The present study investigated the sex difference in swimming performance of female and male winter swimmers competing in the multiple stages of the Winter Swimming World Cup since 2016. We hypothesized that the sex gap in performance of winter swimming in water below 9 °C would decrease with an increasing swimming distance in combination with lower water temperatures and that this decrease would be dependent on the swimming stroke used. The main results were that female athlete presented significantly longer race times for all strokes and water categories than male swimmers in 25 m and 200 m events. The difference between the sexes was greater in the 25 m than in the 200 m for all strokes and water temperatures. Female athletes presented longer race times in FW or CW than in IW for butterfly stroke, while male athletes presented no difference among water categories in 25 m events. For freestyle stroke, male athletes presented longer race times in FW than CW or IW, while female athletes presented no difference among water categories. Male and female athletes presented no difference in race time among water categories for head-up breaststroke.
Influence of Sex on Different Race Distances
A part of our hypothesis was that women would decrease the performance gap in winter swimming over longer distances. The study shows that male athletes were significantly faster than female swimmers in butterfly, freestyle, and head-up breaststroke races over 25 m across the three water temperature (CW, FW, IW) categories. The finding that men were faster than women in sprint distances is most probably due to the differences in anthropometric characteristics such as body height [21], body composition [22] (e.g., body mass, body mass index, fat mass, body fat percentage, visceral adipose tissue level, muscle mass, total body water), muscle thickness [23] and muscle size [24] of men compared to women. In general, women's body composition showed lower values in the body mass index, fat mass per kg, muscle mass, visceral adipose tissue level but a higher body fat percentage than men. Men may outperform women due to the larger muscle mass and force production ability, resulting in a more significant stroke force in the water [25]. These muscular enhancements result in a rightward shift of the force–velocity curve, leading to faster finishing times [26].
The energy cost to move forward increases with speed, whether on land or water. However, in terrestrial sports, the energy cost is lower than in water since resistive forces in water (hydrodynamic resistance, drag, lower propelling efficiency) are larger than the aerodynamic force on land and need to be overcome. The determinants of the energy costs, like drag and efficiency, as well as energy expenditure in its aerobic and anaerobic components, play a role in the performance of athletes [27]. In sprint races, energy is produced mainly on the anaerobic system compared to endurance events, where the resynthesis of ATP relies more on the aerobic system (65%) than on the anaerobic (35%) [12, 28]. Sex differences for the aerobic system are recorded to be smaller than for anaerobic.
Although muscle mass, anaerobic power and force production are significant predictors of explosive and sprint ability in swimmers [26, 29,30,31], anthropometric characteristics (e.g., body height, body weight, body fat percentage, aerobic capacity) become an important contributor with increasing race distance [21, 32, 33]. The average height for Olympic swimmers in 2016 was 188 cm for men and 175 cm for women [34]. While not measured in the current study, male swimmers tend to be taller than their female competitors. Differences in average height are often associated with higher muscle mass, resulting in longer limb levers and, therefore, a more potent stroking force and faster performance on a sprint distance in swimming [21,22,23,24,25, 34].
Secondly, previous evidence suggested that in longer distances in cold water temperatures, such as the 200 m, we were expecting that women’s higher level of body fat, giving them a better buoyancy and insulation [35, 36], would have a positive effect on their race time compared to men [2, 9, 37]. The present study revealed that the effect size of the sex differences on 200 m was smaller than on 25 m for all strokes and water temperatures, supporting our hypothesis. The assumption was based on scientific research showing female swimmers recorded thicker skinfold scores and, therefore, more subcutaneous fat. This causes women to retain more heat within the body for a longer duration and delay muscle cooling to a fatiguing level so that they may retain their swim speed for a longer time in cold water than males with less body fat [38]. A study investigating the body composition in female and male open water swimmers during a competition in Zurich Lake revealed that women have around 12% more body fat than men, which gives them a better basis in cold water over longer distances [39]. Besides that there is evidence that women have different metabolic and hormonal responses to cold water immersion than men [40]. However, a significant difference in the thermogenic response could not be detected. Cold acclimation showed to increase the brown adipose tissue activity and non-shivering thermogenesis [41]. Thus, one can postulate that women with a larger body fat percentage would benefit from that thermogenesis more than men. Other sources report superior cold tolerance of women compared to men when resting in cold water [42]. Previous experience seems to be an important factor apart from anthropometric characteristics for success in ice swimming, especially over longer distances [43].
Furthermore, research showed that psychological skills training could help swimmers suppress the drive to breathe during cold water immersion [44]. However, in this study comparing the performance of female and male athletes on a sprint distance of 25 m to a middle distance of 200 m, the length of the middle-distance track was relatively short. The positive effect of women’s higher level of body fat, which gives them better insulation in cold water and induces a trunk incline giving the body a more streamlined and efficient position as well as better body buoyancy, might not have developed fully over a distance of 200 m [45]. Besides that, time in water over a distance of 50 m and 200 m might be too short since body core temperature during ice swimming first increases and might not have dropped seriously [16, 43]. Comparing the present study results with elite swimming competitions in average temperatures, for example, at the Olympic Games 2020 in Tokyo, female and male Olympic participants only need half of the time for the same track [46]. Also, sex differences in ice swimmers for 1 km Ice Event were higher than pool swimmers [16]. Future studies need to investigate the performance of men and women on a sprint distance to longer distances (i.e., 450 m, 1000 m). Furthermore, the body composition, core temperature, hormonal activity and especially the fat mass of female and male ice swimmers competing at the international level should be scientifically investigated.
Sex Differences in Swimming Speed for Butterfly Stroke in Relation to Water Category
An interesting finding was that women could not close the sex gap over 25 m butterfly stroke. This is possibly due to the muscular and energetic demands of the butterfly stroke. Compared to freestyle and backstroke, butterfly stroke has a significantly increased energy cost and anaerobic contribution [47]. Consequently, success in sprint butterfly is predicated on an anaerobic capacity determined by body composition [47, 48], specifically muscle mass and net force production ability. As a result, men are likely able to outperform women in such a short event requiring maximal force production.
It is important to note that men are able to produce faster completion times in butterfly stroke over 25 m; it is not possible to compare over longer butterfly distances. Currently, the IWSA only offers the 25 m event distance. Interestingly, the butterfly event has the lowest participation rate compared to freestyle and head-up breaststroke races over 25 m. The number of women competing in this event was considerably lower than the number of men (555 women and 958 men). Therefore, it is possible that this difference may contribute to this significant difference in performance. However, as an additional result of our study, we found that women’s butterfly swimming speed was faster in IW (− 2 °C to + 2 °C) than in FW or CW (over + 2°–9 °C) and closest to the performance of men with a mean time difference of 2.55 s. Regardless of gender, swimmers may exhibit a cold shock response, but the rate of muscle cooling would be reflected in the insulation from subcutaneous fat. Therefore, men's times may slow to a greater extent than women's in IW. In contrast to women, men presented no differences in butterfly swimming speed for the different water categories over the sprint distance of 25 m. Further studies might investigate the sex difference in butterfly swimming over longer distances to reveal whether an improvement in women's performance can be observed over a longer distance in this discipline as well, especially in IW, where the performance gap appeared to be smallest. Besides that, it would be of great interest to examine different characteristics of the swimmers in terms of age and training status.
Sex Differences in Swimming Speed for Freestyle Stroke in Relation to Water Category
We can confirm our hypothesis that the sex gap in water below 9 °C would decrease with an increasing swimming distance since the effect size of the difference between the sexes was greater in the 25 m race than in the 200 m race for all water categories when swimming freestyle. Women's performance was most similar to men during the swim on a middle distance of 200 m in FW with an effect size of 0.22. Interestingly, freestyle is faster than butterfly stroke over 25 m for female athletes only in the cold and freezing water category. In addition, freestyle is the most popular stroke used by swimmers during training [49]. In contrast to freestyle, the breaststroke is a less popular stroke; those with hip and knee problems may struggle to breaststroke without pain [50]. This may explain why more swimmers take part in the 200 m freestyle events (n = 724) of the IWSA World Cup events than in the 200 m head-up breaststroke events (n = 365). Future studies might compare the best female to the best male athletes in this discipline to find the true sex difference. Furthermore, the race times of winter swimmers in the morning and afternoon might be compared since the time of practice is an essential factor in the economy of movement. Studies showed that Olympic swimmers were faster in the late afternoon than in the morning [51].
Secondly, the result of the present study confirms that freestyle is a faster stroke than butterfly or head-up breaststroke for male athletes across all water categories. Interestingly, women's performance during freestyle swimming in relation to the water categories' speed remained unchanged, although men’s freestyle swimming speed over 25 m and 200 m was the slowest in FW (+ 2.1 °C to + 5 °C). Despite adequate preparation and conditioning, rapid changes in environmental conditions (i.e., cold water immersion) have been shown to affect neuromuscular and musculoskeletal systems [2, 52, 53]. Women may have an anthropometric advantage due to shorter stature and a higher body fat percentage, which confers more excellent cold water resistance compared to their male counterparts.
Sex Differences in Swimming Speed for Head-Up Breaststroke in Relation to Water Category
Furthermore, we can confirm our hypothesis that the sex gap in water below 9 °C would decrease with an increasing swimming distance, also for the head-up breaststroke. Men were faster than female swimmers in all water categories, but the effect size of the difference between sexes was greater in the 25 m, ranging from 0.43 to 0.45 than in the 200 m where the effect size between sexes ranges from 0.23 to 0.36. Especially in the IW category, it is noticeable that female swimmers reduced the effect size of sex differences from 0.43 in 25 m races to 0.23 in 200 m races. Generally, head-up breaststroke swimmers' highest tethered swimming force values are recorded [25, 54, 55]. This can be explained by the powerful leg kick of this stroke compared to other swim strokes where the leg's action is primarily to keep body balance [54]. However, the head-up breaststroke appears to be the slowest stroke compared to other swimming strokes [56]. The present study confirms that head-up breaststroke is the slowest stroke for men and women in all water categories. This might be the over-water recovery of the arms and lower continuity of the synchronization of arms and legs. Moreover, female and male athletes presented no difference in race time among the water categories. Future studies should compare the body fat of women and men competing in IW to determine if the smallest difference between sexes in IW is related to body composition.
Limitations and Implications for Future Research
A limitation of this study is the short time frame of four years and the low number of female athletes. A comparatively lower number of swimmers participated in longer events like 450 m and 200 m than in the 25 m. We assume that the sex difference will be even lower with a higher number of successful women. Further studies should include longer distances (i.e., 450 m, 1000 m) to confirm the decreasing sex difference with an increasing distance. Furthermore, this study is exclusively retrospective and descriptive. Potential important variables such as body weight, body height, previous experience, training status, motivation and weather conditions were not considered.