High-intensity interval training: optimizing oxygen consumption and time to exhaustion taking advantage of the exponential reconstitution behaviour of D’

Purpose Accumulating the time near maximum aerobic power \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {{\dot{\text{V}}\text{O}}_{{{\text{2max}}}} } \right)$$\end{document}V˙O2max is considered to be the most effective way to improve aerobic capacity. The aims of this study were: (1) to verify whether postponing the first recovery interval improves time to exhaustion during a high-intensity interval training (HIIT) test, and (2) to verify whether a HIIT protocol with decreasing interval duration (HIDIT) is more effective in accumulating time near \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}\text{O}}_{{{\text{2max}}}}$$\end{document}V˙O2max compared with two classical protocols with short intervals (SIHIIT) and long intervals (LIHIIT). Methods Nine active males (35 ± 11 years, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}\text{O}}_{{{\text{2max}}}}$$\end{document}V˙O2max 52 ± 5 mL·min−1·kg−1) performed a graded exercise test on an athletic track. Critical velocity and D’ were estimated from three to five high-intensity trials to exhaustion. Then, the subjects performed three trials with a single recovery interval after 30 s (Rec30s), after 3 min (Rec3min) and after exhaustion (RecTlim) to verify whether postponing the first recovery interval enhances the time to exhaustion. Finally, the subjects performed the three HIIT protocols mentioned above. Results The time to exhaustion was significantly greater in RecTlim (464 ± 67 s) than in Rec3min (388 ± 48 s) (p < 0.0078) and Rec30s (308 ± 44 s) (p > 0.0001). Additionally, it was significantly greater in Rec3min than in Rec30s (p = 0.0247). Furthermore, the time accumulated near \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}\text{O}}_{{{\text{2max}}}}$$\end{document}V˙O2max was significantly longer in HIDIT (998 ± 129 s) than in SIHIIT (678 ± 116 s) (p = 0.003) and LIHIIT (673 ± 115 s) (p < 0.031). Conclusions During the trials, postponing the first recovery interval was effective in improving the time to exhaustion. Moreover, HIDIT was effective in prolonging the time near \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}\text{O}}_{{{\text{2max}}}}$$\end{document}V˙O2max.


HIDIT
Decreasing intervals HIIT (combining high intensity from 3 min to 30 s and low intensity from 2 min to 20 s) HIIT High-intensity interval training HR Heart rate LI HIIT Long intervals HIIT (3 min high-2 s low intensity) SI HIIT Short intervals HIIT (30 s high-20 s low intensity) Rec 30s Test consisting in a bout of exercise of 30 s followed by a recovery phase and a subsequent bout to exhaustion Rec 3min Test consisting in a bout of exercise of 3 min followed by a recovery phase and a subsequent bout to exhaustion Rec Tlim Test consisting in a bout of exercise to exhaustion followed by a recovery phase and another bout to exhaustion

Introduction
Maximum aerobic power ( V O 2max ) is one of the most important parameters for evaluating the efficiency of the cardiorespiratory system, and it is also one of the most important determinants of endurance performance. Therefore, the purpose of many training protocols is to improve maximum aerobic power by working at intensities close to or equal to V O 2max (Wenger and Bell 1986;Midgley and Mc Naughton 2006). Researchers have proposed different protocols that were able to solicit and maintain the exercise intensity close to V O 2max as long as possible. One of the training methods that allows athletes to extend the duration of exercise at a very high intensity is called high-intensity interval training (HIIT) (Midgley and Mc Naughton 2006;Buchheit and Laursen 2013). HIIT can be set with the help of critical velocity (CV) and D' (maximum distance that may be covered beyond CV (Jones et al. 2019)). CV is the intensity below which a metabolic steady state is still possible (for a more detailed definition see Jones and Vanhatalo (2017)). CV and D′ are phenomenologically equivalent to critical power (the critical threshold used in performances where the intensity is measured in Watts such as cycling) and W′ (the finite amount of work that can be done beyond CP), respectively (Ettema 1966;Broxterman et al. 2015;Jones and Vanhatalo 2017;Jones et al. 2019). During HIIT, D' is consumed when the intensity is above CV. On the other hand, during recovery periods below CV, D' is reconstituted (Billat et al. 2001;Skiba et al. 2014;Jones and Vanhatalo 2017). When D' is fully depleted, only an adequate recovery period allows to perform another high-intensity interval. Nevertheless, while the depletion of D' above CV occurs at a constant rate, its reconstitution takes place with exponential behaviour. In other words, whether D' is close to exhaustion, its recovery is quicker than when it is about to be fully reconstituted. In fact, the rate of recovery decreases as a function of recovery time (Ferguson et al. 2010;Skiba et al. 2014;Caen et al. 2019;Sreedhara et al. 2020;do Nascimento Salvador et al. 2021;Lievens et al. 2021). In a previous work (Vaccari et al. 2020) we showed that this feature can be exploited to extend the time elapsed above 90% of V O 2max (T90%VO 2max ) during HIIT. HIIT protocols with initially longer and then shorter intervals above CV would allow prolongation of T90%VO 2max for two reasons: 1-an initial "priming effect" of oxygen consumption (Jones et al. 2008): indeed, long intervals increase oxygen consumption faster than short intervals (Millet et al. 2003); 2-at the beginning of the exercise, when D' is not widely depleted, its recovery is relatively slow (Skiba et al. 2012). In contrast, the reconstitution of D' will be relatively faster starting with a long interval and thus recovering when D' is already nearly depleted (Skiba et al. 2012). After the first long interval, more frequent recovery periods could be introduced by decreasing the duration of the intervals. In fact, at this stage, even a few seconds of recovery will be more effective than at the beginning. This should allow for the extension of high-intensity exercises for longer and maintain the rate of oxygen consumption high for longer time.
The current study aims to expand our previous work (Vaccari et al. 2020) and tests the hypothesis that: (1) comparing three trials with a single recovery period, postponing the first recovery interval recovery allows to prolong the time to exhaustion at the end of the trial; (2) decreasing the intervals duration during a HIIT session can be more effective in running performance in comparison with HIIT protocol with long or short intervals.

Subjects
Nine active, non-smoking men were recruited (34 ± 11 years; 79 ± 8 kg; V O 2max 52 ± 5 mL·min −1 ·kg −1 ). All of them concluded the entire protocol except one who got injured in the last test. Then, all of his completed tests were taken into account for the analysis. The inclusion criteria were: (1) recreational runner; (2) training volume in the latest 6 months was greater than 3 session per week (or was greater than 40 km per week … vedi tu); (3) none of the athletes had a history of neuromuscular or musculoskeletal impairments at the time of the study that could affect the results.

Study design
The Ethics Committee of the Friuli-Venezia Giulia approved the study (protocol number 30/2021). Participants performed from 9 to 11 test days, separated by at least 48 h in the spring of 2021. During the first visit to the laboratory, an operator explained the purposes and the objectives of the study to each subject and obtained written informed consent. Then, participants underwent medical examinations and performed a maximal running step-incremental test to measure V O 2max and maximal aerobic speed ( vVO 2max ). Although the 1 3 objectives were explained to all subjects, the study hypothesis was not revealed to not influence the results. After the first visit, participants were examined three to five times on different days to determine their velocity-duration relationship (CV and D'). CV parameters were used to set up the HIIT tests. Then, they randomly performed three tests to exhaustion to determine the reconstitution of D' after a period of 2 min of recovery, which was set after high-intensity bouts of 30 s, 3 min or after Tlim (time to exhaustion). Finally, the subjects performed three HIIT tests in a randomized order: long interval HIIT (LI HIIT ), high-intensity decreasing interval training (HIDIT), and short interval HIIT (SI HIIT ) (Fig. 1). During the three HIIT tests, oxygen consumption and heart rate (HR) were measured along with Tlim, and the time above 90% of V O 2max and blood lactate concentration (BLC), HR and rate of perceived exertion (RPE) using the Borg CR-10 scale (Borg et al. 2010) were measured at the end of the tests. Each test was performed at the same time of the day, on an athletic track and separated from the previous one by a minimum of 2 and a maximum of 6 days. Subjects were instructed to avoid the consumption of drinks containing caffeine for at least 8 h before each test and to avoid vigorous physical activity in the 24 h preceding each testing session. Furthermore, they were asked to maintain the same drinking and feeding habits before each test. The nine subjects concluded the entire protocol within 12 weeks from the first visit.

On-track testing session
Incremental tests, velocity-duration relationship trials, D' reconstitution trials, and HIIT tests were performed at the athletic track of Gemona del Friuli, Italy. Every test session was preceded by the same warm-up routine: 10 min running at 10 or 12 km/h (depending on the fitness level), followed by 1-min passive recovery. The running speed was set considering a series of markers that were placed every 20 m on the first lane of the track. Every participant was paced by an operator on a bike that set the right speed passing every mark when an acoustic signal (a beep) was emitted from a device. In other words, each "beep" corresponded to the time in which 20 m should have been covered if one went at the predetermined speed (e.g., at 2.7 m/s a beep every 7.4 s). The test ended when the subject was not able to follow the

Incremental test
The incremental test was performed on the athletic track under medical supervision, and standard safety procedures were followed. During the first visit, an operator instructed the subjects to correctly report the rate of perceived exertion on the CR-10 scale (Borg et al. 2010). The incremental step test started at 10 or 12 km/h depending on the fitness level of the subject and increased by 0.5 km/h per minute throughout the test until voluntary exhaustion. Ventilation, V O 2 and carbon dioxide ( V CO 2 ) were measured breath-by-breath using a portable metabolic unit (K5, Cosmed, Italy). The ventilation was measured by a turbine calibrated before each test with a 3-L syringe. Calibration of O 2 and CO 2 analysers was performed before each test by utilizing calibration gas mixtures of known composition (16.00% O 2 ; 4.00% CO 2 ). The VȮ 2 plateau during the last 30 s was used to verify the achievement of V O 2max , while vVO 2max was considered the minimum velocity at which the V O 2max plateau appeared (Taylor et al. 1955;Poole and Jones 2017).

Velocity-duration relationship
CV and D' were estimated from three to five high-intensity trials to exhaustion performed in different days at 90 to 110% of the vVO 2max . All the tests had to last a minimum of ~ 2 and a maximum of ~ 15 min to be considered valid (Jones and Vanhatalo 2017). To find CV and D', two methods were used (Clarke and Skiba 2013): (1) Distance-time method: the distance covered (D) in each of the separate exercise bouts was plotted against T lim . In this case, the slope of the linear regression represents CV, and the intercept represents D'; (2) 1/time method: velocity was plotted against 1/time. In this case, the slope of the line represents D', and the intercept represents CV. For each subject, the determination coefficient (R 2 ) resulting from both methods had to be higher than 0.95. If not, more trials were performed to increase the precision of linear regression. A minimum of three and a maximum of five trials have been made for each subject. Eventually, the method with higher R 2 was used to determine CV and D'.

D' reconstitution tests
To investigate the reconstruction characteristics of D', the participants performed three trials in a randomized order and preceded by the same warm-up as in the incremental test. The three trials comprised one high-intensity bout (B1) followed by a 2-min active recovery interval and a second high-intensity bout to exhaustion (B2). The first bout of the trial was meant to simulate three different interval durations in which D' was consumed: long bout (Rec 3min , Fig. 1C), short bout (Rec 30sec , Fig. 1D) and exhaustion bout (Rec Tlim , Fig. 1E). The intensity of the high-intensity bouts was the same as the HIIT test and corresponded to the velocity that was supposed to lead to exhaustion in 6 min (360 s) according to the following equation (Jones et al. 2010): The velocity used for the low-intensity bout was 66% of the CV and was the same as that used in the HIIT test.
The recovery interval of 2 min served to the reconstitution of D' (Caen et al. 2019), and the last bout to exhaustion was performed to measure how much the timing of the first recovery bout (after 30 s, 3 min or Tlim) influenced the reconstitution rate of D'. Because it can be theoretically assumed that D′ = 0 at exhaustion, this calculation yielded the D′ reconstitution after the recovery interval.
The amount of D′ expended in the first bout (B1) and in the second bout (B2) (Fig. 1C-E) was calculated by numerical integration of the power values above CV. The difference between D' was calculated from the intensity-duration relationship, and the sum of the expenditure of D' during B1 and B2 allowed us to calculate the amount of D' reconstituted during the recovery interval. Subsequently, it was expressed as a percentage of total D'.

HIIT tests
After the incremental tests, the critical velocity tests, and the recovery trials, subjects performed three HIIT training sessions in a randomized order. The velocity of the work/ recovery bouts and the work/recovery duration ratio were the same in each trial, although the duration of the intervals was changed. The work and recovery intensity were the same as the D' reconstitution tests (see previous paragraph), while the ratio work/recovery time was set at 3/2 for all the tests. The three-interval test was structured as follows: • Long intervals (LI HIIT , Fig. 1F): 3 min at high intensity and 2 min at low intensity repeated until volitional exhaustion of the subject; • High-intensity decremental interval training (HIDIT, Fig. 1G): 3 min at high intensity and 2 min at low intensity; 2 min at high intensity and 1 min and 20 s at low intensity; 1 min at high intensity and 40 s at low intensity; 45 s at high intensity and 30 s at low intensity; and finally, 30 s at high intensity and 20 s at low intensity repeated until volitional exhaustion of the subject; (1) CV 6 = D � 360s + CV.
• Short intervals (SI HIIT , Fig. 1H): 30 s at high intensity and 20 s at low intensity until volitional exhaustion of the subject.
Throughout the HIIT protocols, ventilatory parameters were measured using a breath-by-breath metabolic unit (K5, Cosmed, Italy), and then data were averaged every 5 s. V O 2 and HR were measured during the entire test, while BLC and RPE were measured at the end of each test. An operator collected a capillary blood sample from the fingertip to measure BLC after 3 min of the end of the exercise (Lactate Scout 4, EKF Diagnostics, UK), while the subjects reported RPE consulting the CR-10 scale shown by another operator. At the end, the total time spent above 90% of V O 2max was determined as the sum of each averaged 5 s when V O 2 was equal to or higher than 90% of V O 2max .

Statistics
Statistical analysis was performed using GraphPad Prism 8.0.2 software (IBM, Chicago, USA) with significance set at P < 0.05. Descriptive data are presented as the mean ± SD. HIIT tests and the three sessions to test D' reconstitution were compared by a mixed-effect analysis. Where the analysis found a significant difference, Tukey's multiple comparison test between the three protocols was performed. Effect size (ES) was calculated using Cohen's d (0 < d < 0.20, small; 0.20 < d < 0.50, medium; d > 0.50, large). For our purposes, a sample size of 6 subjects was calculated to have a statistical power of 0.8 to refute the null hypothesis and to obtain an ES of 0.80 with an alpha error of 0.05 and a beta error of 0.20. To have a safe margin 9 subject have been recruited.

Results
Age and weights along with values attained during the incremental test and the velocity-duration relationship trials are shown in Table 1.
The physiological and perceptual responses during the three HIIT protocols are shown in Table 2. There were no significant differences between the three protocols other than T lim and T90%VO 2max . In particular, the T lim of HIDIT was significantly higher than those of SI HIIT (+ 48%; 95% C.I from + 488 to + 154 s; p = 0.001; ES = 1.85, large) and LI HIIT (+ 47%; 95% C.I from + 519 to + 133 s; p = 0.004; ES = 1.60, large), while there were no differences between the T lim of SI HIIT and LI HIIT (Fig. 3B). Furthermore, Table 1 Anthropometrics and physiological characteristic of the participants; data were obtained from incremental test and velocity-duration relationship trialṡ VO 2max : maximal aerobic power; vVO 2max : velocity at V O 2max ; HR max : maximal heart rate; CV: critical velocity; D': amount of work which is possible to do beyond CV expressed as a distance in metres  (Fig. 3A).

Discussion
The main findings of the present study showed that (i) postponing the first recovery interval allows increasing T lim . In fact, Rec tlim was more effective than Rec 3min and Rec 30sec , and Rec 3min was more effective than Rec 30sec in prolonging time to exhaustion; (ii) time above 90% of V O 2max and T lim were higher during HIDIT than HIIT protocols with long (LI HIIT ) and short (SI HIIT ) intervals, despite having identical mean velocity, duration and ratio high/low intensity of the intervals. These results confirm that the reconstitution of D', when falling below the intensity corresponding to the CV, has an exponential behaviour that can be exploited in HIIT protocols to prolong T lim and T90%VO 2max . The intensities used for the HIIT protocols were chosen to optimize the time near to maximum oxygen consumption. The velocity used for the high-intensity intervals (CV 6 ), corresponds approximately to the speed at V O 2max (Billat et al. 1999). The velocity used for the low intensity intervals (66%CV), is slow enough to allow recovery, but fast enough to maintain high the oxygen consumption. The choice of protocols for HIIT with long and short intervals was also made to optimize the time near to maximum oxygen consumption, as emerged from the indications of the review of Buchheit and Laursen (2013). The HIDIT protocol, on the other hand, was designed to progressively arrive from long to short intervals over a not excessively long time.
Several studies performed on cycle ergometers have verified that the reconstitution of W' (used in place of D' when cycling) was exponential (Ferguson et al. 2010;Skiba et al. Fig. 2 Reconstitution as a percentage of D' during 2 min of recovery at velocity corresponding to 66% of CV, timed after 30 s (Rec 30sec ), 3 min (Rec 3min ) and after exhaustions (Rec Tlim ) at the intensity of CV 6 (the velocity that was supposed to lead to exhaustion in 6 min). *significantly different (P < 0.05) Table 2 Data from HIIT protocol with short intervals (SI HIIT ), with decreasing duration of intervals (HIDIT) and with long intervals (LI HIIT ) For all the protocols the same mean speed for high intensity and low intensity were used, and the ratio of the durations of high/low intensities were identical Values are presented as mean ± SD Tlim: time to exhaustion; V O 2peak : peak oxygen consumption during the HIIT protocols; V O 2mean : mean oxygen consumption during the HIIT protocols; V O 2max : maximal oxygen consumption from the incremental test; T90%VO 2max : Time elapsed above 90% of V O 2max ; HR peak : peak heart rate; HR mean : mean heart rate during the work intervals; BLC post: blood lactate concentration 3 min after the end of the protocol; RPE: rate of perceived exertion. *different from HIDIT ( Caen et al. 2019;Sreedhara et al. 2020;do Nascimento Salvador et al. 2021;Lievens et al. 2021), while the present study is the first to verify it on running performance. Furthermore, our study used a different experimental design from previous studies. For example, the studies by Ferguson et al. (2010) and Caen et al. (2019) demonstrated that the reconstitution of W' during recovery was exponential by performing a series of tests with recovery intervals with progressively longer durations. Thus, they verified that after reaching exhaustion, at the beginning, the reconstitution of W' was very fast and then gradually slowed down. In recent years, this topic has been of interest to several studies that have tried to better explain the behaviour of W', both during bouts above critical power (CP, used in place of CV when cycling) and below CP. The studies by Skiba and colleagues (Skiba et al. 2012(Skiba et al. , 2014(Skiba et al. , 2015Chidnok et al. 2013;Broxterman et al. 2015) led to the creation of a model that allows us to predict the behaviour of W' (i.e., the Model W' bal). Recent studies have tried to question it. For example, the study by Chorley et al. (2018) showed that repeated exhaustion exercises slow down the reconstruction of W'. The study by Caen et al. (2019) showed that the speed of reconstitution of W' depends on how quickly it was depleted by the previous bout: the faster it is consumed, the faster it rebuilds. Furthermore, the study by Lievens et al. (2021) showed how the reconstitution of W' was slower than expected in the heavy domain. Finally, the study by Sreedhara et al. (2020) showed that the intensity of recovery was even more important than the duration of recovery. In agreement with previous studies on cycling, our study confirmed the exponential trend of the reconstitution of D' during running performance. Moreover, it showed that this feature can be exploited to increase exercise tolerance by postponing the first recovery interval.
Unexpectedly, in the Rec 30sec test, the total time over CV was even less than that estimated by the velocity-duration relationship tests (Fig. 2). This means that the 2 min under CV did not allow for recovery but probably even consumed a portion of D'. Unfortunately, we cannot explain this result through our data, and there are no other studies that have verified the behaviour of W' (cycling) or D' (running) after such a short interval above CV. A possible explanation is that the first 30 s were not sufficient to activate the aerobic system adequately. Therefore, the recovery carried out in the following 2 min (i.e., at 66% of the CV) was not sufficiently low to allow D' reconstitution. Indeed, the study by Sreedhara et al. (2020) showed that at an intensity close to CP, the reconstitution of W' is much slower than one might expect, even at approximately 90% CP intensity. In some cases, a depletion of W' occurs instead of a reconstitution.
We previously observed that a HIDIT protocol can increase the T90%VO 2max in cyclists compared to HIIT protocols with short or long intervals (Vaccari et al. 2020). However, the results of this study were slightly different from the results of the present study. Indeed, on cyclists, HIDIT did not significantly increase T lim , but the average V O 2 during the HIIT with long intervals was higher than HIIT with short intervals. In the present study, we wanted to verify that HIDIT was advantageous also in running performance, since the kinetics of V O 2 are not the same in running and cycling (Hill et al. 2003). Although in recent years several studies have tried to propose strategies to increase the time close to V O 2max during HIIT, researchers have focused mainly on the fast start strategy to exploit the priming effect (Billat et al. 2013;De Aguiar et al. 2013;Lisbôa et al. 2015;Bossi et al. 2019;Rønnestad et al. 2019Rønnestad et al. , 2021Beltrami et al. 2021). Additionally, most of the studies were performed on cycle ergometers. On the other hand, the studies by Rønnestad et al. were conducted on crosscountry skiers and confirmed that a fast-starting strategy can increase the average V O 2 (Rønnestad et al. 2019(Rønnestad et al. , 2021 and the time above 90% of the V O 2max (Rønnestad et al. 2021) compared to a traditional HIIT session. Moreover, the study by (Beltrami et al. 2021) is particularly interesting because the authors compared a fast start protocol and a traditional HIIT protocol in runners and cyclists. The authors showed that only in cycling performance the average VȮ 2 and the time above 90% of the V O 2max were greater. Conversely, when subjects were asked to perform the same protocol by running, the V O 2 kinetics were not different between the two HIIT protocols. Although the tests by Beltrami and colleagues did not lead to exhaustion, there seem to be some similarities between the fast start strategy and HIDIT during running. In fact, our data show that there is no difference in the average V O 2 between HIDIT and traditional protocols. Therefore, for the same duration, the two protocols would not have shown differences over time above 90% of V O 2max .

Practical applications
In adult active males, training protocols starting with long intervals followed by short intervals could be useful to accumulate time close to V O 2max and consequently improve it. HIDIT would not seem physiologically more fatiguing than other classical protocols. Thus, it could be used in a general preparation phase in the same way as traditional HIIT protocols. Women were not included among the participants, this precluded us from verifying whether HIDIT can also be successfully applied to women's training.

Conclusions
In conclusion, during the HIIT trials performed for this study, postponing the first recovery interval below CV contributed to increasing T lim . Furthermore, active males achieved longer time above 90% of V O 2max and longer T lim by performing a HIDIT running protocol than HIIT protocols with long and short intervals, despite having similar values of BLC and RPE at the end of the trial.