy expenditure during open water swimming has not yet been explored in an ecological environment. This study aimed to investigate and compare energetics and kinematics of 5 km swimming, in both swimming pool and open water conditions. Through four independent studies, oxygen uptake ( $$\dot{V}\text{O}$$ 2) kinetics, heart rate (HR), blood lactate concentration ([La−]) and glucose level (BGL), metabolic power ( $$\dot{E}$$ ), energy cost (C) and kinematics were assessed during 5 km front crawl trials in a swimming pool and open water conditions. A total of 38 competitive open water swimmers aged 16–27 years volunteered for this four part investigation: Study A (pool, ten females, 11 males), Study B (pool, four females, six males), Study C (pool case study, one female) and Study D (open water, three females, four males). In the swimming pool, swimmers started with an above average swimming speed (v), losing efficiency along the 5 km, despite apparent homeostasis for [La−], BGL, $$\dot{V}\text{O}$$ 2, $$\dot{E}$$ and C. In open water, swimmers started the 5 km with a below average v, increasing the stroke rate (SR) in the last 1000 m. In open water, $$\dot{V}\text{O}$$ 2 kinetics parameters, HR, [La−], BGL, respiratory exchange ratio and C were affected by the v and SR fluctuations along the 5 km. Small fluctuations were observed for energetic variables in both conditions, but changes in C were lower in swimming pool than in open water. Coaches should adjust the training plan accordingly to the specificity of open water swimming.

Critical velocity (CV), anaerobic distance capacity (ADC), maximal instantaneous velocity (V (max)), and aerobic inertia (tau) were calculated from two (CV(2par) and ADC(2par)), three (CV(3par), ADC(3par), and V (max 3par)), and four-parameter model data (CV(4par), ADC(4par), V (max 4par), and tau), which were obtained from six different times and distances (50, 100, 200, 400, 800, and 1,500 m) swum in front crawl stroke under maximal intensity. Fourteen swimmers (14-15-year-old; sprint and endurance groups, each group n = 7) volunteered in this study. CV values were not influenced by the groups. The model effects showed that CV(2par) was higher than CV(3par) and CV(4par) regardless of the group used. In addition, CV(3par) and CV(4par) were similar. ADC seems to be better estimated using both three- and four-parameter models. V (max) was higher in the sprint group regardless of the model used. The models effects showed that the V (max 4par) was higher than the V (max 3par) regardless of the group. Sprint and endurance groups showed similar tau values. The analysis of the models (F test, coefficient of determination R (2), and adjusted coefficient of determination R (adjusted) (2) ) showed that the three-parameter model was more appropriate among the applied models. Although the four-parameter model showed better correlation for the endurance group, the inclusion of tau (fourth parameter) did not significantly improve the quality of adjustment. However, it is important to emphasize the availability of another parameter for the study of bioenergetics in swimming and other sports.