The main findings of the present study were, firstly, that elite national and international freestyle swimmers were on average 2.0 ± 0.6% faster on short course than on long course. Exceptions were national women with faster swimming speeds in 1,500 m long course events than in short course and international men with faster swimming speeds in 800 m long course than in short course. Secondly, swimming speed of international men and women increased significantly in short and long course during the 2000–2012 period, whereas only men national swimmers were able to improve on short and long course events. Thirdly, sex-related differences in swimming speed increased over time for national swimmers except for 1,500 m distance but did not change significantly for international swimmers at any course length and distance.
Faster swimming speed in freestyle in short course
The results of the present study generally supported the hypothesis that swimming speeds of male and female freestyle swimmers would be faster on short course than on long course. These results confirm results of previous studies examining effects of course length on performance in 200 m freestyle (Keskinen et al. 2007; Lowenstein et al. 1994). In those races, swimmers spend about twice as long turning and gliding in 25 m pools than in 50 m pools (i.e. 31.5 s versus 13.5 s, on average) (Keskinen et al. 2007). Each turn results in recovery time leading to a decrease in lactate production and an increase in lactate clearance in the upper body and arm muscles which are used for a regular stroke (Craig, 1986; Keskinen et al. 2007; Lowenstein et al. 1994; Telford et al. 1988; Wirtz et al. 1992). Swimmers with a good turning performance have a particularly large advantage in short course. Men with higher peak leg power than women gain more advantage from turning and benefit more from swimming on short course (Dore et al. 2005). Thus, percent differences in swimming speeds tended to be greater for international men than for international women (Table 1). However, the trend was not clear in swimmers at national level.
Decrease in sex-related differences with increasing race distance in freestyle in short and long course
The decrease in the sex-related difference in swimming speed observed with increasing distance is well known for freestyle events on long course, and is attributed to an increasing economy of female swimmers with increasing race distance (Tanaka and Seals 1997). Greater economy in women is due to smaller body size, less body drag, greater percent fat, lower body density, and shorter lower limbs, resulting in a more horizontal and streamlined swim position compared to male swimmers (Hinrichs 2007; Lavoie and Montpetit 1986; Pendergast et al. 1977; Toussaint et al. 1988). Greater swimming economy in women could provide an additional advantage in long course where phases of regular stroke are twice as long as in short course (Keskinen et al. 2007). This might explain our finding that national female swimmers were even faster on long course than on short course in 1,500 m freestyle.
Different temporal trends at international and national level across years
The hypothesis that freestyle swimming performance would improve over time was supported by the results for FINA finalists in both long and short course and for national men over long course, which was not surprising in light of freestyle swimming records set in recent events (FINA, 2012b). Stanula et al. (2012) identified an even longer trend showing that performance in Olympic freestyle swimming events of 50 m to 1,500 m on long course improved during 1896 to 2008.
Improved swimming performance is partly attributable to technological advances, such as deeper, deck-level pools, more effective anti-wave lane ropes, and improved swim suits (FINA, 2010; Nevill et al. 2007). The lack of fundamental improvement in national swimmers, compared to that in international swimmers could reflect less use of advanced technologies or less intense training (Mujika, 1998; Mujika et al. 1995). Particularly short course performance is more dependent on start and turning times than stroking, and therefore, requires more complex technical skills than long course performance (Keskinen et al. 2007; Smith et al. 2002). Both less use of advanced technologies and less intense swim training might also have contributed to the consistently slower performance of Swiss swimmers. FINA regulated the use of full-body, polyurethane swimsuits in 2009, and the Swiss Swimming Federation followed this regulation. Interestingly, no Swiss national record in freestyle swimming on short course has been broken since 2009 for men and 2008 for women (Schweizer Schwimmverband, 2012).
In addition to technological advances, improved swimming techniques and/or training methods, and increased access to the sport by a larger number of athletes probably contributed to the observed improvement in swim performance (Schulz and Curnow 1988; USA-Swimming, 2011). Psychology and motivation, which also affect athletic performance, might differ between national and international level competitors explaining differences of temporal trends in swimming performance (Johnson et al. 2009; Miller 1993).
Male and female FINA finalists showed similar improvement in swimming speed during 2000–2012, which explains the lack of a change in sex-related differences. Nevill et al. (2007) similarly reported that the sex-related differences in various swimming and running events were remarkably stable during the last 60 years. However, our results further showed that the sex-related difference in Swiss long-course events increased over time, because the performance of Swiss men improved, while performance of Swiss women did not. The sex-related difference in Swiss short-course events did not change, because neither men nor women showed improvement on short course. Less improvement over time in Swiss than in FINA swimmers might be a result of the fewer athletes competing at the national level, reducing competitive pressure and selection for faster swimmers.
Limitation, implications for future research and practical applications
Interpretation of these results is limited to some extent by the observational and cross-sectional study design. Moreover possible influences of anthropometric (Kukolj et al. 1999; Latt et al. 2010; Zampagni et al. 2008), biomechanical (Keskinen et al. 2007; Latt et al. 2010), and physiological (Latt et al. 2010) factors were not considered. However, this drawback is compensated for by the large study population, which provided sufficient power to detect small differences between groups that were statistically significant even after implementation of various statistical models to correct for potential confounders, such as multiple participations of athletes and age. Additional studies are required to determine whether course length has similar effects on other swim styles. These studies should include anthropometric, biomechanical, and physiological factors to fully understand the effects of course length on swim performance. For the first time our data provide extensive evidence over the complete range of official freestyle race distances that pool length plays a tremendous role in determining freestyle swimming performance. The results of the present study further indicate that performance in freestyle swimming is still improving at international level while remained unchanged national. Swimmers at national level should aim at more vigorous and optimized training programs to close the performance gap between elite national and international performance level.