Characteristics of the interventions

Interventions lasted between 2 and 12 weeks, with a frequency of 3-4 sessions per week. Exercise protocol varied between the studies as shown at (Table 2). 8 studies [29,32,35,38,45,46,48,50] included sprints with durations of 30s, whereas the remaining studies included sprints with durations ranging from 6-20s. In addition, the work: rest ratio varied from 9s to 4min. Studies reported interval training as: SIT [29,32,33,36,39-41,43,46,48,50], HIIT [30,31,37,42,44,45,49,51,53,54], interval spring training (ISE) [47], supramaximal exercise training (SET) [52], repeated sprint exercise (RSE) [34], sprint interval training in normoxia (SitN) [35] and repeated sprint training in normoxia (RSN) [38]. Four studies compared HIIT/SIT protocols to non-exercise groups [29,42,52,53], whereas the remaining studies compared HIIT/SIT protocols with other exercise interventions reported as: traditional exercise recommendations (TER) [43], continuous moderate intensity (MIT) [45], MICT [31,32,46,48,54], moderate to vigorous intensity continuous training (MVCT) [30] and moderate intensity endurance training (END) [50]. Some other studies included 3 or more groups, most frequently comparing HIIT/SIT to control group and to another exercise/dietetic intervention group [33,3541,44,47,51]. The included studies present clinical heterogeneity in terms of comparator interventions (Table 2); therefore, the results are presented separately in three categories: (1) SIT/ HIIT compared to control-group without exercise, (2) SIT/HIIT compared to another exercise intervention, (3) SIT/HIIT compared to two or more exercise interventions.

Findings from the systematic review

The results of the included studies are summarised in (Table 2). Among the 26 studies, 24 studies assessed body composition changes following SIT/HIIT interventions with 11 reporting significant improvements in several body composition parameters [33,34,36,37,39,41,42,45,47,51,54] compared to the others which did not [27,31,35,40,43,44,46,48,50,52,53]. Moreover, 23 studies assessed cardiorespiratory fitness and 19 studies reported significant improvements post-intervention of HIIT/SIT [2934,36,37,39,40,42,44-51], whereas 4 studies stated no changes from baseline or any significant changes [38,43,52,53]. While 6 studies reported significant changes in some blood markers following HIIT/SIT [35,37,41,45,47,53], 7 studies reported no significant changes [31,36,40,42,43,51,52]. In addition, 9 studies assessed physiological parameters [29,36,40,42,43,45,46,50,52] and only 5 of them reported significant changes for some variables [29,42,46,50,52].

Studies that compared SIT/HIIT to control-group (no exercise)

The results of the included studies are summarised in (Table 2). Regarding the studies that compared SIT/HIIT to a non-exercise group [29,42,52,53], only one out of four studies did not report body composition outcomes [29]. Among the studies comparing HIIT/SIT to a non-exercise group, significant reductions in body composition parameters were reported in one study by Heydari et al. (2012), including BW, BMI, WC, FM (Kg), BM (%), along with improvements in FFM (Kg) [42]. However, another study by Jabbour et al. (2017a) reported no changes in body composition following 2 weeks [52] and 6 weeks of sprinttraining [53]. Moreover, all of these studies reported maximal oxygen consumption (VO2peak) as an outcome [29,42,52,53], with two out of three studies showing significant improvements of VO2peak [29,42]. Regarding cardiovascular parameters, Trilk et al. (2010) and Heydari et al. (2012) [29,42] reported significant reductions in resting HR and stroke volume (SV) post-intervention, while cardiac output (CO) remained unaffected. Changes in glucose levels varied across studies, with Jabbour et al. (2017b) showing significant reductions in glucose but wasn’t significantly different between the groups [53]. Systolic blood pressure (SBP) was significantly improved in one study, while diastolic blood pressure (DBP) did not differ between baseline and 2 weeks postintervention [52].

Studies that compared SIT/HIIT to another intervention

Moreover, studies which compared SIT/HIIT to another exercise-group, reported various findings regarding body composition. In some studies, both HIIT/SIT and other exercisegroup demonstrated similar improvements in body composition parameters [45,49,54].

For example, Tsirigkakis et al. (2021) [49] compared 2 different HIIT protocols lasting for 8 weeks and found comparable improvements in body composition, while Fisher et al. (2015) reported similar outcomes between HIIT and MIT-groups over a 6 weeks period [45]. On the other hand, Umamaheswari et al. (2017) reported significant improvements in body composition parameters including [(BF (Kg), FFM (Kg) and BF%], specifically in the SIT/HIIT group compared to MICT [54]. However, conflicting results emerged from other studies, with some of them reporting no significant improvements following HIIT/SIT interventions [27,31,43,46]. Notably, Higgins et al. (2016) found that the improvements of these parameters were greater in SIT-group compared to MICT-group [27], whereas Cocks et al. (2013) found that only MICT-group improved significantly these parameters [46]. Additionally, some specific protocols such as END, MICT and MVCT, were associated with greater improvements in body composition parameters compared to HIIT/SIT in certain studies [30,48,50]. Regarding cardiorespiratory fitness, most studies reported significant improvements following interventions, with some variability in the outcomes. While some studies found no significant differences in VO2peak improvements between HIT/ SIT and other exercise-groups [30,31,46,49], others reported significant differences favouring either HIIT/SIT or alternative protocols such as END [45,50]. For instance, Fisher et al. (2015) observed significant greater improvements in MIT-group [45], while Higgins et al. (2016) reported significant improvement in VO2peak in SIT-group [27].

Findings regarding other physiological outcomes such as blood pressure, resting HR and lipid blood profile varied across studies. Some studies reported no significant changes in these parameters following HIIT/SIT interventions [43,45,46], while others observed reductions, more specifically in SBP [50]. Moreover, conflicting results were observed in studies assessing blood lipid profile, with some studies reporting no significant changes [30,31] and others observing reductions in specific blood lipid parameters [45].

Studies that compared SIT/HIIT to two or more groups

In studies with two exercise-groups and one control-group without exercise, significant improvements in cardiorespiratory fitness were observed post-intervention, with variations in outcomes among the exercise groups. More specifically, Tong et al. (2018) reported greater improvement of VO2peak in SIT-group than HIIT-group [33]. Moreover, Sim et al. (2015) reported no significant difference among exercise-groups (HIIT vs MICT) [44]. Similarly, studies with three exercise groups and one control-group reported significant improvements in cardiorespiratory fitness. Hu et al. (2021) reported comparable significant improvements in cardiorespiratory fitness across exercise groups [39], similar to others [34,36], while Gaherman et al. (2016) reported slightly higher improvements in VO2max in the group combining SIT with green tea compared to SIT-group alone [47] and Dupuit et al. (2020) who found lower cardiorespiratory values in MICT-group compared to HIIT-groups [37]. In addition, Carmacho-Cardenosa et al. (2020) reported significant improvements in cardiorespiratory fitness only in HIIT-group under hypoxia [38].

Regarding body composition outcomes, mixed results were found. While some studies reported significant improvements in body composition parameters without significant differences between exercise groups [33,44], others found differences among groups. Among studies with three exercise groups, significant improvements were found in body composition parameters, but with differences between interventions. For instance, Martins et al. (2016) reported significant improvements in several body composition parameters but the improvements did not significantly differ between groups [51]. Moreover, Cooke et al. (2022) stated significant improvements in body composition parameters in the group combining SIT with intermittent fasting diet, with no significant differences between diet and diet-exercise in combination [40]. Similarly, Zhang et al. (2021) stated significant improvements in body composition parameters following 12 weeks of exercise, with no differences among exercise groups [41]. In addition, the effects of HIIT/SIT on lipid profile were inconsistent across studies. While some studies found no significant changes in blood lipid profile parameters such as TC [40,51] and glucose [36], others reported significant reduction in TC or triglyceride levels in SIT and HIIT-groups [41]. For example, Gaherman et al. (2016) demonstrated significant improvements in TC only in SIT-group [47]. Similarly, Zhang et al. (2021) noted reductions in glucose, triglycerides and TC levels in SIT and HIIT-groups compared to the other exercise groups [41]. Carmacho-Cardenosa et al. (2019) also reported significant reductions in triglycerides during detraining period following SIT [35]. Some studies, such as this one of Cooke et al. (2022) found no significant change in blood pressure parameters following HIIT/SIT interventions [40].

Quality and risk of bias assessment

According to Cochrane scores as described previously, 2 out of the 26 studies reviewed were rated as at low risk (high quality), 11 were rated as at high risk (low quality) and the remaining 13 studies were rated as raising some concerns (moderate quality) (Figure 2). 19 studies were rated as raising some concerns in domain 1, due to missing information of randomization process. 16 studies were also rated as raising some concerns in domain 2, due to deviations from the intended intervention because blinding was not possible for those who participated in exercise intervention groups. 21 studies were rated as raising some concerns in domain 4, due to bias in measurement of the outcome because blinding of assessors and participants was not performed. All studies were rated as at low risk in domain 3 and domain 5.

Figure 2: Risk of bias assessment using Cochrane’s risk-of-bias 2 (RoB 2) tool.

Discussion

This systematic review examined the effects of HIIT with short work bouts (≤30s) on body composition, cardiorespiratory fitness levels and other physiological parameters in overweight and obese individuals. Due to the rising rates of obesity and its association with well-documented health risks, identifying effective exercise interventions is important.

This review suggests that HIIT with short burst of intense activity, observed notable but mixed improvements in body composition parameters conductive to overall metabolic health and reduced obesity-related complications. More specifically, several studies in this systematic review reported significant improvements in body composition parameters following HIIT/SIT interventions, highlighting the efficacy of these exercise modalities in promoting changes in overall body composition. These findings support previous research demonstrating the effectiveness of HIIT/SIT in inducing alternations in body composition among individuals with excess body weight [14,17,55-57]. It was observed that exercising at least 3 times per week can yield to improvements related to body composition parameters. Research, indicates that a higher frequency (≥3) of exercise can lead to an enchantment of fat loss due to the increased catecholamines response, elevated b-adrenergic receptors and heightened fat oxidation compared to low frequency workouts [58]. SIT involves high intensity activity which can lead to a greater calorie burn both during training and post-training, leading to fat loss and improved body composition. SIT can trigger the release of lipolytic hormones such as catecholamines, cortisol, glucagon, growth hormone and due to the high demand of exercise, ATP resynthesis for sustaining exercise primarily relies on glycolytic metabolism [59]. Moreover, HIIT/SIT is more effective in at activating lipolysis because it leads to a higher excess post-training oxygen consumption (EPOC) [60]. On the other hand, lipolysis in continued training primarily results from elevated concentrations of lipolytic hormones which stimulate fat breakdown in adipose tissue causing triglycerides to be broken down into free fatty acids (FFA) and then transported to active muscle cells and mitochondria where they are undergo b-oxidation [59]. This action is depended upon the duration of exercise as intensity remains moderate and consistent [61]. For this reason, this systematic review along with other research mainly did not observe significant differences in body composition parameters between HIIT/SIT and MICT. The majority of the included studies did not meet the exercise recommendations for physical activity (150 minutes of moderate intensity physical activity per week) for the prevention of weight loss/gain. While it is generally accepted that at least 60 minutes of moderate to vigorous aerobic activity every day is necessary for weight loss, the absence of improvement in some studies may be due to the relatively low total energy expenditure [62]. Notably, this pivotal aspect of SIT is not merely the shorter workout duration, but rather the higher overall energy expenditure achieved through out these exercise intervals involving vigorous intensity. Moreover, due to differences in active and passive recovery periods among these protocols the mechanism of the positive effects on body composition may varies. Nevertheless, it was reported that work bouts lasting ≤60s per repetition were found to improve body composition parameters [63,64], while longer work bouts did not show a significant effect [56]. Another aspect that it should be considered is the exercise modality, as previous research imply that running may activate larger and more muscle groups compared to a stationary cycle ergometer which was used in the protocols of the included studies in this systematic review, thus leading to a more effective increase in metabolic rate in contrast to cycling resulting to a greater BF loss [56].

Continuous training is frequently suggested to enhance VO2max, however HIIT impose more significant physiological demands on the cardiovascular system and seem to elicit a VO2max reaction that is similar to or potentially better than continuous training [65]. In this systematic review, the majority of the findings related to VO2max indicated that HIIT/SIT demonstrated similar or slightly superior improvements over continuous training protocols. The findings of this systematic review, extents the evidence that high intensity exercise with shorter burst time (≤30s) is effective for improving cardiorespiratory fitness. According to Wilsoff et al. (2009), it was reported that the degree of adaptation in VO2max and the function, structure of cardiomyocytes is depended on the intensity of exercise [65]. This improvement is believed to occur from mechanisms that involve cardiac contractility which leads to alternations to stroke volume [66]. Moreover, it involves the transportation of glucose and the enhancement of skeletal muscle oxidative capacity and therefore, increase the generation of ATP through improved mitochondrial function [67]. Considering the health significance associated with VO2max [68], the results of the systematic review underscore the valuable ability of SIT to enhance cardiorespiratory fitness efferently, with just 3 sessions on average per week.

While it’s been observed that reductions in BP post-training appear to correlate with exercise intensity [69], previous results from systematic review and meta-analysis examining the effectiveness of HIIT in improving BP have produced conflicting findings [7073]. Moreover, HIIT shows similarities to MICT in reducing arterial stiffness [72] and studies have demonstrated its superiority in enhancing endothelial function [74,75]. Some studies in this systematic review assessed BP and while the majority of them did not report any changes [43,45,46], one reported that a SIT protocol can improve SBP [52]. This improvement in endothelial function may contribute to decreased peripheral vascular resistance, potentially mitigating the impedance mismatches between central and peripheral vessels. This finding suggests that the protocol with shorter interval (≤6s) and “all out” intensity by Jabbour et al. (2017) [52] probably promoted a higher parasympathetic reactivation, which is characterized by maximum effort exertion. This intensity leads to accelerated blood flow and elevated subsequently nitric oxide (NO) levels in the structures of endothelium. This increased level of NO enhances vasodilation capacity, promoting more effective vascularization and increased peripheral compliance and this process lead in a reduction of BP [76].

HR-resting is a crucial indicator of health and well-being, serving as a significant marker for various health conditions [77]. While it was assessed only by 4 studies [29,42,46,50] in this systematic review, all of these studies reported significant reductions following HIIT/SIT protocols. The mechanisms contributing to the decrease of resting HR may involve the reduction of the activity of sympathetic nervous system (SNS), therefore improving SV and reducing resting-HR [78].

In addition, glucose was examined by some studies in this systematic review and only 3 studies reported significant improvements following HIIT/SIT [41,52,53]. The results of this systematic review are in some way in agreement with previous studies assessing individuals with [79,80] and without diabetes [81]. Nevertheless, the baseline metabolic health of the overweight and obese participants, including their baseline glucose level coupled with the diverse nature of high intensity protocols, in which each exercise parameter is manipulated differently and could lead to varying acute and chronic responses. High intensity exercise allows the increase of fat oxidation during its application and this adjustment seems to be partially linked to an improvement of glycolytic activity within the muscle, leading to increased breakdown of glucose in order to produce energy to perform the exercise [53].

Research suggests that approximately one-third of cardiovascular diseases could be linked to elevated levels of TC, due to its role in the pathogenesis of atherosclerosis [82]. While some of the included studies reported that following high intensity protocols significantly improve blood lipid parameters [35,37,40,41,45,47], others reported no changes at all [31,42,43,52]. The results of this systematic review are mixed regarding the impact of HIIT/SIT in lipid profile of obese and overweight individuals, but they are in line with other systematic reviews reporting no significant changes following HIIT [83,84]. In the systematic review they were included studies with interventions duration ranging between 2-5 weeks, which may be very brief to produce significant changes [31,43]. Another reason for these mixed results of the systematic review can be explained by earlier studies which indicated that the levels of cholesterol and their respective normal ranges can differ depending on age of the individual, gender and ethnicity, consequently affecting the interpretation of TC levels as a disease risk factor [85]. The studies included in the systematic review involved individuals of various age groups. Studies have demonstrated a significant relationship between exercise intensity and the levels of growth hormone, epinephrine and fat oxidation. Both these hormones exhibit lipolytic properties and when combined with fat oxidation could have improvements on blood lipids [86]. Given that HIIT/SIT demands higher exercise intensities compared to MICT, it is possible that the greater levels of growth hormone, epinephrine and fat oxidation associated with the application of high intensity may influence lipid profile in a different way than MICT. Moreover, changes in the autonomic nervous system that occur during the application of intermittent exercise might lead to better cardiovascular and hemodynamic responses. More specifically, these approaches are effective in reducing the blood flow to fat tissues because there is an increase in the activity of a specific type of receptor called a2-adrenergic receptors and when they are activated, they signal the fat cells to release fewer fat acids into the bloodstream [87]. This reduction in fat acid could lead to lower levels of blood lipids, such as TC and triglycerides which are well-known risk factors for developing cardiovascular diseases.

As an overall, HIIT/SIT interventions compared to a nonexercise group showed mixed results in body composition and cardiovascular parameters, with some studies demonstrating significant improvements in cardiorespiratory fitness and certain metabolic markers. Moreover, when HIIT/SIT interventions were compared to another exercise intervention, while demonstrated promises for improving body composition and cardiorespiratory fitness, their efficacy compared to other exercise modalities, remains somehow inconclusive. However, the efficacy of HIIT/ SIT in improving these health parameters may vary depending on the duration of intervention, burst work duration of interval and as well as participants characteristics.

The present systematic review has some limitations that are worth nodding. The included studies exhibited variations in the duration of intervention, frequency, total duration of the training program, duration and number of bouts of intervals, the type of recovery periods (active/passive). These differences may bring difficulties in consolidating the results and reaching conclusive conclusions. It was reported earlier that some studies had relatively brief duration of intervention, which has been shown that is probably not sufficient to observe significant changes in various health care parameters of overweight and obese individuals. Another aspect was the participants characteristic, which varied in terms of age and gender and could influence the response of exercise training.

Conclusions

This systematic review has explored the application of HIIT/ SIT with very short intervals in overweight and obese population. These findings suggest that while SIT holds promise as an effective approach for improving various health parameters of this clinical population, there remain several gaps in the literature. While some included studies demonstrated positive results, such as improvement in cardiorespiratory fitness and body composition, others reported inconclusive results or negligible effects. Moreover, the heterogeneity of the studies in terms of protocol parameters, participants characteristics, poses challenges related to the synthesis of the findings and conclusions. The methodological limitations observed across the studies they were related to small sample size, inadequate blinding and randomization procedures, underscoring the need for future high quality studies in this clinical population. Despite these limitations, the overall findings of this systematic review suggest that SIT with a shorter duration of intervals can be a valuable exercise intervention for those with obesity.

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