The main findings of the present study were, firstly, that the sex-related difference in swimming speed was significantly greater for freestyle than for breaststroke over 50 m, 100 m, and 200 m race distances for Swiss swimmers, but not for FINA finalists. Secondly, the sex-related difference for both freestyle and breaststroke swimming speeds decreased significantly with increasing race distance for Swiss and FINA athletes. Thirdly, race distance did not affect the age of peak swimming performance by women in breaststroke, but the age of peak performance was four years older for FINA women than for Swiss women. Men achieved peak swimming performance in breaststroke at younger ages for longer race distances, and the age of peak swimming performance was six years older for FINA men than for Swiss men. Fourthly, in freestyle swimming, race distance did not affect the age of peak swimming performance for Swiss women, but the age of peak swimming performance decreased with increasing race distance for Swiss men and for both sexes at the FINA World Championships.
Due to the observational and cross-sectional study design interpretation of present results is limited to some extent. Furthermore, possible influences of anthropometric, biomechanical and physiological measures could not be considered [19–21]. However, this drawback is compensated for by the large study population providing sufficient power for the statistical analyses.
The sex-related differences in peak swimming speed
Results for elite Swiss swimmers supported the hypothesis that sex-related differences in peak swimming performance were smaller for breaststroke than for freestyle swimming. This finding can be partly attributed to the biomechanics of the swimming styles, particularly the front crawl, which is the fastest technique and typically used for freestyle swimming . At high swimming speeds, women have a higher stroke rate and shorter stroke length than men, resulting in a poorer performance, and this effect is more pronounced in the front crawl than in breaststroke . Furthermore, Sánchez  found that men have a higher stroke index (SI) than women, and that the SI is higher in freestyle than in breaststroke, which could translate into a higher swimming efficiency for women in breaststroke swimming. Indeed, Havriluk  found that women have a higher level of technical efficiency than men in breaststroke swimming, relative to the sex-related difference in freestyle swimming. Sex-related differences in body drag could also affect performance differences between swimming styles. Breaststroke is the swimming style with the greatest body drag, while freestyle has the least drag . Female swimmers have a lower degree of body drag than men [25, 26], and should have a greater gender advantage in breaststroke than in freestyle swimming. Nevertheless, women lack the absolute power to achieve comparable performance times  and consistently underperform men in both freestyle and breaststroke. This might be especially important in short races, where anaerobic capacity and upper extremity muscle power are most influential.
In contrast to the Swiss data, the FINA data did not support the hypothesis that sex-related differences are smaller for breaststroke than for freestyle swimming highlighting the fact that the performance difference between men and women is not solely of sex-related nature. Different skill levels at national and international level must also be taken into account. This is further supported by studies of performance determining factors, like stroke rate, arm lag time, and simultaneous arm-leg propulsion time . These factors differ not only with sex but also with performance level and event. FINA competitors represent a much larger pool of elite athletes than the Swiss pool and have greater opportunities for training, particularly women. Top FINA women might be sufficiently stronger freestyle swimmers than top Swiss women to close the gender gap to some extent. In fact, the sex-related difference in freestyle swimming was smaller for FINA athletes in 50 m races than for Swiss athletes in 200 m races (10.3% vs. 12.4%, respectively). At international women have the same access to swimming training compared to men, which leads to a higher training load compared to national level. Mujika et al.  described a positive correlation of training load on performance in competitive swimming which might explain this circumstances .
Data for both Swiss and international swimmers supported the hypothesis that sex-related differences in both breaststroke and freestyle decline with increasing race distance. This result confirmed previous findings of Tanaka and Seals , who concluded from freestyle records that women swim more efficiently than men, and so show relative improvement in performance as race distance increases. The more economic swimming in women has been attributed to smaller body size, which reduces drag, as well as shorter legs, a greater percentage of body fat, and lower density, which results in a more horizontal and streamlined position [8, 9, 12].
The age of peak swimming performance
Results of the present study did not support the hypothesis that the age of peak swimming performance is similar for breaststroke and freestyle swimming. Both Swiss and international women and men exhibited peak swimming performance at younger ages in breaststroke than in freestyle events. However, when 10-year age classes were used to analyse the data, this difference was not seen showing that the use of the finest possible age scale is important to detecting such differences.
Both Swiss and FINA data corroborated previous findings that, in freestyle swimming over distances of 50 m to 1,500 m, women generally achieve their peak swimming performance at younger ages than men . Earlier maturation and puberty in women [31, 32] account at least partly for this difference. Maximal increases in bone width, mineral content, and density occur earlier in women than in men . The growth pattern of metacarpal bones also shows a two-year difference between sexes . Lean body mass, which primarily reflects muscle mass, begins to increase during early puberty in both sexes, but females gain more fat-free mass than males . Fat mass increases more during later puberty in women than in men , which might increase swimming efficiency, as mentioned previously.
One exception to the relatively consistent sex-related difference in the age of peak performance was in the 200 m breaststroke. Based on the age group with the numeric fastest swim, both Swiss and FINA men achieved their peak swimming speed at a younger age than women. However, the reason for that finding remains unclear and warrants further investigation. With regard to the statistical power of the underlying analysis we consider this finding rather incidental, in particular as there is no obvious physiological explanation behind.
Top female and male athletes in the FINA World Championships were 4–6 years older in breaststroke and 3–4 years older in freestyle than top Swiss swimmers. This difference can be explained by the higher performance required to successfully compete at international level. Skills and experience that take years of high training load are usually required to close the performance gap between national and international levels, and achieve qualification times for international events .
Finally, results of the present study showed that the age of peak swimming performance by women in breaststroke and freestyle swimming was independent of race distance in the 50 m to 200 m range. Schulz and Curnow  found peak swimming performance for women in 100 m freestyle events at about the same age (19.4 years) as the present study, but at a younger age (17.6 years) for 400 m races . In contrast, Tanaka and Seals  found that women achieved their fastest swimming times for longer freestyle distances (1,500 m) at 30–35 years, and short distances (50 m) at 20–30 years . However, Tanaka and Seals excluded the 10–19 years age group from their analysis, and Schulz and Curnow  included rather old data in theirs. The present study did not include distances greater than 200 m, so further study is needed to determine the actual age of peak performance for long freestyle and breaststroke races.