In recent years, there have been numerous studies investigating swimming speed trends in different Olympic disciplines [1–3]. Berthelot et al. analyzed 3,263 swimming world records established for all quantifiable official contests since the first Olympic Games. They distinguished an increase in swimming speed until the 1970s, where a plateau in swimming speed was reached. It was hypothesized that the maximum possible physiological sports performance for human species will be reached in one generation, which implies that half of all world records will not be improved more than 0.05% by 2027 . Nevill et al. investigated whether swimming world records were beginning to plateau in 100 m, 200 m and 400 m freestyle swimming. They reported a similar plateau effect between the 1980s and 1990s. Notwithstanding, swimming speed increased again by roughly 2% at the beginning of the 21st century . This unforeseen swimming speed improvement was investigated and reported by several studies [4–6].
Smith et al. analyzed on scientific tools used in physiological and psychological disciplines. They concluded that athletes could be distinguished on the basis of their psychological skills and emotional competencies. Colwin  investigated on training processes and how they improved across the years. It was summarized that in any country the future starts with inspired coaches and not the administrators or scientists. Beside physiological, psychological characteristics as well as trainings processes, Berthelot et al. focused on material science in swimming by measuring the impact of the three successive generations of swimsuits on human performance. As outcome of this study three bursts of swimming speed improvements were reported occurring in 2000, 2008 and 2009.
Summarizing the findings from Colwin , Smith et al. and Berthelot et al., mainly four factors were identified to explain the unforeseen swimming speed improvements at the beginning of the 21st century: First, the better evaluation of swimmers by physiological parameters, psychological skills, and emotional competencies , second, more efficient training processes based on better training control , third, deeper pools and more effective ‘antiwave’ lane ropes  and, fourth, new drag-reducing swimsuits .
Considering differences between female and male elite athletes, swimming speed remained stable between 1957 and 2006. Interestingly, female athletes improved their swimming speed in 100 m, 200 m and 400 m freestyle faster than their male counterparts during the 1960s and 1970s, but never outperformed their male counterparts . Between 1991 and 1995, Tanaka et al. reported a decrease in sex difference with increasing race distance. In detail, for freestyle swimmers, the sex difference in swimming speed decreased from 19 ± 1% for 50 m to 11 ± 1% for 1,500 m. Despite this development, sex differences between the top-six finalists at each FINA World Championship and the Olympic Games in 100 m freestyle and 100 m backstroke swimming increased again between 2000 and 2005 . This led to an overall increase in sex difference between 1981 and 2006. Buhl et al. compared medley and freestyle swimming speeds for national (i.e. top ten elite Swiss athletes) and international swimmers (i.e. top eight FINA World Championship athletes) between 1994 and 2011. These authors reported that the sex difference for national and international athletes in 400 m medley as well as freestyle was lower compared to the 200 m distances. Wolfrum et al. focused on national and international breaststroke and freestyle disciplines and reported a decrease in the sex difference with increasing race distance. Relating to sex difference, Rüst et al. also reported a decrease in sex difference with increasing race distance from 50 m to 800 m amongst Swiss elite freestyle swimmers ranked on the Swiss high score list between 2006 and 2010. However, they reported that for 1,500 m freestyle, the sex difference increased compared to 800 m freestyle. Buhl et al. compared medley and freestyle swimming speeds for national (i.e. top ten elite Swiss athletes) and international swimmers (i.e. top eight FINA World Championship athletes) between 1994 and 2011. For both, national and international athletes, the sex difference decreased with increasing race distance in both individual medley and freestyle .
Beside studies on the swimming speed and sex difference trends in swimming speed at indoor pool competitions, also open-water long-distance races were investigated for the same purpose [12–18]. Vogt et al. studied the sex difference of elite open-water swimmers competing in 10 km swimming competitions at European Championships, FINA World Championships, World Cup races and the Olympic Games between 2008 and 2012. The study’s outcomes showed that the swimming speed remained stable for the best elite female and male athletes. Even compared to other long-distance races (e.g. ultra-running, ultra-cycling) the sex difference in 10 km swimming with 7% was remarkably low. Eichenberger et al. analysed the 26.4 km ‘Marathon Swim’ held in Lake Zurich, Switzerland. During the last decade, the sex difference remained stable at ~11.5%. The same authors analyzed the ‘Zurich 12 h Swim’ between 1996 and 2010, whereby results showed that the annual best swimming speed was not significantly different between male and female athletes . A similar event, namely the 46 km ‘Manhattan Island Marathon Swim’ was analyzed for sex difference in swimming speed . The observed time period was from 1983 to 2013. As result these authors stated that the best women were approximately 12-14% faster than the best men. Rüst et al. investigated on the 36 km ‘Maratona del Golfo Capri-Napoli’  and the 32 km ‘Traversée internationale du lac St-Jean’ event  regarding sex difference trends in swimming speed. For the ‘Maratona del Gofo Capri-Napoli’ and the ‘Traversée international du lac St-Jean’ the time period observed was from 1954 to 2013 and from 1955 to 2012, respectively. In conclusion, the fastest women reduced the gap with the fastest men at the 36 km ‘Maratona del Golfo Capri-Napoli’ linearly from ~40% to ~5.6%  whereas the sex difference in swimming speed remained unchanged at 8.8 ± 5.6% at the 32 km ‘Traversée internationale du lac St-Jean’ competition . Zingg et al. investigated on sex difference trends in swimming speed for elite male and female swimmers competing in 5 km, 10 km and 25 km open-water FINA World Cup races held between 2000 and 2012. These authors reported that the sex difference in swimming remained stable in 5 km, decreased linearly in 10 km and increased linearly in 25 km.
To the best of our knowledge, no studies investigated swimming speed and sex difference trends in swimming speed in all swimming disciplines held at FINA World Championships and Olympic Games for very recent years. So far, studies have only focused on swimming speed and sex difference trends in swimming speed in a limited range of disciplines and race distances. The present study is the first to focus on a complete evaluation of swimming speed with a focus on sex difference in swimming speed of all indoor swimming events held at FINA World Championships and the Olympic Games.
Therefore, the aim of the present study was to examine the trends in swimming speed and sex difference in swimming speed in all swimming disciplines held at FINA World Championships and the Olympic Games between 1992 and 2013. We hypothesized (i) an improvement in swimming speeds in all disciplines across the years, (ii) a stability of sex differences in swimming speed over the period from 1992 to 2013, and (iii) a decrease in sex difference in swimming speed with increasing race distance.