Cardiorespiratory Fitness and Performance Adaptations to High-Intensity Interval Training: Are There Differences Between Men and Women? A Systematic Review with Meta-Analyses

High-intensity interval training (HIIT) involves repeated bouts of exercise performed in the vigorous intensity domain interspersed with periods of complete rest or low-intensity exercise. High-intensity intervals can last anywhere from 5 s to 8 min and are generally performed above the second ventilatory threshold which elicits a heart rate response between 80 to 100% of heart rate maximum [1]. Variables such as the interval duration, intensity, and the number of work and recovery bouts can be manipulated to develop a myriad of different HIIT protocols. When the work rate increases towards the upper bound of the typical intensity range for HIIT, a specific form of HIIT referred to as ‘sprint interval training’ (SIT) occurs. This involves very brief work bouts, usually around 8–30 s, that are repeated and performed at supra-maximal intensity (greater than that associated with 100% of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max) or ‘all-out’ efforts [2]. These relatively short intervals are also interspersed with either long or short recovery periods [3].

Previous studies have shown that HIIT can elicit equal or greater improvements in maximal oxygen uptake (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max) compared with continuous training [4–7], particularly with the use of higher intensities and longer work intervals [8, 9]. Currently, HIIT is increasingly prescribed as a potential therapeutic intervention to address a variety of chronic illnesses including cardiovascular disease, cancer, and metabolic syndrome, due to the robust evidence showing significantly enhanced cardiorespiratory fitness [6, 10–13]. The approach appears to be a viable strategy for fostering mental, psychological, and cognitive health and may reduce the severity of anxiety and depression [14–19]. Lastly, HIIT can be undertaken without the need for expensive gym equipment or access to commercial exercise training facilities. Overall, HIIT appears to be a feasible alternative to traditional endurance training for improving cardiorespiratory fitness, and may facilitate these changes with a surprisingly low training volume [20, 21].

Anatomical and physiological differences between men and women are believed to underlie differences in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max and endurance performance [22]; however, differences in the adaptation to chronic training are less well known. Studies in which biological sex was treated as an independent variable have been considered crucial towards improving the understanding of overall human health, and also for enabling more personalized, sex-specific training regimens [23]. Compared with men, the absolute aerobic capacity of trained women is 10–25% lower, however, when maximal oxygen uptake (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max) is adjusted relative to body weight, the difference can be reduced to around 5–10% [24–26]. After normalization of body weight, the remaining difference could be due to lower blood hemoglobin concentration, cardiac dimensions, and total blood volume [24, 27–29]. For example, women's hearts and major blood vessels are typically smaller than those of men of the same body weight, ethnicity, and chronological age [22, 30–33]. Similarly, various studies have identified respiratory system limitations in women. Compared with men, height- and weight-matched women appear to have smaller lung sizes [34–36]. Furthermore, the diameter of the conducting airways is lower and the number of alveoli is less than in men, both of which negatively affect airflow and efficiency of gas exchange during heavy exercise [36–39]. Although some research indicates that performance could be impacted by sex differences in lung volume, but not airway anatomy and mechanics [22, 40, 41], such differences likely still contribute to physiological limitations to oxygen transport and thus would tend to exert a negative impact on exercise performance in women compared with age-, height- and/or weight-matched men.

Another physiological sex difference that has the potential to influence exercise response is that the less fatigable type I muscle fibers tend to be more abundant in women [42]. As such, there is evidence that for the same period of high-intensity exercise, women tend to experience less peripheral muscle fatigue-related contractile dysfunction than men, which translates to greater fatigue resistance and faster recovery [43]. Due to the differences in muscle fiber type percentages, women oxidize more fat and less protein and carbohydrate at matched relative intensity compared with men [44], whereas men possess higher glycolytic capacity [45, 46], which would therefore tend to alter intracellular homeostasis to a greater extent in men versus women at an equivalent relative intensity. Moreover, in response to HIIT or SIT, some studies have reported that females present with lower blood lactate levels [47]. Other findings have demonstrated that anaerobic capacity, estimated by energetic equivalents of the phosphagen and glycolytic pathways, may be lower in women when compared with men after a supramaximal effort [48]. Collectively, these studies suggest that women are less prone to peripheral muscle fatigue and have a greater tendency towards more aerobic metabolism than men.

To date, many reviews investigating the role of sex differences on acute exercise responses and chronic adaptation have been narrative in nature [2, 49]. One review concluded that attenuated blood lactate accumulation, lower protein synthesis, and mitochondrial biogenesis occur in women relative to men following SIT [2]. One systematic review, which also included a meta-regression [50], examined the effects of low-volume HIIT on cardiorespiratory fitness in adults and found moderate improvements in the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max of active and sedentary participants, without presenting a conclusion regarding a sex-specific response to HIIT. A more recent meta-analysis concluded that HIIT is an efficient method of decreasing total abdominal and visceral fat mass without differences between men and women, but it did not investigate cardiorespiratory fitness as an outcome [51]. A meta-analysis by Diaz‑Canestro and Montero [52] found significantly larger increases in both absolute and relative \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max after moderate-intensity endurance training in men compared with women; however, this review did not investigate the effects of HIIT as an intervention. Overall, the findings of these reviews indicate the potential for sex to impact health outcomes and cardiorespiratory fitness adaptations to exercise training, yet to date, no definitive conclusions can be drawn regarding how sex differences influence the adaptation to HIIT. Therefore, the objective of this systematic review with meta-analyses was to examine the influence of biological sex on the relative magnitude of adaptations in cardiorespiratory fitness and performance, following either HIIT or SIT interventions.

The main finding of the current review with meta-analyses is that men and women improve fitness and performance outcomes to a similar extent following equivalent HIIT and SIT interventions. In particular, meta-analyzed outcomes for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max and power_AT revealed strikingly similar small to moderate increases for men and women. These findings are consistent with those of Weston and colleagues [50], who found moderate improvements in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max in active and sedentary adults in response to HIIT. Until recently, there has been insufficient research to conclude whether or not sex differences exist in fitness adaptations to HIIT interventions. Our findings expand on the work of Weston and colleagues [50], who could not come to a conclusion regarding a sex-specific response to HIIT, by demonstrating near-identical effect sizes in women and men and a lack of significant between-group differences for the primary meta-analysis of any outcome.

The sensitivity analysis that was undertaken to estimate raw mean differences found that baseline \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max in men was significantly higher, the equivalent of 1.06 L·min⁻¹ or 5.88 mL·kg⁻¹·min⁻¹, compared with women, as expected. The pre-post analysis using a raw mean difference indicated significant overall improvements of approximately 0.23 L·min⁻¹ and 3.40 mL·kg⁻¹·min⁻¹, without the presence of significant between-group differences for men and women. This analysis indicates that the general results were not altered by the use of a standardized mean difference designed for dependent data and provides more practical estimates of effect. The results of this analysis should be taken with some caution, however, since the meta-analytical approach assumes independent data. In particular, the low levels of heterogeneity in the pre-post analyses appear to be underestimated.

While larger effect sizes for PPO were demonstrated for women compared with men across all sub-groupings, this difference only reached the threshold of significance in participants who were well trained at baseline. Despite the presence of this apparent difference, it must be noted that the well-trained male and female sub-groups only consisted of two studies and had a small sample size of only 18 men and 19 women. Baseline training status accounted for all significant heterogeneity in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max and PPO outcomes in moderately trained and well-trained men and women. While decreases in the variability of outcomes for participants who were trained at baseline could make sense from a physiological standpoint as fitness and performance outcomes move closer to a theoretical physiological ceiling, it is unclear from the current analysis what factors contributed to the variability of outcomes for untrained participants.

A significant between-group difference was present overall for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max when sub-grouped by baseline training status, with smaller effect sizes present for the moderately trained groups. This likely reflected some confounding from shorter intervention lengths in the moderately trained groups. All except one study within the moderately trained groups had an intervention length ranging between 2 and 4 weeks, while studies in the well-trained category had a range of 6–8 weeks. Consistent with this, the sub-grouping by intervention length revealed significant between-group differences with smaller effect sizes seen for interventions of ≤ 4 weeks in duration for both sexes. While the female data demonstrated a gradual increase in effect size with longer interventions, the male data demonstrated similar effect sizes but greater variability with all sub-groups longer than 4 weeks in duration. Interestingly, these findings are consistent with the study by Hirsch et al. [72], who highlighted potential differences in the rate of adaptation between men and women, with significant changes in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max occurring during the first 4 weeks for the men in their study, while the significant changes in women occurred during the second 4 weeks.

Conversely, the sub-group analyses for mean group age and intervention type (HIIT versus SIT) did not significantly influence the heterogeneity in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max or PPO outcomes and revealed no significant between-group differences. Furthermore, the observed sensitivity of the mean age sub-group analysis indicated fundamental issues with the robustness of this grouping, and as such these results should be interpreted with caution. Overall, it appears that baseline training status and intervention length may be important factors influencing the variability of outcomes in response to HIIT and SIT interventions for both sexes, rather than age or intervention type.

Except for conflicting evidence regarding fatigue and speed/power decrement, the qualitative analysis of results of the additional performance outcomes (outlined in Table 7) demonstrated a similar lack of differences between men and women regarding the magnitude of change for most performance outcomes. Generally, the included studies supported current knowledge that women are less fatigable than men [66, 73, 76, 91, 95, 96] prior to HIIT. While some of the studies included in this review indicated that men may be able to adapt this to a greater extent than women through HIIT [91, 93], some studies found significant improvements in fatigability or power output through repeated efforts in women only [95, 97], while still other studies found no sex differences at all regarding the change in fatigability [66, 73, 76, 96].

Despite the notable lack of sex differences in the magnitude of adaptation of fitness and performance outcomes in response to HIIT, some of the findings outlined in Tables 7 and 8 indicated potential differences regarding the underlying mechanisms contributing to these improvements. Most notably, all significant sex × HIIT interactions reported in the primary studies that related to central cardiorespiratory adaptations favored men, with women generally demonstrating smaller, and often non-significant changes. Examples of this included the observed increases in peak cardiac output, peak stroke volume, and peak cardiac index in men only after SIT, as demonstrated by Bostad et al. [80]; the significant sex × HIIT interaction for oxygen pulse (the amount of oxygen ejected from the ventricles with each cardiac contraction) with a greater increase in men compared with women as demonstrated by Marterer and colleagues [67]; the increases in accumulated oxygen uptake and decreases in accumulated oxygen deficit after HIIT seen only in men, as demonstrated by Weber and Schneider [74]; and finally, the increases in minute ventilation and decreases in maximum heart rate and the ventilatory equivalents for oxygen and carbon dioxide in men only, as demonstrated by Mucci and colleagues [70]. These findings, together with the increase in coupled and uncoupled mitochondrial respiratory capacity that was demonstrated in men only, as outlined by Chrøis et al. [81], indicate that improvements in oxygen delivery and uptake in men may play a greater role in achieving fitness and performance adaptations after HIIT compared with women. While adaptations to high-intensity exercise via improvements in oxygen delivery and uptake may seem counter-intuitive on initial contemplation, previous studies have indicated that the greater oxygen availability during exercise in women provides an advantage with regard to the fatigability of muscular contractions, even at maximal intensities [43, 99]. Consistent with this, it appears that the sex differences in fatigability can be eliminated under ischemic conditions [100]. As such, it is conceivable that such adaptations may contribute to improvements in maximal fitness and performance outcomes. Similar findings have also been reported in a previous review of responses to endurance training, where increases in left ventricular end-diastolic volume and stroke volume increased to a greater extent in men compared with women [101]. Although this meta-analysis reported outcomes using mean differences, thereby reporting absolute changes in these outcomes, the results reported in the primary studies included in the current review indicate that these differences may also exist in these changes when considered relative to baseline.

In contrast, there was a relative lack of evidence surrounding the mechanisms of adaptation that account for the equivalent improvements in fitness and performance outcomes in women. Interestingly, the study by Hoffmann and colleagues [73] reported a significant correlation where a change in threshold power (LT₂) accounted for 77% of the performance improvement in the 40 km cycling time trial for women. This same result was not observed for men, nor were there any significant correlations between change in time trial performance and measures of heart rate or blood lactate at LT₂, absolute or relative peak power output, or incremental time to fatigue. Another study in the current review [97] found a significant increase in mean power during repeated Wingate tests and greater increases in the cross-sectional area of type IIb muscle fibers in women only in response to four weeks of SIT. Although these are the findings of only two studies, which could have been influenced by exercise mode since both used cycle-based testing and training protocols, some additional information can be gathered from outcomes that have been presented in other contexts. Spina and colleagues [102] compared mechanisms of adaptation in older men and women who participated in 9–12 months of moderate to vigorous uphill walking and running-based endurance training and found that 66% of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max adaption in older men was accounted for by a 15% increase in stroke volume in combination with a 7% increase in arteriovenous oxygen difference at maximal exercise. In contrast, this study found no change in stroke volume in older women and the whole change in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max could only be contributed to an increase in peripheral oxygen extraction. In another example, similar to the relationship noted by Hoffmann and colleagues [73], one cross-sectional study [103] found that 60% of the variability in 10-km performance for highly trained female runners aged 23–47 years was explained by running velocity at the lactate threshold. The authors noted, however, that this relationship was age-dependent, with \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max explaining 74% of the variability in performance in women aged 37–56 years. The potential differences highlighted here alongside the strikingly similar effect sizes seen for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max in the current meta-analysis suggest some potential sex differences in the adaptive responses to HIIT/SIT that may warrant further investigation.

Despite highlighting potential differences in the adaptative responses to HIIT and SIT in men and women, these appear to be only different means to the same end. Improvements in maximal cardiorespiratory fitness along with many other performance measures outlined in the results of the current review were found to occur to a similar magnitude in men and women. In contrast to this, a previous meta-analysis by Diaz-Canestro and Montero [52] reported greater increases in absolute and relative \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max in men compared with women in response to moderate-intensity endurance training. This discrepancy between the results of these two meta-analyses may be influenced by the differing interventions (endurance training vs HIIT/SIT) or the methodological differences between the two meta-analyses. The earlier review by Diaz-Canestro and Montero [52] presented absolute change in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max (mL·kg⁻¹·min⁻¹ and mL·min⁻¹) using raw mean differences, whereas the current review primarily focused on changes relative to baseline reflected as standardized effect sizes, percentage change, and sex × HIIT interactions (see also the comment by Senefeld and colleagues [104]). Despite these differences in overall approach, the sensitivity analysis using a raw mean difference in the current review persisted to indicate that there were no significant differences between men and women for either absolute or relative \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max in response to HIIT/SIT interventions. Overall, these findings seem to indicate that sex differences in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max response may be protocol dependent, and could warrant further investigation.

The current review has a number of limitations. Firstly, in order to minimize confounding from different exercise protocols (exercise dose) across studies, only studies that presented both male and female data were included in the current review. While this ensures that exercise dose is normalized between male and female sub-groups and provides relatively similar numbers of men and women within each analysis, the majority of the included studies were small, which limits the number of participants within the meta-analyses overall. Since studies with small sample sizes tend to be associated with larger effect sizes and greater error, some of the effect sizes seen here have the potential to be overestimated or unduly influenced by a small number of participants. Despite this, the focus of the current review was to identify differences between the relative change in outcomes for men and women rather than quantifying effect sizes. The general effects of HIIT, particularly with relation to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max, have been demonstrated for mixed male and female groups in previous meta-analyses [4, 9, 50], and the current analysis strongly indicates that sex differences in the relative change in these outcomes are minimal.

Another potential limitation of the current meta-analysis could be that while prescribed training doses were matched between men and women in all studies, there was an inability to properly assess the actual dose received in many studies and whether this was consistent with the prescribed dose (dose delivered). Many studies provided only basic details regarding compliance to prescribed exercise, with only a few reporting that this remained consistent between men and women. Despite this, many of the included studies appeared to have been tightly controlled interventions, in which case the differences between the prescribed and actual training doses are unlikely to have been substantially different.

In addition to the small pooled sample size and challenges associated with assessing the fidelity of the interventions, the use of pre-post meta-analysis techniques and multiple effect sizes from individual studies in the same analysis (such as the matched male and female sub-group data) have been widely used, but also widely debated in the literature [105–107]. While the use of dependent pre-post data has been largely accounted for with the use of correlation coefficients in the meta-analyses, sufficient data was not available to calculate correlation coefficients for all studies, therefore many of these were imputed. Furthermore, while the design of the current review ensures that prescribed exercise dose is matched between male and female groups, the wide range of intervention lengths and exercise protocols included in the literature makes it difficult to precisely examine the influence of different protocols on these outcomes. Overall, the approaches used within the current review with meta-analyses aimed to minimize statistical and methodological errors as much as possible in the face of the unique set of challenges associated with the research question; however, the results should be considered within the constraints of the limitations that are outlined here.

Finally, while the current review provides some insight into generalized similarities and differences between men and women regarding physiological adaptations to HIIT interventions, evaluation of the influence of hormonal status on outcomes was outside of the scope of this review. While the influence of hormonal fluctuations in women may be somewhat offset by the current focus on adaptation over several sessions, generally spanning weeks or months, these concepts appeared to be largely overlooked in the included studies and should be a focus of future primary and secondary research. Despite the limitations outlined here, the findings from the current review will be critical in order to fill the research gaps and to promote better optimization of exercise prescription and health for both women and men.

The current review with meta-analyses aimed to clarify sex differences in the adaptations of fitness and performance outcomes in response to HIIT and SIT interventions. The main findings of this review indicated that the magnitude of change in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max and power_AT in response to HIIT interventions is similar for men and women. While a borderline significant sex difference was found for PPO in well-trained men and women, the sub-groups consisted of small sample sizes and therefore should be interpreted with caution. Additionally, qualitative analysis of performance outcomes and concurrent measures of physiological adaptation indicated potential differences in the underlying mechanisms of adaptation for men and women. Lastly, it appears that baseline training status and intervention length may play a role in influencing the variability of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{V}$$\end{document}V˙O_2max and PPO outcomes in both sexes, including significantly smaller effect sizes for interventions with a duration of 4 weeks or less.