The parameters of the power-time relationship, termed critical power (CP) and the curvature constant (W'), have been used to analyze the physiological responses and endurance performance during high-intensity exercise (Poole et al., 1988). CP has been considered the lower boundary of the severe-intensity domain and the W' determines the amount of external work that can be performed above CP, irrespective of the rate of its expenditure (Jones et al., 2010). By definition, all severe-intensity work rates (i.e., >CP) performed until voluntary exhaustion drive pulmonary oxygen uptake (VO₂) to a maximal value (i.e., maximal oxygen uptake—VO₂max) (Jones et al., 2010). However, during exhaustive exercise performed above the upper bound of the severe intensity domain, exercise duration would be too short to permit attainment of VO₂max Caputo and Denadai (2008). Several studies have demonstrated that endurance exercise performance within severe-intensity domain was coincident with the depletion of the W', accumulation of metabolites associated with fatigue (i.e., PCr, Pi, and H⁺), and attainment of VO₂max due to VO₂ slow component (VO₂SC) development (Fukuba et al., 2003; Chidnok et al., 2013). Indeed, VO₂SC has been associated with loss in muscular efficiency (Jones et al., 2011) and has been negatively related with endurance performance (Zoladz et al., 1995; Murgatroyd et al., 2011; Barbosa et al., 2014a).

VO₂ kinetics and muscle [PCr] responses to high-intensity exercise have been reported to present both fundamental and slow component phases (Rossiter et al., 2002) being intrinsically linked. Indeed, Rossiter et al. (2002) have reported similar values of the time constant (τ) of the fundamental component ([PCr] = 38 s; VO₂ = 39 s), as well as the relative amplitude of the slow component ([PCr] = 13.9%; VO₂SC = 15.3%) of muscle [PCr] and VO₂ during high-intensity exercise. It has been proposed that progressive intramuscular depletion [PCr] during exhaustive exercise performed within severe intensity domain provides the appropriate stimulus to oxidative phosphorylation, determining the development of VO₂SC and, consequently, the attainment of VO₂max (Rossiter et al., 2002). Thus, both creatine phosphate depletion and development of the VO₂SC seem to be intimately associated with endurance performance during constant-work rate exercise (CWR) performed within the severe intensity domain.

While this scenario is well established during CWR exercise, very little information is available during intermittent exercise, which has been considered an important tool in training programs aiming to improve aerobic fitness in health and in disease (Laursen and Jenkins, 2002; Hwang et al., 2011). Indeed, intermittent exercise can improve performance comparing to CWR during high-intensity exercise (Millet et al., 2003; Chidnok et al., 2012), since the former allows resynthesis of intramuscular substrates ([PCr]) and/or clearance of fatigue-related metabolites (i.e., reconstitution of W') (Chidnok et al., 2013). However, several aspects seem to influence endurance performance during high-intensity intermittent exercises. For instance, endurance performance is progressively shorter when the work-recovery “duty-cycle” (e.g., 10:20 s, 30:60 s, 60:120 s, and 90:180 s) (Turner et al., 2006) and/or exercise intensity performed during active recovery is increased (i.e., light, moderate, heavy and severe) (Chidnok et al., 2012). These aspects influence PCr kinetics (Chidnok et al., 2013) and hypothetically, the changes of the rates at which VO₂ increases during high-intensity intermittent exercises (i.e., VO₂SC). Indeed, Chidnok et al. (2012) have demonstrated that enhanced endurance performance during severe-intensity intermittent exercise could be explained by the reconstitution of W' during recovery intervals performed at lower-intensity domains (i.e., light and moderate). At this condition, the reconstitution of W' was associated with a blunted increase in both VO₂ and integrated EMG with time, supporting the hypothesis that VO₂SC kinetics influences endurance performance during intermittent exercise. However, as discussed above, endurance performance during severe intermittent exercise is markedly modulated by both work-recovery duration and exercise intensity performed during active recovery. Thus, the possible relationship between VO₂SC and endurance performance during intermittent exercise performed with different durations (e.g., short vs. long) and recovery type (i.e., passive vs. active) remains elusive, and further studies are warranted.

However, an important issue must be considered when the possible influence of VO₂SC on endurance performance is investigated. Knowing that work-recovery duration influences endurance performance during severe intermittent exercise (Turner et al., 2006), it appears appropriate to compare exercise duration before (short condition) and after (long condition) the emergence of VO₂SC. However, many studies have verified that both the emergence and the amplitude of VO₂SC (and possibly the [PCr]) present a large intra-individual variation (Murgatroyd et al., 2011; Barbosa et al., 2014b). Thus, it would be interesting to analyze the responses of VO₂ kinetics and endurance performance during severe intermittent exercise, with both the duration of exercise and recovery periods being determined based on the individual VO₂SC kinetics response.

Thus, the current study was undertaken to compare the endurance performance and VO₂SC kinetics during high-intensity intermittent exercise performed with different work:recovery duration (short vs. long) and recovery types (passive vs. active) in active individuals. It was hypothesized that: (a) endurance performance would be improved during the exercise with passive recovery, regardless of the duration of the repetition, and; (b) endurance performance would be improved during the intermittent exercise with short duration, regardless of the recovery type. We also hypothesized that the possible interaction between exercise duration and recovery type during intermittent high intensity exercise would influence the changes to the rates at which VO₂ increases with time (VO₂SC kinetics) and consequently, endurance performance.

Table 1 presents the mean ± SD values of the variables obtained during the incremental test for both PR and AR groups. No significant difference was found between the groups (p > 0.05).

The VO₂ response profiles of a representative subject obtained during the different exercise conditions for both PR and AR groups are depicted in Figure 2. Based on the VO₂ kinetics parameters obtained during CWR, the repetition duration for the Short (PR = 105 ± 29 s; AR = 132 ± 39 s) and Long (PR = 252 ± 50 s; AR = 253 ± 56 s) tests were not significantly different between the groups (p > 0.05).

Figure 3 presents the mean ± SD values of end-exercise VO₂ measured during the different exercise conditions for both PR and AR groups. There was a significant main effect for the exercise condition on end-exercise VO₂ values (F = 5.47, p = 0.009, η² = 0.25), but no effect of group (F = 1.53, p = 0.23, η² = 0.08) or interaction was detected (F = 1.25, p = 0.29, η² = 0.07). The end-exercise VO₂ values obtained during CWR (PR = 3236.9 ± 405.8 mL.min⁻¹; AR = 3488.6 ± 415.9 mL.min⁻¹) were higher than those attained during Short (PR = 2995.2 ± 337.7 mL.min⁻¹; AR = 3205.7 ± 447.2 mL.min⁻¹) and Long (PR = 3053.3 ± 276.1 mL.min⁻¹; AR = 3149.6 ± 476.3 mL.min⁻¹) tests (p < 0.05).

The mean ± SD values of tlim and VO₂ slope during CWR and intermittent exercises for the PR and AR groups are presented in Table 2. A group vs. exercise condition interaction (F = 11.08, p = 0.000, η² = 0.40) indicated longer tlim obtained during intermittent exercises (Short and Long) than CWR for both groups (p < 0.05). Considering the duration of the work and recovery type, tlim at Short was significantly longer than at Long only for the PR group (p < 0.05). Group effect (i.e., PR vs. AR) was significant only when comparing the Short intermittent protocols (p < 0.05), with no significant difference for Long conditions (p > 0.05). There was a significant main effect for the exercise condition on VO₂ slope values (F = 95.98, p < 0.000, η² = 0.90), but no group effect (F = 1.86, p = 0.19, η² = 0.16) or interaction was detected (F = 0.02, p = 0.99, η² = 0.01). VO₂ slope was significantly greater at CWR than Short and Long conditions (p < 0.05).

The mean ± SD values of [La] and HR during CWR and intermittent exercises for the PR and AR groups are presented in Table 3. There was a significant main effect for the exercise condition on [La] values (F = 4.72, p = 0.01, η² = 0.22), but no effect of group (F = 0.05, p = 0.81, η² = 0.04) or interaction was detected (F = 1.76 p = 0.18, η² = 0.09). The [La] was significantly lower at Short than CWR and Long condition (p < 0.05). A group vs. exercise condition interaction (F = 5.00, p = 0.01, η² = 0.23) indicated that HR was lower during Short than Long and CWR only for the PR group (p < 0.05).

This, we believe, is the first study to compare the endurance performance and VO₂SC kinetics during severe-intensity intermittent exercise performed with different durations and recovery types in active individuals. The data demonstrate that endurance performance during severe-intensity intermittent exercise is negatively influenced by active recovery only during shorter (~120 s) intermittent exercise. Interestingly, slopes describing the increases in VO₂ with time (i.e., VO₂SC) and end-exercise VO₂ were reduced during intermittent exercise (i.e., CWR vs. intermittent exercise). However, VO₂ kinetics (VO₂SC and end-exercise VO₂) were similar between work:recovery duration (short vs. long) and recovery type (passive vs. active) analyzed in the present study, therefore rejecting our original hypothesis. Thus, the relationship between VO₂ kinetics (VO₂SC and end-exercise VO₂) and endurance performance observed during CWR exercise (Jones et al., 2010; Barbosa et al., 2014a) seems to be differently regulated during severe-intensity intermittent exercise.

It has been widely reported that endurance performance during high-intensity intermittent exercise is improved when compared with CWR exercise (Demarie et al., 2000; Millet et al., 2003; Chidnok et al., 2012). However, both endurance performance and metabolic response are influenced by the characteristics of the protocol utilized during high-intensity intermittent exercise. Turner et al. (2006) analyzed the influence of duty cycle duration with the same work:recovery ratio (10:20 s, 30:60 s, 60:120 s, and 90:180 s) on pulmonary gas exchange and blood lactate dynamics during intermittent cycling exercise performed at 120% IVO₂max. At this condition, a greater metabolic response (elevated blood lactate concentration and attainment of VO₂max) and exercise intolerance (i.e., subjects could not complete 30 min of exercise) were observed only for the longer duty cycles (i.e., 60:120 s, and 90:180 s). Although our intermittent exercise protocol presents different characteristics (e.g., work:recovery = 2:1 and exercise intensity = 95% IVO₂max), it was also verified a reduced endurance performance during longer duty cycles performed with passive recovery. The intramuscular PCr concentration ([PCr]) kinetics both during and following high-intensity exercise presents a curvilinear profile and seems to be closely linked with VO₂ kinetics (Rossiter et al., 2002). For instance, under the conditions of the present study, is very likely that the amplitude of [PCr] restoration during the 240 s recovery intervals (Long protocol) was not doubled than what was presented when 120 s periods of recovery (Short protocol) were allowed. Moreover, Chidnok et al. (2013) demonstrated that [PCr] restoration become longer as the intermittent protocol continued. Thus, [PCr] is progressively lower immediately before each repetition, particularly when duty-cycle duration is lengthened. The metabolites generated by muscle contraction at this condition, such as Pi, ADP, and AMP, increase glycolytic flux and consequently, glycolytic H⁺ (Adams et al., 1990; Conley et al., 1997) and lactate (Karpatkin et al., 1964) production. Low values of muscle [PCr] and pH (i.e., high values of [H⁺]) and consistently high values of [Pi] and [ADP] have been associated with fatigue development during high-intensity exercise (Jones et al., 2008; Vanhatalo et al., 2010).

Another factor that can influence both endurance performance and metabolic response is the activity pattern performed during the recovery intervals between each bout (Chidnok et al., 2012). Using the CP model, Chidnok et al. (2012) demonstrated that endurance performance during intermittent exercise was enhanced only when the recovery intervals were performed below CP. Active recovery performed below CP allows a partial PCr reconstitution and/or clearance of fatigue-related metabolites (Chidnok et al., 2013), with the former being apparently more important to enhance endurance performance during high-intensity intermittent exercise. Indeed, both endurance performance (Chidnok et al., 2012) and PCr reconstitution (Chidnok et al., 2013) are higher during intermittent exercise with passive recovery than during active recovery performed bellow CP condition. Thus, a lower PCr reconstitution can explain, at least in part, the impaired endurance performance during short condition performed with active recovery, as observed in the present study.

However, a different scenario emerges from the data obtained during the Long intermittent exercise protocol. At this condition, endurance performance was not modified by the active recovery periods. Two different mechanisms, which can occur simultaneously, could help explain this phenomenon. Firstly, the negative influence of active recovery on PCr reconstitution could be time-dependent, i.e., longer duty-cycle could allow more similar PCr reconstitution than a shorter one. The curvilinear PCr recovery profile supports this hypothesis (Harris et al., 1976). Secondly, the clearance of lactate and H⁺ ions within muscles might be higher during the longer duty-cycle. A higher muscle pH can reduce, directly or indirectly (a more favorable metabolic milieu for PCr reconstitution), fatigue during high-intensity exercise. Alternatively, it is possible that [PCr] kinetics both during and following high-intensity intermittent exercise would contribute progressively less to endurance performance when the duty-cycle duration is lengthened.

The end-exercise VO₂ was not significantly different between CWR exercise and VO₂max measured during the incremental test. This is consistent with the fact that exhaustive exercise performed within the severe intensity domain (i.e., above CP) is characterized by the development of the VO₂SC, which is truncated at VO₂max. Some interventional (e.g., endurance training and priming exercise) (Jones et al., 2007; Caritá et al., 2014) and correlational studies (Barbosa et al., 2014a) have produced evidences that both VO₂ kinetics (a proxy for intramuscular PCr kinetics) (Rossiter et al., 2002) and VO₂max attainment is related to endurance performance during high-intensity exercise. Thus, it was hypothesized that VO₂SC trajectory, which reflects the interaction between VO₂SC and VO₂max attainment, could explain the endurance performance during high-intensity intermittent exercise. Indeed, it was demonstrated that VO₂SC trajectory was faster during CWR exercise than during intermittent exercise, regardless of duration and recovery type. However, similar to the results found by Chidnok et al. (2012), VO₂SC trajectory was not significantly different among intermittent exercise, and end-exercise VO₂ was lower during these conditions than at CWR exercise. Thus, substrate utilization/accumulation, VO₂ kinetics (VO₂SC trajectory and end-exercise VO₂) and endurance performance during high-intensity exercise seem to present different relationship during CWR and intermittent exercise. Priming high-intensity exercise has previously been reported to reduce the amplitude of VO₂SC and an increase in apparent W' during subsequent exercise (Caritá et al., 2014, 2015; Dekerle et al., 2015). In this context, each preceding intermittent exercise bout may have “primed” the muscle (i.e., reduces the amplitude of VO₂SC and/or raise the W') during subsequent bouts. These modifications are consistent with enhanced endurance performance, and could help to explain the apparently different metabolic regulation imposed by the interaction between intervals duration and recovery type during intermittent exercise.

Our experimental protocol (i.e., exercise intensity, work:recovery durations and recovery types) was specifically designed to investigate the hypothetical association between intermittent endurance performance and VO₂SC kinetics. Similar to previous studies (Caputo and Denadai, 2008; Barbosa et al., 2014a), both CWR and intermittent exercise were performed at 95% IVO₂max. As demonstrated in the present study, exhaustive exercise performed at this intensity is characterized by the development of the VO₂SC and VO₂max attainment. Some studies have utilized the “percentage delta” (for details please see Lansley et al., 2011) aiming to select a predetermined exercise intensity domain (i.e., heavy or severe) and/or to standardize the exercise intensity between subjects. Indeed, when compared to a more traditional method (e.g., %VO₂max), this approach allows a lower inter-subject variability of physiological responses to CWR exercise (Lansley et al., 2011). However, for the first time, the present study have normalized the wok:recovery durations based on the individual VO₂SC kinetics response. Thus, we are confident that the inter-subject variability of physiological responses during the intermittent exercise was attenuated. Finally, this study presented a possible limitation, since the effect of passive and active recovery on intermittent exercise was analyzed using 2 different groups of active individuals. Hypothetically, this experimental design could be influenced by the individual variability on both endurance performance and VO₂SC kinetics. However, PR and AR groups have presented similar data during incremental (VO₂max, IVO₂max, 95% IVO₂max and LT) and CWR exercise (endurance performance and VO₂SC kinetics). Therefore, the possibility of inter-subject variability influencing the recovery types comparisons was probably reduced. This limitation comes from the heavy testing required to be undertaken by each subject to test our research hypothesis. It is important to note that a short-term training program (6 sessions) involving high-intensity exercise (repeated all-out sprint training) have reduced the amplitude of the VO₂SC and increased tolerance to high-intensity exercise in recreationally active subjects (Bailey et al., 2009). Thus, if a repeated measures design has been utilized in our experimental approach, a confounding factor could be added to our analysis, since the volunteers would have to perform 6 bouts of severe-intensity exercise.

The present study showed that under our experimental conditions (i.e., exercise intensity, work:recovery durations and recovery type), intermittent exercise enhances endurance performance during severe-intensity exercise, independently of intervals duration and recovery type. Passive recovery is superior in relation to active recovery to enhance endurance performance only during shorter duty-cycles. Although VO₂SC trajectory is attenuated during high-intensity intermittent exercise, its alteration does not seem to explain the interaction effects of intervals duration and recovery type on endurance performance. Moreover, the end-exercise VO₂ was lower during intermittent exercise than at CWR exercise. Thus, severe-intensity intermittent exercise performed with different intervals duration and recovery type seems to modify the relationship between endurance performance and VO₂ kinetics observed during CWR exercise. Further studies using a repeated measures design are required to examine the effect of severe-intensity intermittent exercise on both endurance performance and VO₂SC in trained individuals. A threshold in the duration of the recovery, from which PCr resynthesis and/or W' reconstitution would be less affected by active recovery could be identified. This can help to explain and confirm our main results, giving support to elaborate a more sophisticate interval training programs for different populations.

Study design: BD and CG. Data acquisition and analysis: LB, BD, and CG and Writing the paper: LB, BD, and CG.

Supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (grant 2009/07700-2 and grant 2016/22907-6), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação para o Desenvolvimento da Unesp (FUNDUNESP).