## 4 Discussion
The net influence of a lower initial HR in the 12 IP condition on HR achieved in the final minute of the trial affected the female participants more than the male participants. This is the first study to show a sex-based difference in the HR response to different interim periods, although others have shown that a longer rest period in 5 \({}^{\circ}\)C air affects the peak HR achieved at the end of a rowing or running time trial in a mixed-sex cohort [13]. Overall, it is known that theslope of the HR/VO\({}_{2}\) relationship is flatter (i.e., less steep) in colder air conditions [39], and our results indicate that a sex-based difference in accumulated cold stress may also exist, especially when looking at the 12 IP condition. There is limited research to compare to our findings but what might be considered is that males and females with no rest period have a similar average HR (152 male vs. 150 female) when running for 40 min in 10 \({}^{\circ}\)C conditions [40]. In addition, severe cold (\(-\)15 \({}^{\circ}\)C) with no rest period showed no difference in male and female HR responses to an 8-min exercise trial [26]. Thus, our results suggest that a severe cold air rest period may affect the physiology of thermoregulation more in females, and this should be confirmed in future research. Although not statistically significant, female participants did report a 2-fold greater cold discomfort score (were \"severely cold\") compared to males (were \"slightly to moderately cold\") at the beginning of the performance trial in the 12 IP cooling condition. This was also associated with females also being \"cold\" on the thermal state scale in the 12 IP condition compared to males being \"cool\" at the start of the performance trial, a finding that aligns with current perspectives on female responses to whole-body cooling [41]. To the best of our knowledge, this is one of the first studies to measure cold discomfort during a physical activity trial and the first to report a sex-based difference. The original research on cold discomfort found that after 25 min of passive \(-\)20 \({}^{\circ}\)C exposure in one layer of insulative clothing, participants rated themselves as severely cold [36]. In contrast, despite the benefits of two extra layers of insulative clothing and a vigorous warmup, the 12 IP interim period significantly increased cold discomfort in female but not male participants in the present study. It has been shown that the onset of shivering (using a passive cooling model) is significantly shorter in females and related to a low BMI [42], which might physiologically explain some of the differences between sexes in this study. Compared to the males' BMI in this study (21.6 \(\pm\) 1.8), the female participants' average BMI was 18.1 \(\pm\) 1.7 (range: 15.8 to 20.4), which is associated with low adiposity and lean muscle mass [43]. This lower BMI may have then reduced any protective effect that adipose and lean mass might have on delaying the \"threshold of cold perception\" onset [42]. Given that \"feeling cold\" as reported in the 12 IP condition by females has been associated with significant alterations in the skin temperature and core temperature preservation [44], the core temperature results in this study do not fit this conclusion. In fact, female participants in this study did not have, on average, a lower core temperature at T1 compared to males (Figure 5), which highlights, in this case, that perceptions of the thermal state and comfort are not necessarily related to body temperature (i.e., feeling colder is not tied to an actual decreased core temperature). Collectively, these perceptual responses reveal some key sex-based differences in the 12 IP condition that have consequences on competition preparedness, especially in female endurance athletes, and how they perceive cold discomfort and the thermal state prior to a race. The performance trial had the desired effect of being strenuous enough to induce feelings of \"severe\" heavy legs overall with no differences between sexes or conditions.