Effects of Aerobic Exercise Versus High-Intensity Interval Training on V̇O2max and Blood Pressure

Today, more than 20% of the world's population suffers from hypertension, a major risk factor for heart disease. Therefore, lifestyle modifications such as dietary change, smoking cessation, and exercise are often prescribed to hypertensive patients as a first-line treatment. This study aims to examine and compare the effects of different exercise regimens on the cardiovascular system, particularly that of high-intensity interval training (HIIT) and lower-to-moderate-intensity aerobic exercise (aka aerobic exercise). After researching various databases and extracting 4,724 studies, 196 were viable within the exclusion criteria related to exercise's effects on blood pressure and maximal oxygen uptake (V̇O2max). Of these, 36 studies were selected as viable, and their data is herein outlined. In addition, the results provided by these studies were summarized, respectively, and the raw data were analyzed using a two-tailed unpaired t-test. Through this study, we aim to clarify whether HIIT or lower-to-moderate-intensity aerobic exercise differ in their effects on improving cardiovascular health. It was observed that HIIT was non-inferior to lower-to-moderate-intensity aerobic exercise in the reduction of ambulatory blood pressure of hypertensive or normotensive individuals. However, HIIT was more effective at increasing cardiorespiratory fitness by means of V̇O2max than aerobic exercise was. Considering the significant time-to-completion difference between both exercise modalities, it is remarkable that HIIT has the same benefits as lower-to-moderate-intensity aerobic exercise on blood pressure and higher efficiency in increasing V̇O2max.


Introduction And Background
The importance of exercise and its impact on human health has been extensively researched and welldocumented [1]. The American College of Sports Medicine's (ACSM) position on exercise and hypertension (HTN) recommends dynamic aerobic endurance training for at least 30 minutes daily, preferably supplemented with dynamic resistance exercise [2]. Nonetheless, the effects of exercise training may vary with different exercise modalities (e.g., endurance training or resistance exercise) and dose parameters, specifically program length, session duration, frequency, and workload or intensity. As such, the optimal exercise training prescription remains unclear [3]. As new trials arise and our databases expand, it is vital to challenge our current knowledge and answer more elaborate questions. In a global community where cardiovascular disease is an increased risk and a major cause of death, researching and comparing the benefits of different exercise regimens on the cardiovascular system is beneficial.
The aim of this investigation was to compare the effects of high-intensity interval training (HIIT) versus low-to-moderate intensity continuous training on ambulatory blood pressure through a systematic review and analysis of both randomized controlled trials and other systematic review papers. HIIT generally refers to repeated sessions of relatively brief intermittent exercise, often performed with an "all-out" effort or at an intensity close to that which elicits VȮ 2 max (i.e., ≥90% of VȮ 2 max) [2]. VȮ 2 max is the highest value of oxygen consumption attained on a particular graded exercise test [2]. This is most commonly an incremental or other high-intensity test designed to bring the subject to the limit of tolerance and, as such, an appropriate measure of exercise intensity [4]. This paper will summarize the mean findings and effects of the respective exercises on various measures.
To understand the effects of HIIT on the cardiovascular system, measures of cardiovascular functioning, such as systolic blood pressure (SBP) and diastolic blood pressure (DBP), were observed and analyzed before and after the exercise regimens. SBP is a measure of the pressure in blood vessels when the heart contracts, while DBP relates to the pressure in the arteries when the heart relaxes between beats [5]. This investigation aims to determine the magnitude of change in SBP and DBP in subclinical and hypertensive populations after one session of exercise or after several weekly sessions. The null hypothesis states that there is no significant difference between HIIT and low-to-moderate intensity continuous training with respect to the effects on the cardiovascular system. Further, the study aims to examine whether the magnitude of change in SBP and DBP was related to exercise program characteristics, such as the number of sessions, duration, intensity, or exercise mode. Additionally, parameters, such as the effects of exercise on VȮ 2 max and angiogenesis, were also noted when available, under the premise that the body works harder after a bout of exercise to restore homeostasis [6]. Doing so increases VȮ 2 max over time and promotes the release of vascular endothelial growth factors from skeletal muscle, promoting angiogenesis.

Data Sources
A total of 4,724 studies were identified through our database search. These were extracted from EBSCO, Pubmed/Medline, Cochrane, EMBASE: Excerpta Medica Database, and DARE electronic databases. After removing duplicates and two initial screening phases (title/abstract and full text), 4,431 studies were withheld based on the exclusion criteria. Of the remaining 293 articles, 92 viable articles published between the years 1990 and 2022 were identified. The others were excluded based on statistically insignificant information and/or failure to meet established parameters. Of these 92, a total of 56 articles were excluded as they were not relevant to the scope of this review. A total of 36 articles were then analyzed for the means of this paper.
Key search terms included exercise or aerobic exercise or high-intensity interval training and HTN or blood pressure or VȮ 2 max or moderate-intensity continuous training, or low-to-moderate intensity continuous training.
The screening process is summarized in Figure 1.

Inclusion criteria
Studies were included in this research upon meeting the following criteria: the study must be written in English and must discuss the effect of low-to-moderate intensity continuous training or HIIT on blood pressure or VȮ 2 max on human adults (>18 years old). The study must be experimental, or a systematic review or meta-analysis of experimental studies on the effects of exercise on blood pressure or VȮ 2 max, or include measurements of the mentioned parameters. The study must include normotensive and/or hypertensive patients. The study must have been published after 1990. Studies excluded from this research were those that did not clearly state the number of participants; studies that did not include blood pressure or VȮ 2 max parameters; studies with diabetic patients or hypertensive patients with comorbid diseases; and studies were also excluded if they were reports, letters, or comments.

Data extraction
The authors gathered to formulate a research question and drafted a research proposal that included a protocol to be followed throughout the review. It included the databases, types of data, and parameters to be used prior to collecting data. The search terms set in the inclusion/exclusion criteria above were followed upon extraction and review of the studies. Each author was responsible for searching at least one of the aforementioned databases. After the extraction, each author was responsible for ensuring that all citations for their respective studies were included in the appropriate format, there were no duplicates, and all studies were accessible through the respective databases. It was ensured that all data acquired were from credible academic databases and that all material had been cited lawfully. Having multiple reviewers ensured that information was reported accurately. The selected participants and values were obtained from per-protocol numbers. Individuals that failed to complete their respective regimens were excluded from calculations. Data from papers that failed to provide statistically significant results or were simply not reported were excluded from calculations.
The authors extracted and summarized all related data from the selected studies and detailed it in Tables below.

Statistical analysis
The statistical analysis was conducted using a two-tailed unpaired t-test for computing variances and pvalues, which were set for a statistical significance <0.05. Mean values were calculated using the data for each of the groups, HIIT vs. low-to-moderate intensity continuous training for each parameter VȮ 2 max, SBP, and DBP.

Results
All studies included in this analysis employed a HIIT intervention or a low-to-moderate intensity continuous training regimen. The number of study participants in HIIT groups ranged from 6 to 183. Most participants were young (18-45 years old) men and women, both normotensive and hypertensive. The HIIT protocols ranged from a single acute session to longer-term multiple sessions (four HIIT sessions, lasting 4 minutes per session, three times per week for 12 weeks). Low-to-moderate intensity continuous training modalities (at ~65% VȮ 2 max) included treadmill running, swimming, and cycling. HIIT exercise modalities (at >90% VȮ 2 max) included bouts of exercise at high intensity interspersed with low-intensity activity.  HIIT: High-intensity interval training; V O2max: Maximum rate of oxygen uptake; SBP: Systolic blood pressure; DBP: Diastolic blood pressure; HR: Heart rate; HRpeak: Heart rate peak; HRmax: Maximum heart rate; VT2: Second ventilatory threshold; vVT2: Velocity associated with VT2; vV O2max: Velocity at maximal oxygen uptake; PPO: Peak power output.

Effect of HIIT on the cardiovascular system
Please note that the sections with no values have been marked with a (-) to indicate that the featured study did not provide a value for the respective parameter. In addition, values that were not statistically significant (p-value >0.05) were also excluded and marked with a (-). The number of sessions with ranging values describes meta-analyses with varying modalities included in their respective studies.
* Studies that describe hypertensive patients have been marked with an asterisk. after working out for 37+ sessions was an 11.38% increase. Papers in this session that provided no VȮ 2 max values were excluded from the calculations. A total of 13 studies described the effects of HIIT on blood pressure. Of these, the mean for the SBP after working out with the HIIT regimen for 1-10 sessions showed a 5.85 mmHg decrease in blood pressure. The mean for the SBP after working out for 11-36 sessions was a 6.84 mmHg decrease. The mean for the SBP after working out for 37+ sessions was a 5.33 mmHg decrease. The mean for the DBP for 1-10 sessions was a 3.30 mmHg decrease. The mean for the DBP (11-36 sessions) was a 3.56 mmHg decrease, and the mean for the DBP for 37+ sessions was a 4.82 mmHg decrease. Values that have been replaced by a (-) in this session were excluded, given that no measurements were provided to meet the necessary parameters or no statistical significance was found by the researchers of their respective papers.  HIIT: High-intensity interval training; V O 2 max: Maximum rate of oxygen uptake; SBP: Systolic blood pressure; DBP: Diastolic blood pressure.
*Papers used for calculations given that the respective parameters were met and statistical significance was provided in the results.
It was observed that HIIT increases VȮ 2 max by an average of 11.52% with a p-value of 0.0107. However, the extent of the increase varies depending on the number of sessions performed. We observed that while 11-36 sessions over the extent of approximately 12 weeks, this is to say, a maximum of three sessions a week yielded a higher increase in VȮ 2 max than performing more than three sessions a week or performing HIIT consistently over a prolonged period of time. The same distribution in values was observed in the mean reduction of SBP. Performing 11-36 sessions over a 12-week period appeared to have a greater effect on SBP than performing more sessions, or less than these, for that matter. The overall reduction of SBP by using HIIT in both hypertensive and normotensive patients was -6.00 on average, with a p-value of 0.0053; the authors noted that the greatest reductions were observed in hypertensive patients as described by the aforementioned studies.  Please note that the sections with no values have been marked with a (-) to indicate that the featured study did not provide a value for the respective parameter. In addition, values that were not statistically significant (p-value >0.05) were also excluded and marked with a (-). The number of sessions with ranging values describes meta-analyses with varying modalities included in their respective studies.

Effect of low-to-moderate intensity continuous training on the cardiovascular system
* Studies that describe hypertensive patients have been marked with an asterisk. Table 4 summarizes the results of the obtained means for the effects of low-to-moderate intensity continuous training on the cardiovascular system regarding blood pressure and VȮ 2 max. After working out for 1-10 sessions, there was a 3.9% average increase from the established VȮ 2 max baseline. The average result for the VȮ 2 max after working out for 11-36 sessions was a 14% increase, and the average result for the VȮ 2 max after working out for 37+ sessions was an 8.2% increase. Papers in this session that provided no VȮ 2 max values were excluded from the calculations. A total of 25 studies described the effects of low-tomoderate intensity continuous training on blood pressure. Of these, the mean for the SBP after working out with the low-to-moderate intensity continuous training regimen for 1-10 sessions showed a 7.0 mmHg decrease in blood pressure. The mean for the SBP after working out for 11-36 sessions was a 4.7 mmHg decrease. The mean for the SBP after working out for 37+ sessions was a 4.7 mmHg decrease. The mean for the DBP after working out for 1-10 sessions was a 4.8 mmHg decrease. The mean for the DBP (11-36 sessions) was a 3.2 mmHg decrease, and the mean for the DBP for 37+ sessions was a 2.8 mmHg decrease.
Values that have been replaced by a (-) in this session were excluded given that no measurements were provided to meet the necessary parameters or no statistical significance was found by the researchers of their respective papers.  HIIT, High Intensity Interval Training; V O 2 max, maximum rate of oxygen uptake; SBP, systolic blood pressure; DBP, diastolic blood pressure.
*Papers used for calculations given that the respective parameters were met and statistical significance was provided in the results.
It was observed that low-to-moderate intensity continuous training increases VȮ 2 max by an average of 8.73%, with a p-value of 0.041. The extent of the increase, similar to what was observed in HIIT, varied depending on the number of sessions performed. We observed that while 11-36 sessions over the extent of approximately 12 weeks, this is to say, a maximum of 3 sessions a week, yielded a higher increase in VȮ 2 max than performing more than three sessions a week or performing low-to-moderate intensity continuous training consistently over a prolonged time. This value distribution was not observed in the mean reduction of SBP. Performing 11-36 sessions over a 12-week period appeared to have the same effect as performing 37+ sessions, both effects being reportedly lower than the initial drop in SBP observed with 1-10 sessions. The overall reduction of SBP by using HIIT in both hypertensive and normotensive patients was -5.47 on average with a p-value of 0.002.

Discussion
With the widespread increase in morbidity and mortality in hypertensive patients, more importance has been placed on preventing the disease and finding better ways to manage it [1]. One of the most commonly given adjunct treatments is a lifestyle change, including modifications to diet, alcohol intake, smoking exposure, and exercise regimen [1]. In addition, numerous studies have shown evidence that supports the use of increased physical activity [1] to decrease and control the development of HTN [42]. To understand multifactorial pathways for how exercise reduces the risk of and protects against cardiovascular disease, parameters such as systemic blood pressure and VȮ 2 max were utilized. These were used to compare the efficiency of HIIT (at ~90% VȮ 2 max) versus low-to-moderate intensity continuous training (at ~66% VȮ 2 max).

HIIT versus low-to-moderate intensity continuous training on VO 2 max
It was noted in this study that the mean values for the effect of HIIT on VȮ 2 max gradually increased per the number of sessions completed. However, as described herein, it was also noted that while there was a benefit to increased sessions, this benefit was to a lesser extent than that of performing approximately three exercise sessions a week. There was an overall increase in VȮ2max for HIIT and aerobic exercise. However, HIIT proved to have a more significant effect than traditional aerobic training on VȮ2max (p<0.02). These results are consistent with recent studies, which have found that "high-intensity aerobic interval training resulted in significantly increased VȮ2max compared with long slow distance and lactate-threshold training intensities (P <0.01)" [43].
A two-tailed unpaired t-test was performed to compare the average effect of HIIT on VȮ 2 max versus the average effect of aerobic training on VȮ 2 max, resulting in P<0.0204. It was observed that HIIT was superior to low-to-moderate intensity continuous training in increasing VȮ 2 max at all session intervals. However, no statistical significance was found regarding the superiority of either HIIT or low-to-moderate intensity continuous training and their respective effects on reducing SBP or DBP. This is to say that in the matter of blood pressure reduction, HIIT was found to be non-inferior to low-to-moderate intensity continuous training. Of note, the average time for the performance of HIIT modalities was approximately 10-15 min for the majority of papers included in this study, whereas the time taken to perform low-to-moderate intensity continuous training sessions ranged in the 40-60 minutes mark. Figure 2 compares the increase in average VȮ 2 max based on the type of exercise, HIIT vs. aerobic exercise, and the number of sessions.

HIIT versus low-to-moderate intensity continuous training on blood pressure
The pooled data suggests to the authors that while both HIIT and low-to-moderate intensity continuous training decrease the blood pressure of both hypertensive and normotensive patients, there is no statistically significant difference as to the extent to which they do so. Meaning HIIT is non-inferior to lowto-moderate intensity continuous training in reducing blood pressure, thus rejecting the hypothesis that HIIT may have a higher impact on blood pressure than aerobic exercise. These findings are consistent with previous studies, which have found that, in contrast to cardiorespiratory fitness, exercise's acute and chronic effects on resting and ambulatory BP appear not to be influenced by exercise intensity [44,45]. In other words, while higher-intensity exercise regimens appear to have a significant effect on cardiorespiratory fitness, it has been observed that the effect on blood pressure reduction is not intensity-dependent. It is of note that studies have shown that the most common barrier to patient compliance to prescribed patient exercise is the lack of time [44]. The decreased time it takes to achieve HIIT versus low-to-moderate intensity continuous training does make the prospect of considering HIIT more attractive to individuals with a busier schedule.
When looking at mean values of the number of sessions performed in aerobic exercises, the mean values did not alter significantly with gradual session quantity increase; this is to say that patients did not experience any statistically significant decrease in blood pressure per session. The pooled data suggests to the authors that implementing either HIIT or low-to-moderate intensity continuous training regimens over long periods or at a higher frequency within a limited period does increase cardiorespiratory fitness in terms of VȮ 2 max, as well as decreasing blood pressure. However, these benefits do not appear to exhibit direct proportionality between the extent of the benefit and the number of workout sessions. Some studies [20] noted the difference in BP between HTN and non-HTN patients; the decrease in SBP was observed to be more significant if the patients were hypertensive before the exercise regime. Furthermore, there were studies in which a portion (~24%) of the subjects in the HIIT group became normo-tone (<130 mmHg systolic).
In contrast, a smaller portion (4.34%) low-to-moderate intensity continuous training group and one person in the control group obtained normotensive values after the study period [21]. These findings are also consistent with other studies, which showed that more intense HIIT modalities yielded a higher reduction in SBP and DBP than lower-intensity HIIT modalities [24]. While there is no significant difference between HIIT and low-to-moderate intensity continuous training in blood pressure reduction, higher-intensity modalities appear to have a slightly greater effect on the blood pressure reduction of previously hypertensive patients.
HTN is a progressive disease capable of establishing many life-threatening complications, including, but not limited to, congestive heart failure, strokes, renal failure, or myocardial infarctions [46]. Some of the most common treatments are lifestyle changes, including modifications to diet, alcohol intake, smoking exposure, and exercise regimen, often combined with drug therapy [46]. Patient compliance is a major issue that arises from prescribing exercise as a lifestyle modification [47]. The most commonly used low-to-moderate intensity continuous training regimens involve moderate activity and require a sustained duration of at least thirty minutes [3]. This review presents statistical evidence rejecting the null hypothesis that "there is no significant difference between exercise modalities of short duration such as HIIT and longer duration such as traditional aerobic exercise, concerning the effects on the cardiovascular system." HIIT requires significantly less time to perform and may result in higher patient compliance. The differences in studies between HIIT compared with low-to-moderate intensity continuous training at lower intensities were analyzed in this study.

Limitations
The authors of this systematic review are aware that there is a limited volume of data and participants in studies concerning the effects of exercise on HTN. An unconsidered issue was the multifactorial pathways of how exercise affects blood pressure, as these are not yet fully understood and may play an important role in the disease progression. The lack of differentiation between younger and older adults and the lack of details portraying the differences between genders is also considered a limitation of this study. Some research studies included acute exercise impact performance with specific VȮ 2 max on the cardiovascular response for a specific recovery time. However, they failed to mention any pre-existing health conditions that might have impacted the final results. Additionally, there was a failure to assess the effect of the intervention of regular physical activity, eating habits, and continued compliance with HIIT and low-to-moderate intensity continuous training protocols daily in all the included studies. In most of the HIIT studies, there was no documentation of the time between the last training session and the blood pressure recording. There was also a lack of reporting regarding adherence to the designed regiment or frequent supervision of adverse events. Differences in the natural aging process, such as hormonal changes, can have undocumented effects on bodily processes, and as such, differences in data may be misleading. The authors concluded that some of the limitations contained enough data values that were a suitable match for the assigned parameters. In research, it can prove difficult to find the necessary data for the guidelines of a specific protocol. This may have significantly impacted the results of the mean value comparison by not having sufficient data to perform a complete comparison among all the studies included in this review. The authors, however, strived to maintain the data as accurate as possible through a rigorous methodology and multi-layered verification of all calculations to avoid skewing the results.

Conclusions
The described findings provide compelling reasons to recommend HIIT as an alternative to patients being prescribed exercise as a treatment. This would better fit scheduling and time constraints while significantly reducing the risk of developing hypertension and, in some cases eliminating high blood pressure. However, future research must be done regarding how exercise affects the cardiovascular system, as well as research describing the effect of various exercise regimes on preventing other comorbidities. With this article, we intend to confirm that there are viable, time-efficient alternatives to the traditional low-to-moderate intensity continuous training regime, which is so often recommended as part of lifestyle modifications for hypertensive and pre-hypertensive patients.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.