Effects of Passive or Active Recovery Regimes Applied During Long-Term Interval Training on Physical Fitness in Healthy Trained and Untrained Individuals: A Systematic Review

Background Intermittent exercise programs characterized through intensive exercise bouts alternated with passive or active recovery (i.e., interval training), have been proven to enhance measures of cardiorespiratory fitness. However, it is unresolved which recovery type (active or passive) applied during interval training results in larger performance improvements. Objectives This systematic review aimed to summarize recent evidence on the effects of passive or active recovery following long-term interval exercise training on measures of physical fitness and physiological adaptations in healthy trained and untrained individuals. The study protocol was registered in the Open Science Framework (OSF) platform (https://doi.org/10.17605/OSF.IO/9BUEY). Methods We searched nine databases including the grey literature (Academic Search Elite, CINAHL, ERIC, Open Access Theses and Dissertations, Open Dissertations, PsycINFO, PubMed/MEDLINE, Scopus, and SPORTDiscus) from inception until February 2023. Key terms as high-intensity interval training, recovery mode, passive or active recover were used. A systematic review rather than a meta-analysis was performed, as a large number of outcome parameters would have produced substantial heterogeneity. Results After screening titles, abstracts, and full texts, 24 studies were eligible for inclusion in our final analysis. Thirteen studies examined the effects of interval training interspersed with passive recovery regimes on physical fitness and physiological responses in trained (6 studies) and untrained (7 studies) individuals. Eleven out of 13 studies reported significant improvements in physical fitness (e.g., maximal aerobic velocity (MAV), Yo-Yo running test, jump performance) and physiological parameters (e.g., maximal oxygen uptake [VO2max], lactate threshold, blood pressure) in trained (effect sizes from single studies: 0.13 < Cohen’s d < 3.27, small to very large) and untrained individuals (effect sizes: 0.17 < d < 4.19, small to very large) despite the type of interval training or exercise dosage (frequency, intensity, time, type). Two studies were identified that examined the effects of passive recovery applied during interval training in young female basketball (15.1 ± 1.1 years) and male soccer players (14.2 ± 0.5 years). Both studies showed positive effects of passive recovery on VO2max, countermovement jump performance, and the Yo-Yo running test. Eleven studies examined the effects of interval training interspersed with active recovery methods on physical fitness and physiological parameters in trained (6 studies) and untrained individuals (5 studies). Despite the type of interval training or exercise dosage, nine out of eleven studies reported significant increases in measures of physical fitness (e.g., MAV) and physiological parameters (e.g., VO2max, blood pressures) in trained (effect sizes from single studies: 0.13 < d < 1.29, small to very large) and untrained individuals (effect sizes: 0.19 < d < 3.29, small to very large). There was no study available that examined the effects of active recovery on physical fitness and physiological responses in youth. Conclusions The results of this systematic review show that interval training interspersed with active or passive recovery regimes have the potential to improve measures of physical fitness and physiology outcomes in trained and untrained adults and trained youth. That is, the applied recovery type seems not to affect the outcomes. Nonetheless, more research is needed on the effects of recovery type on measures of physical fitness and physiological adaptations in youth.


Background
Maximal oxygen uptake (VO 2max ) is an important physiological determinant of sports performance particularly in metabolically demanding sports such as middle and long-distance running, cycling, rowing, cross-country skiing, etc. [1][2][3][4][5].Training-induced improvements in VO 2max depend on factors, such as age, sex, and training status [6][7][8][9].Accordingly, training protocols with the goal to improve VO 2max should be individualized according to the needs of the athlete [10][11][12][13].Nevertheless, the available literature provides some guidance on the programming of exercise protocols to enhance VO 2max .For instance, for the training modalities exercise frequency, intensity, time, and type of exercise (FITT-principle), there is evidence that training at or near an individual's VO 2max may be an ideal stimulus to increase VO 2max [3,14].Regarding the type of training, it seems that intermittent exercise protocols consisting of intensive exercise bouts alternated with passive or active recovery regimes can improve cardiorespiratory fitness [4][5][6][15][16][17][18].
Interestingly, high-intensity interval training (HIIT) has not only been applied in sub-elite and elite athletes to enhance their aerobic capacity but also in recreational athletes and even in patients (e.g., individuals with obesity, diabetes, etc.) [19][20][21][22][23].Despite its widespread use, there is no common definition of HIIT; in the context of performance, HIIT can be defined as intermittent bouts of exercise realized at high-intensity, and in the context of health, HIIT can be characterized as intermittent exercise performed at low or moderate intensity [24].As such, the dosage of HIIT protocols varies greatly and differs in exercise intensity, duration of intervals, number of repetitions, recovery types, work-to-rest ratio, and rest time between interval bouts [10,11].The application of HIIT protocols is particularly popular in intermittent sports, such as team sports (e.g., soccer) or racket sports (e.g., tennis), to improve measures of physical fitness [25,26].Physiological adaptations following HIIT are based on complex molecular (e.g., the expression of PGC-1α mRNA) and cellular mechanisms (e.g., mitochondrial density and biogenesis) [27,28].
During the performance of HIIT sessions, immediate post-exercise recovery (i.e., after each repetition or interval), represents an important restorative process (e.g., physiological, psychological) that impacts on the magnitude of the training-induced physiological adaptations [29,30].Both the type of recovery (active, passive) as well as the recovery time influence maximal performance during each interval and the overall physiological stress [31,32].

Active recovery at low-to-moderate intensities during
HIIT may enable larger adaptive potential during the next HIIT exercise bout than passive recovery, but the experimental data to support this claim are inconclusive [33].While some studies report a greater magnitude of adaptation with active recovery regimes [4,5,34,35], other studies indicate that the type of recovery does not have an impact on training-induced adaptations [36].A recent systematic review focused only on acute physiological, perceptual, and performance effects of recovery mode applied between repeated-sprints during running and cycling protocols reported that passive recovery reduced physiological and perceptual demands and reduced loss of performance compared to active recovery in repeatedsprints running, with limited data on cycling studies [37].In contrast, another systematic review on the effects of recovery mode on performance limited to mean and peak power, time to exhaustion, and distance covered during an interval exercise session only indicated that passive recovery aids in maintaining performance during interval exercise [38].
Accordingly, it seems timely to systematically summarize the literature to identify whether interval training with active or passive recovery shows larger adaptive potential after a long-term exercise training.The objective of this systematic review was to gather recent evidence on the effects of the recovery type (passive or active) applied during long-term interval training on measures of physical fitness and physiological adaptations in healthy trained and untrained youth and adult individuals.
The PICOS approach (Population, Intervention, Comparator, Outcomes, Study design) was followed to identify inclusion criteria (Table 1).Only randomized controlled trials and controlled trials that examined the effects of passive or active recovery during long-term interval training, for at least 3 weeks, on measures of physical fitness and physiological adaptations in trained and untrained youth and adult individuals were eligible for inclusion.The following criteria were a priori defined for studies to be eligible for inclusion in this systematic review article: (1)  Studies were excluded if they (1) did not meet the minimum requirements of an experimental study design (e.g., case reports), (2) did not meet the minimum requirements regarding training design (e.g., lack of information on training methodology or testing sessions), (3) were not written in English, and (4) involved individuals with clinical concerns.Published review articles were also excluded from our analysis.
In the current review, "trained individuals" refers to athletes who exercised at least three times per week, and "untrained" refers to individuals who did not meet the World Health Organization minimum activity guidelines of 60 min per day (youth) and 150 min per week (adults) [40].

Literature Search Strategy
We searched nine databases including the grey literature (Academic Search Elite, CINAHL, ERIC, Open Access Theses and Dissertations, Open Dissertations, PsycINFO, PubMed/MEDLINE, Scopus, and SPORTDiscus) from inception until February 2023.The following key terms (and synonyms searched for using the MeSH term database) were included and combined using the operators "AND", "OR",: [(exercise OR training OR "exercise training" OR "interval exercise" OR "intermittent exercise" OR "interval training" OR "intermittent training" OR "high-intensity interval exercise" OR "high-intensity interval training") AND (recovery OR "recovery mode" OR "passive recovery" OR "active recovery") AND ("physical fitness" OR "physiological adaptation*" OR "physiological response*")].
In addition, the reference lists and citations (Google Scholar) of included studies were explored further to detect additional related studies.Since the scope of this systematic review article is large in terms of outcome measures (e.g., physical fitness, physiological adaptations), we performed a systematic review rather than a meta-analysis, as a large number of outcome parameters would have produced substantial heterogeneity.

Study Selection
The final screening was done by two investigators (FR and AJ) based on the relevance of the inclusion and exclusion criteria and the identified items for assessing the effects of passive or active recovery after long-term interval training (at least 3 weeks) on measures of physical fitness and physiological adaptations in trained and untrained individuals using PICOS criteria.If the title of the article was potentially relevant, the manuscript was screened at the abstract level.When abstracts indicated potential inclusion, full texts were reviewed.A third-party consensus meeting was held with another author (HZ) if the two reviewers were not able to reach a consensus.

Quality and Risk of Bias and Assessment
The methodological quality of the included studies was assessed using the Physiotherapy Evidence Database (PEDro) scale (http:// www.pedro.fhs.usyd.edu.au), which has good reliability and validity [39].The PEDro scale has eleven possible points that examine external validity (criterion 1) and internal validity (criteria 2-9) of controlled trials and whether there is sufficient statistical information for interpreting the results (criteria 10-11).A cut-off score of six on the PEDro scale was used to differentiate between low and high methodological quality [41].Two independent researchers (FR and ABA) assessed the quality of the studies, and if any unambiguity arose, a third researcher (HZ) was contacted to reach a unanimous decision.

Statistics
The percent change (Δ%) was calculated (if not available in the study) for each study to evaluate the magnitude of the effects using the following equation: where Mpost represents the mean value after intervention and Mpre the baseline mean value.
Effect sizes (ES) were computed for single studies but were not aggregated across studies to present standardized effects of exercise training on the outcome variables (e.g., physical fitness and physiological adaptations).As an ES measure, we consistently used Cohen's d [42] by dividing the raw ES (difference in means) by the pooled standard deviations: Values for ES were defined as trivial (< 0.2), small (0.2-0.6), moderate (0.6-1.2), large (1.2-2.0), and very large > 2.0 [43].Results for each outcome variable are presented with several observations (N), Δ%, and ES.The data analysis was processed using SigmaStat 3.5 software (Systat, Inc, USA).The ES and Δ% were analyzed in studies where sufficient data were available.A significant difference was indicated when the 95% confidence interval (CI) of the ES did not overlap zero.

Study Selection
We identified 23,815 studies related to the effects of long-term interval training on physical fitness and physiological parameters in healthy trained and untrained individuals (Fig. 1).After the screening of titles, abstracts, and full texts, 24 studies were eligible to be included in our final analysis.The characteristics of the included studies are summarized in Table 2 Thirteen studies (229 participants) examined the effects of interval training interspersed with passive recovery regimes on physical fitness and physiological performances in trained (6 studies) and untrained (7 studies) individuals.Eleven studies (501 participants) examined the effects of interval training interspersed with active recovery methods on measures of physical fitness and physiological parameters in trained (6 studies) and untrained individuals (5 studies) (Table 3).

Effects of Interval Exercise Training Using Passive Recovery on Measures of Physical Fitness and Physiological Adaptations in Trained and Untrained Individuals
Table 4 summarizes the 13 studies that examined the effects of long-term interval training interspersed with passive recovery on measures of physical fitness and physiological parameters in both trained and untrained youth and adult individuals.Six studies [4,26,[47][48][49][50] involved trained individuals and the seven remaining studies comprised untrained subjects [51][52][53][54][55][56][57].Irrespective of the type of interval training or exercise protocol (type of exercise, duration, or intensity of exercise training), eleven out of 13 studies reported increases in measures of physical fitness (e.g., maximal aerobic velocity [MAV], Yo-Yo running test, jumping) and physiological parameters (e.g., VO 2max , lactate threshold, blood pressures) in both trained (effect size for single studies: 0.13 < d < 3.27, small to very large) and untrained adults as well as trained youth (effect size: 0.17 < d < 4.19, small to very large).
Two studies were identified that examined the effects of passive recovery applied during interval training in young female basketball players aged 15.1 ± 1.1 years [49] and in male soccer players aged 14.2 ± 0.5 years.[48].Both studies showed positive effects of passive recovery on VO 2max , countermovement jump (CMJ), and the Yo-Yo intermittent running test level 1 (YYIRTL-1).

Effects of Interval Exercise Training Using Active Recovery on Measures of Physical Fitness and Physiological Adaptations in Trained and Untrained Individuals
Table 5 summarizes the findings of eleven studies that examined the effects of interval training interspersed with active recovery on physical fitness and physiological parameters in trained and untrained adults.Six studies [4,5,[58][59][60][61] involved trained individuals and the five remaining studies incorporated untrained subjects [44-46, 62, 63].
Irrespective of the type of interval training or exercise protocol (type of exercise, duration, or intensity of exercise training), nine out of 11 studies reported increases in physical fitness (e.g., MAV) and physiological parameters (e.g., VO 2max , lactate threshold, blood pressures)

Discussion
Our main finding was that irrespective of the recovery type (passive or active) long-term interval traininginduced enhancements in measures of physical fitness and physiological parameters in trained and untrained males and females aged 14-48 years.

Effects of Interval Exercise Training Using Passive Recovery on Measures of Physical Fitness and Physiological Adaptations in Trained and Untrained Individuals
Our analysis showed that on average, exercise performance was increased after long-term interval exercise training with passive recovery in healthy-trained individuals.Nine studies [4, 26, 47-50, 58, 59, 63] were of high quality and included both sexes [4, 26, 47-50, 58, 59] and reported that passive recovery had a large positive effect on VO 2max and physical fitness using aerobic exercise training at 90-100% of VO 2max .Eleven studies used aerobic interval training as an intervention [4, 26, 47-51, 53, 54, 56, 57], while another two studies used either sprint interval training [52] or repeated sprint ability as interventions [55].Most included studies showed large effect sizes of passive recovery during long-term interval exercise training on VO 2max .Two studies showed a small [26] or trivial [4] impact on VO 2max .For jump performance, researchers from two studies reported a trivial [49] to large [48] effect after long-term interval exercise training with passive recovery on CMJ performance in youth male soccer [48] and youth female basketball players [49].
Results from these studies indicate positive effects of passive recovery on VO 2max , CMJ, and the YYIRTL-1.This result has to be interpreted with caution due to the limited number of available studies.More research is needed on the effects of passive recovery on measures of physical fitness and physiological adaptations in youth.
Our analysis indicated small improvements in measures of physical fitness after long-term interval exercise training with passive recovery in healthy untrained individuals.Indeed, the seven included studies [51,52,54] were of high quality, included both sexes, and the population was restricted to healthy individuals [52,[54][55][56][57], overweight, and obese [51,53] individuals, aged 18-38 years old.Three studies with passive recovery reported slight positive training-induced changes in VO 2max [52][53][54]56], jump tests [55][56][57], muscle strength [55], and body composition [52].Researchers from one study [51] reported a very large positive impact on the body mass index in overweight participants aged 18.0 ± 1.5 years.after a 30 s/30 s training program with a passive recovery.
The largest training-induced effects on VO 2max and physical fitness were observed in trained athletes compared to untrained individuals aged 18-38 years old.To better appreciate the impact of passive recovery on high-intensity interval exercise training, it is important to understand how this recovery mode relates to VO 2max and physical fitness.Accordingly, Ben Abderrahman et al. [4] showed that the longer time limit observed in trained adults could mainly be explained by the resynthesis of a higher proportion of the muscle phosphocreatine used during the 30 s intensive runs at 105% of MAV during the passive recovery.Another potential explanation for the observed result might be the difference in body mass between trained (74.2 ± 10.3 kg) and untrained individuals (67.0 ± 6.5 kg) [4].
The studies included in this systematic review reported that weight-bearing high-intensity interval exercises have a greater positive impact on anthropometrics [51] and cardiovascular [52] parameters compared with physical fitness and VO 2max in healthy non-obese or obese untrained individuals aged 18-38 years old.
Further, it was previously demonstrated that passive recovery facilitates a greater interval performance in trained athletes aged 20-25 years old [4,47,50].The fact that athletes completed exercise training with large VO 2max increases and small changes in peak HR [50] and blood lactate [47] suggests that athletes could perform more bouts in this condition and, therefore, accumulate more time spent at high %VO 2max levels compared to untrained individuals.Regarding the effects of passive recovery during HIIT sessions on exercise performance, some studies [24,38] indicated that, compared to active recovery, passive recovery was associated with a greater time to exhaustion (i.e., the accumulation of more work intervals or time spent at high intensities close to VO 2max ), but also a higher mean velocity/power development during work intervals when the number of bouts was fixed and the intensity self-regulated [24,38].

Effects of Long-Term Interval Exercise Training Using Active Recovery on Measures of Physical Fitness and Physiological Adaptations in Trained and Untrained Individuals
Our analysis showed that on average, exercise performance is slightly increased during long-term high interval exercise training with active recovery in healthy trained individuals.Indeed, six included studies [4,5,50,58,60,61] were of high quality, included both sexes, and the population was restricted to young athletes aged 19-25 years.The studies, using aerobic interval training at 100-110% of MAV, found that active recovery had a small but positive effect on VO 2max [4,60] and a large, positive effect on MAV [4,5,61].Some other studies used sprint interval training [45,46,63], or maximal repeated sprint ability [5,61] and observed significant small to large effects on VO 2max .
Our analysis showed that on average, exercise performance can be improved (large to very large effects) after long-term interval exercise training with active recovery in healthy untrained individuals.Indeed, the four included studies [45,46,62,63] were of high quality, including both sexes and restricted to healthy [46] and overweight/obese [45,62,63] subjects.Those studies found that active recovery had large to very large positive effects on VO 2max [45,56,62,63] and was associated with a trivial change in body composition [62] using interval exercise training at 80-100% of power output or HR max .Three studies used maximal interval exercise training [45,46,63], and one study applied 80-90% HRmax during interval exercise training [62].
It is generally accepted that active recovery during long-term high-intensity interval exercise training has a very large effect on VO 2max in healthy or overweight untrained individuals compared to athletes [64].In other words, these VO 2 and mechanical efficiency data suggest that untrained individuals benefit more from maximal interval exercise training with active recovery than athletes [23].
The largest increase in VO 2max was reported in the study from Trapp et al., [46].A possible explanation for this result might be related to the population and sex (inactive healthy females) with wide variations in VO 2max values.Moreover, the training duration was 15 weeks with 3 weekly sessions, which is a greater volume compared to the studies of Poon et al., [62] (3 times/week over 8 weeks) and Smith-Ryan et al., [44] (3 times per week over 3 weeks).
To the best of our knowledge, there is no study available that examined the effects of active recovery on physical fitness and physiological adaptations in youth.

Study Limitations
There are some limitations to the current systematic review that should be noted.First, the studies examined were highly heterogeneous.In fact, the study populations varied in terms of sample size, sex, age (only two studies involved young individuals), and country of recruitment.Second, the training program's duration (3-15 weeks) and volume (15-45 min per session) were variable.Third, to our knowledge, no study currently available has reported the variation by effect size in physical fitness and physiological adaptations between trained and untrained individuals during high-intensity interval exercise training with passive or active recovery.Finally, due to the small number of studies included in our analysis, we were unable to consider sex and age as moderators of active recovery on physical exercise and VO 2max .Furthermore, except for the work of Rhibi et al. [5], the majority of studies did not measure lactate concentration, which could provide more information on the relation between active recovery and lactate clearance during high-intensity exercise training.

Practical Applications
The findings of our systematic review suggest that interval training, irrespective of the intensity level, has the potential to improve selected measures of physical fitness (e.g., MAV) and physiological responses (e.g., VO 2max , blood pressure) similarly in trained and untrained adults and trained youth, regardless of the type of exercise and exercise dosage.More specifically, our findings suggest that the type of recovery (active or passive) applied during interval training results in similar training-induced outcomes, irrespective of the training status (trained, untrained) and sex (males, females).Thus, when longterm interval training programs (≥ 3 weeks) are performed, coaches and athletes can use either passive or active recovery modes.The decision should be based on the overall exercise programming parameters of the respective interval training.High exercise workloads may demand passive recovery whereas low workloads may favor active recovery.

Conclusions
To conclude, our findings suggest that irrespective of the recovery mode (passive or active), long-term interval exercise training has the potential to enhance physical fitness and physiological adaptations in trained and untrained male and female adults.More research examining the effects of passive or active recovery on measures of physical fitness and physiological adaptations in youth is recommended.

Table 2
Characteristics of the studies that examined the effects of interval training with active or passive recovery on measures of physical fitness and physiological adaptations in trained and untrained individuals

Table 2
(continued) MAV Maximal aerobic velocity, HIIT High intensity interval training, SIT Sprint interval training, IL-6 Interleukin-6, TnF alpha Tumor necrosis factor alpha, BF Body fat, BMI Body mass index, CRP C-reactive protein, SSG Small sided soccer game, VO 2max Maximal oxygen consumption, HR Heart rate, rpm Revolution per minute, PER Periodized interval training, Row-HIIT Traditional HIIT using rowing, MM-HIIT Multimodal HIIT, RPE Rating of perceived exertion, SCT Supramaximal cycling test, VIFT Maximum speed reached in the last stage of the 30-15 intermittent fitness test, 1 RM one-repetition maximum, IE Intermittent exercise, HIIRT High-intensity interval resistance training

Table 3
Physiotherapy Evidence Database (PEDro) score of the included longitudinal studies

Table 4
Effects of interval training using passive recovery on physical fitness and physiological adaptations in trained and untrained individuals

Table 4
(continued) Yo-Yo intermittent recovery test level 1, Row-HIIT Traditional HIIT using rowing, MM-HIIT Multimodal HIIT, RPE Rating of perceived exertion, SCT Supramaximal cycling test, VIFT Maximum speed reached in the last stage of the 30-15 intermittent fitness test, 1 RM One-repetition maximum, IE Intermittent exercise, HIIRT High-intensity interval resistance training CMJ Countermovement jump, LT Lactate threshold, PPO Peak power output, AT Anaerobic threshold, PVV Plasma volume variation, VT Ventilatory threshold, MAV Maximal aerobic velocity-alpha, BF Body fat, BMI body mass index, CRP C-reactive protein, SSG Small sided soccer game, VO 2max Maximal oxygen consumption, BP Blood pressure, HR Heart rate, YYIRTL-1

Table 5
Effects of interval exercise training using active recovery on physical fitness and physiological adaptations in trained and untrained individuals

Table 5
(continued) LT Lactate threshold, PPO Peak power output, AT Anaerobic threshold, a-v do2 Maximal arteriovenous difference, HOMA _IR Homeostasis model assessment of insulin sensitivity, PVV Plasma volume variation, VT Ventilatory threshold, MAV Maximal aerobic velocity, IL-6 Interleukin-6, TNF alpha Tumor necrosis factor-alpha, BF Body fat, BMI Body mass index, CRP C-reactive protein, SSG Small sided soccer game, VO 2max Maximal oxygen consumption, BP Blood pressure, HR Heart rate, YYIRTL-1 Yo-Yo intermittent recovery test level 1, Row-HIIT Traditional HIIT using rowing, MM-HIIT Multimodal HIIT, RPE Rating of perceived exertion, SCT Supramaximal cycling test, VIFT Maximum speed reached in the last stage of the 30-15 intermittent fitness test, 1 RM One-repetition maximum, IE Intermittent exercise, HIIRT High-intensity interval resistance trainingin trained (effect size: 0.13 < d < 1.29, small to large) and untrained individuals (effect size: 0.19 < d < 3.29, small to very large).