Physiological aspects and energetic contribution in 20s:10s high-intensity interval exercise at different intensities

Introduction

High-intensity interval exercise (HIIE) is repeated efforts with intensity above 90% of the intensity related to maximal oxygen consumption (iV⋅O_2MAX) followed by active or passive recovery (Buchheit & Laursen, 2013b; MacInnis & Gibala, 2017). Effort and recovery duration and intensity are the mainly manipulated variables during HIIE, which distinctly affect acute and chronic metabolic responses (MacInnis & Gibala, 2017). However, the inconsistency and variability in HIIE protocols may limit its external validity and the data extrapolation to different populations (Viana et al., 2018b).

One of the most popular HIIE structure (Tabata, 2019) was proposed by Tabata et al., (1996), which is also one of the most inconsistently applied protocols (Gentil et al., 2016). It is seven to eight repetitions of 20 s of effort and 10 s of passive recovery (20s:10s) performed until the participant was unable to keep at least 85 rpm. This HIIE model is at an intensity equivalent to 1.7 times the measured V⋅O_2MAX, which was calculated by the extrapolation of the linear relationship between submaximal exercise intensity and oxygen uptake (7-8x 20s @170V⋅O_2MAX: 10s of passive recovery). Previously, in this type of HIIE, acute studies observed high oxygen consumption (Viana et al., 2018c), elevated glycolysis, pronounced glycogen depletion (Scribbans et al., 2014), and high parasympathetic inhibition (Schaun & Del Vecchio, 2018). Intermittent efforts at 170% of iV⋅O_2MAX, obtained with a graded exercise test, are indicated for sprint interval training or repeated sprint training, but not for short HIIE (Buchheit & Laursen, 2013b). In a study carried out with physically active young men on magnetic bikes, Viana et al. (2018c) verified that the 170% of iV⋅O_2MAX intensity allowed an average of only four repetitions, and induced a very short period at high oxygen consumption rates. However, to date, the effect of this type of exercise in highly trained cycling athletes habituated to this exercise model is unknown.

Notwithstanding, HIIE models based on Tabata et al. (1996) have been widely used to improve the metabolic profile or increase physical fitness (Bonafiglia et al., 2017; Domaradzki et al., 2020; Logan et al., 2016; Ma et al., 2013; McRae et al., 2012; Scribbans et al., 2014). However, many investigations (Domaradzki et al., 2020; Logan et al., 2016; Ma et al., 2013; Scribbans et al., 2014) have limitations in describing effort intensity, using all-out efforts or different from the intensity corresponding to 170% of iV⋅O_2MAX, therefore, differs from the original protocol (Tabata et al., 1996; Viana et al., 2018a).

Regarding energetic responses, the authors claimed that the 20s:10s protocol reached maximal aerobic and anaerobic demands (Tabata, 2019; Tabata et al., 1996). This statement was because, at the end of the protocol, the subjects reached their maximal accumulated oxygen deficit (MAOD) and an oxygen uptake equal to their V⋅O_2MAX. However, the model applied has been previously questioned (Bangsbo, 1992), and the acute physiological impact of different intensities in 20s:10s HIIE on cardiorespiratory and neuromuscular variables is unknown. Information about the contribution of energetic systems during the 20s:10s protocol is essential to understand its physiological demands, and to date, we are aware of no studies have analysed such responses.

Therefore, considering that this knowledge is relevant to the exercise organisation, as it allows to drive physiological stimuli according to the training status, the objectives of the present study were to measure the physiological demands (oxygen uptake, heart rate, and blood lactate), to assess the contribution of energy systems, as well as neuromuscular parameters (total sprint number, mean, and maximum power output) in the 20s:10s HIIE protocol in three different intensities, in cycling athletes and non-athletes.

Materials & Methods

Results

Independent t-tests indicated that athletes had lower maximal heart rate (p = 0.04) and higher absolute oxygen uptake (p = 0.02), age (p = 0.04), and peak power output (p < 0.001) than non-athletes.

The analysis of variance for the multivariate test (number of sprints, power output, blood lactate, peak heart rate, and percentage of maximal heart rate) resulted in significant differences between groups (F = 9.62; p = 0.001) and intensities (F = 384.05; p < 0.001), with no interactions. The results of univariate tests indicated that the number of sprints and peak heart rate were higher in lower intensities, while peak power output was higher at higher intensities (Table 1). The mean duration of each protocol was: 488, 258 and 127 s, for 115%PPO, 130%PPO, and 170%PPO, respectively. Besides, athletes reached higher levels of blood lactate concentration than non-athletes.

Considering absolute contribution (Fig. 1C), there were significant differences between energetic systems (F = 20.86; p < 0.001) and intensities (F = 12.65; p = 0.001), with no interactions between systems and groups, intensities and groups, nor systems × intensities × groups. Pairwise comparisons using Bonferroni posthoc test localised differences between the three energetic systems, with a p-value lower than 0.01, as well as between the three intensities, with p-values lower than 0.001. Repeated measures showed a linear trend for decrease the participation of energetic system contribution (aerobic, lactic, and alactic) for kcal (F = 98.49; p < 0.001), kJ (F = 98.48; p < 0.001) and litres of O₂ (F = 98.45; p < 0.001), as well as a linear reduction of kcal amount (F = 110.2; p < 0.001), kJ amount (F = 110.22; p < 0.001) and litres of O₂ consumed (F = 109.87; p < 0.001) considering all intensities (115%, 130%, and 170%). For the total amount of kcal, kJ, and litres of O₂, no differences were found between non-athletes and athletes, but there were significant differences between intensities (F = 22.81; p < 0.001), with no interactions between groups and intensity. As a partial analysis by each energetic system, in total amount were found a linear trend for intensity in kcal (F = 106.52; p < 0.001), kJ (F = 106.52; p < 0.001) and litres of O₂ (F = 109.9; p < 0.001), with p-values lower than 0.001 for all comparisons (115%PPO vs 130%PPO; 115%PPO vs 170%PPO and 130%PPO vs 170%PPO).

Concerning relative energetic contribution system, there was no effect of group (non-athletes vs cycling athletes), but there were significant differences between intensities (F = 39.3; p < 0.001), and systems (F = 411.0; p < 0.001), with no significant interactions between group and intensity, group and energetic system. There was no interaction between intensity and energetic systems contribution (F = 47.81; p < 0.001). Finally, we found no interaction between group, intensity, and energetic system.

Considering the intensity and energetic systems (Fig. 1A), at 115%PPO and 130%PPO, the aerobic contribution was higher than lactic (p < 0.001) and alactic (p < 0.001), with no difference between lactic and alactic. At 170%PPO, the aerobic contribution was different from lactic (p < 0.001) and alactic (p = 0.02), and lactic was different from alactic (p = 0.04). Additionally, the aerobic contribution was different considering the three selected intensities, 115%PPO was higher than both 130%PPO (p < 0.001) and 170%PPO (p < 0.001), and 130%PPO was higher than 170%PPO (p < 0.001). The lactic contribution was higher at 170%PPO in comparison to 115%PPO, and alactic contribution was higher at 170%PPO in comparison to 115%PPO (p < 0.001) and 130%PPO (p = 0.02).

In absolute values (Fig. 2A), we found significant differences for intensity (F = 8.35; p = 0.05), with increased O₂ consumption at 130%PPO (p-value = 0.005 in comparison to 115%PPO and 170%PPO), and higher O₂ consumption in athletes (F = 6.20; p = 0.02), with no interactions between intensities and groups (F = 4.36; p = 0.05). Figure 2B shows that the three intensities reached different relative values to V⋅O_2MAX (F = 3.25; p = 0.05), and the post-hoc test pointed difference between 115%PPO and 130%PPO (p = 0.008), with no differences between non-athletes and athletes, nor interactions. Additionally, there was a quadratic trend in these relative values (F = 11.30; p = 0.005).

Discussion

The present study aimed to analyse and compare the energetic system contribution in the 20s:10s HIIE with three different intensities in cycling athletes and non-athletes. As main findings, we found: (i) the relative dominance of the aerobic system in all three intensities when compared with lactic and alactic systems; (ii) the inability to perform the initially proposed number of sprints associated with 170%PPO when the load is calculated from graded exercise test; (iii) the 130%PPO promoted higher oxygen consumption, but only in cyclists.

Previously, Gaitanos et al. (1993) showed that the lactic system participation reduced significantly from the first to the tenth sprint of 6s:30s, and, at the end of the exercise, the energy was predominantly from the alactic system, followed by an increase in aerobic contribution. Trump et al. (1996) applied intermittent exercise with different effort:pause structure (30s:240s). However, the results have a similar reduction in anaerobic systems participation and an increase in the aerobic system participation. Recently, Panissa et al. (2018) studied a short-form of HIIE, and their findings also reinforced the raising of aerobic contribution during HIIE. Most of the findings regarding energy systems contributions could be related to training volume, total effort duration, or, specifically in HIIE, the sum of repeated efforts (Buchheit & Laursen, 2013b). As these variables increase, the contribution of the aerobic system also rises (Gastin, 2001; Glaister, 2005; Trump et al., 1996).

Based on this, it was not surprising that the protocols showed high aerobic contribution (Gaitanos et al., 1993). Even the low exercise duration, found in the 170%PPO condition, had an average length of approximately 120 s. About 1/3 of this time involved pause periods, in which aerobic contribution is very high (Brooks & Mercier, 1994). The more substantial aerobic contribution found in the 115%PPO would be explained by the relatively lower required anaerobic contribution for lower exercise intensities. Furthermore, 115%PPO allowed more repetitions, contributing to a longer duration in the session (488 s, vs 258 and 128 in 130%PPO and 170%PPO, respectively). We also observed that, as the intensity increases, the anaerobic systems exert a higher relative, but not absolute, contribution. This higher relative contribution might have occurred because, when exercising at higher intensities, it allowed a lower number of bouts, reducing the total duration of the effort.

Despite the high blood [La-], frequently higher than 11 mmol.L⁻¹, the relative glycolytic contribution was not so high, probably since each lactate unit corresponds to only 3 ml.O₂. kg⁻¹ (Bertuzzi et al., 2016; Bertuzzi et al., 2007; Campos et al., 2012), and the participants had higher oxygen consumption than accumulated lactate, even after respective conversion. This fact reinforces the hypothesis that measuring the glycolytic contribution of a given exercise considering only the blood [La-] might not be recommended (Bertuzzi et al., 2016), or the glycolytic contribution is quite small during some HIIE (Gaitanos et al., 1993; Trump et al., 1996). Another possibility is that a greater amount of lactate was metabolised within the muscle when the exercise was longer (Brooks, 1986); possibly reducing the lactic contribution. It is important to highlight that the assumption of equating 1 mmol.L⁻¹ of lactate to 3 ml.O₂. kg⁻¹ stands for submaximal exercises. However, many other studies used that to assess supramaximal exercises lactic contribution (Bertuzzi et al., 2007; Campos et al., 2012; Lopes-Silva et al., 2015). Further comparisons using direct measurements (i.e., muscle biopsy) should be made to confirm the reliability and replicability of this method.

The athletes presented higher blood [La⁻] for all intensities, therefore, also presented a higher absolute lactic contribution, this occurred probably due to the greater anaerobic capacity of this population (Ponorac et al., 2007), which contributed to the higher power output during the efforts. Regarding oxygen consumption in athletes, the highest value was in 130%PPO. Considering that maintaining higher V⋅O₂ values during exercise is essential for V⋅O_2MAX increase (Midgley & Mc Naughton, 2006), the optimal intensity for aerobic power development in athletes, in this time structure, would be near to 130%PPO. Previously, on a treadmill, other studies demonstrated that for 30s:15s training in active young men and 15s:15s in middle-aged runners, the intensities of 110%vV⋅O_2MAX and100%vV⋅O_2MAX, respectively, presented the higher oxygen consumption (Aguiar et al., 2013; Billat et al., 2001). Otherwise, subjects maintained the V⋅O₂ closed to 90–95% of the V⋅O_2MAX, which is lower than the 100% reached in the original study and questioned the author’s statement that the ’Tabata training’ should be considered as “one of the most energetically effective exercise training protocols for maximally improving both the aerobic and anaerobic energy-supplying systems” (Tabata, 2019).

Interestingly, at the initially suggested intensity of 170% V⋅O_2MAX (Tabata et al., 1996), most of the athletes and non-athletes were unable to complete the 7-8 sprints. This aspect was previously questioned (Viana et al., 2018c), and these incompatibilities are possibly due to the differences between the tests used to obtain the training loads (Tabata, 2019), which put some light in this debatable point from the findings of the present investigation. While Tabata et al. (1996) used an obsolete, unpractical and questionable protocol that requires a large number of 10-min visits to the laboratory (Bangsbo, 1992; Medbo et al., 1988), Viana et al. (2018c) we used the traditional, worldwide-used, and very practical maximal graded exercise test (Buchheit & Laursen, 2013b).

Even using a similar graded testing protocol, our findings are different from those found by Viana et al. (2018c), in which the intensity of 115%PPO allowed the accomplishment of 7 ± 1 sprints, our results suggest that it is possible to perform this same number of bouts at a higher intensity (130% PPO). Probably this difference is due to the type of ergometer used since previous authors used a magnetic locking model. In contrast, the present study used a mechanical braking device, similar to that used by Tabata et al. (1996). Such differences further reinforce the problem of generalisation of interval protocols reported in previous studies (Viana et al., 2018c), especially regarding the protocol proposed by Tabata et al. (Tabata et al., 1996; Viana et al., 2018a).

There seems to be an apparent conflict in the structural variables of the HIIE protocol with the 20s:10s effort:pause structure as commonly used (Tabata et al., 1996). According to previous studies, we classify as a short HIIE model (effort and pause block lasting less than one minute). However, the intensity close to 170%V⋅O_2MAX is characteristic of Sprint Interval Training models (Buchheit & Laursen, 2013a). Notwithstanding, the effort:pause ratio for sprint interval training is 1:4-8 due to the need to recover anaerobic pathways to maintain an elevated power output (Brooks & Mercier, 1994; Glaister, 2005), whereas in the classic 20s:10s protocol this ratio is 2:1. This conflict might explain why it was not possible to reach 7–8 bouts when using 20s:10s, with a load equivalent to 170%V⋅O_2MAX, even in athletes.

Finally, despite the methodological issues that we should consider regarding this specific HIIE protocol (Gentil et al., 2016; Tabata et al., 1996; Viana et al., 2018c), its potential to induce positive physiological changes should be recognised (Ma et al., 2013; Miyamoto-Mikami et al., 2018; Scribbans et al., 2014; Tabata et al., 1996). Both “classical” studies by the “Tabata” group showed that participants could achieve relevant aerobic and anaerobic improvements in a very high time-efficient manner (Tabata et al., 1996). More recently, the same group showed that this protocol could significantly increase aerobic power, maximal accumulated oxygen deficit, and thigh muscle cross-sectional area (Miyamoto-Mikami et al., 2018). Further longitudinal studies should investigate this 20s:10s protocol at an intensity range of 115 to 130%PPO in order to raise the time near and at V⋅O_2MAX.

Conclusions

In conclusion, to a 20s:10s HIIE protocol, the aerobic contribution is predominant, independently of the intensity applied in a range from 115%PPO to 170%PPO. Despite that, the lower the intensity, the higher is the aerobic contribution, and 130%PPO is the suggested intensity to induce higher V⋅O2 in trained cyclists. Finally, to reach about eight sprints, we propose the intensity of 130% the power at V⋅O_2MAX obtained in a graded test, either for athletes or non-athletes.

Supplemental Information