Low-volume high-intensity interval training (LV-HIIT) is considered a practical and tolerable protocol for healthy and clinical populations (Gibala et al., 2012). LV-HIIT is performed at a 'vigorous' to 'near maximal' efforts (Weston et al., 2014), usually between 85-95% of maximal heart rate ([HR.sub.max]) (Gibala et al., 2012). LV-HIIT protocols involve a low amount of work at 'vigorous' to 'near maximal' efforts, which usually lasted 10 minutes or less. For example, 10 x 60 s intervals at 90% of [HR.sub.max] interspersed by 60 s of recovery (Gibala et al., 2014). From a physiological perspective, LV-HIIT has demonstrated efficacy to improve cardiorespiratory fitness (Currie et al., 2013; Gillen et al., 2013) and cardiometabolic risk factors in several populations (Ciolac et al., 2009; Hood et al., 2011; Little et al., 2010; 2011).
However, the psychological responses to interval training have been the focus of an intense and polarized debate (Biddle and Batterham, 2015; Hardcastle et al., 2014; Stork et al., 2017). Affective response (i.e. feeling of pleasure/displeasure) during interval training has been debated because it is considered an important factor related to exercise adherence (Garber et al., 2011). Previously, some studies have shown a negative affect during LV-HIIT (Frazao et al., 2016; Olney et al., 2018; Thum et al., 2017), while other studies have reported a positive affect (Jung et al., 2014; Kilpatrick et al., 2015; Martinez et al., 2015). It seems that this disagreement about the affective response to LV-HIIT may be related to the different designs of the protocols used, mainly regarding the different intensities and durations of the intervals and recovery periods (Stork et al., 2017). LV-HIIT protocols involving interval duration [less than or equal to] 60 s and performed at an intensity [less than or equal to] 85% of [HR.sub.max] elicit a positive affective response (i.e. feeling of pleasure) (Martinez et al., 2015). On the other hand, LV-HIIT protocols involving interval duration [greater than or equal to] 60 s performed at an intensity greater than to 85% of [HR.sub.max] elicit a negative affective response (Frazao et al. 2016; Olney et al. 2018; Thum et al. 2017). However, it remains unclear whether only the work-recovery duration modulates the affective response when the LV-HIIT protocols are matched by the work-recovery ratio and the total work performed.
It should be noted that the physiological responses during interval training may be modulated by the work-recovery duration (Buchheit and Laursen, 2013). Overall, shorter intervals elicit lower oxygen uptake, HR, and blood lactate concentration when the total work performed is matched (Tschakert et al., 2015; Tucker et al., 2015). Different combinations of work-recovery durations and intensity may generate a balance or imbalance between lactate production and clearance (Tschakert and Hofmann, 2013; Tschakert et al., 2015). We have observed previously that the affective response during HIIT is more negative when the HR and perceived exertion are higher, which occurs especially in the end of the exercise session. We argue that it occurs when a metabolic imbalance is present (i.e. higher lactate production than clearance) (Frazao et al., 2016; Oliveira et al., 2013). More recently, we have demonstrated that the affective response to 60/60 s LV-HIIT is negatively correlated with time spent above respiratory compensation point (Farias-Junior et al. 2018). Taken together, it seems that work-recovery combinations that elicit a state of metabolic imbalance are associated with negative affective response to HIIT. The present study has investigated whether a LV-HIIT session performed with shorter work-recovery duration (i.e. 30 s) elicits lower physiological response and, as a result, lower perceived exertion and affective response less negative than a LV-HIIT protocol performed with longer work-recovery duration (i.e. 60 s).
A total of 18 participants were approached for the study and 10 men completed the study (age: 26.6 [+ or -] 4.8 years; BMI: 25.6 [+ or -] 2.3 kg/[m.sup.2]; V[O.sub.2peak] 49.3 [+ or -] 5.3 mL/kg/min; [V.sub.max]: 16.4 [+ or -] 1.9 km/h; [HR.sub.max]: 193.3 [+ or -] 7.5 bpm) (see Figure 1). The participants were recruited from the invitation disclosed in university settings, e-mail and online social networks. Individuals who agreed to participate in the study completed an in-lab interview for eligibility confirmation. The study was conducted from June 2016 to October 2016. Inclusion criteria were: i) men aged from 18 to 35 years; ii) apparently healthy according to the Physical Activity Readiness Questionnaire; and iii) having experience in treadmill running. Exclusion criteria were: i) BMI 30.0 kg/[m.sup.2]; and (ii) injury during the study period; iii) use of medication that affects cardiorespiratory function. The participants were informed about all procedures related to the study, and gave written informed consent. The study was approved by the Ethics Committee of University (protocol 706.789/2014).
We conducted a randomized, counter-balanced order trial including two interventions. The trial compared oxygen uptake, carbon dioxide output, respiratory exchange ratio, HR, rating of perceived exertion, and affect between two LV-HIIT protocols with different work-recovery durations (i.e. 60/60 s vs. 30/30 s), but matched by work-recovery ratio and total work performed (i.e. 1:1 and 10 minutes, respectively). The study was reported in accordance with the CONSORT Statement guidelines (Boutron et al., 2017). Each participant performed the following procedures: i) initial screening; ii) maximal graded exercise test; and iii) a single session of 60/60 s LV-HIIT and 30/30 s LV-HIIT. Initially, the participants were screened using the Physical Activity Readiness Questionnaire. Afterward, they underwent a clinical examination where body weight (kg) and height (m) were measured. Body mass index (BMI) was calculated as weight (kg) divided by the square height in meters (kg/[m.sup.2]). After 48 h of the initial screening, participants performed a maximal graded exercise test on a treadmill. At the end of the maximal graded test, the two experimental sessions (60/60 s LV-HIIT and 30/30 s LV-HIIT) were scheduled with one-week interval between each one. A computer-based randomization (http://www.randomization.com) was used to determine the order of the exercise sessions. Only the participants were blinded to the order of interventions. Figure 1 shows the flowchart of the study. All procedures were performed in the afternoon (between 1:00-4:00 p.m.). Participants were asked to avoid moderate-vigorous physical activity, caffeinated products, and alcohol consumption as well as to maintain a good sleeping pattern and normal dietary habits 24 h before the graded exercise test to volition exhaustion and experimental sessions.
Graded exercise test to volitional exhaustion
Participants performed a graded exercise test to volition exhaustion on a motorized treadmill (RT350, Movement[R], Sao Paulo, Brazil) to determine the maximal velocity ([V.sub.max]), [HR.sub.max] and peak oxygen uptake (V[O.sub.2peak]). The test started at 4 km/h for 1 minute, followed by fixed increments of 1 km/h per minute until volitional exhaustion. HR was continuously recorded throughout the test using a HR monitor (RS800cx, Polar Electro[R], Oy, Kempele, Finland). Oxygen uptake was continuously recorded using a breath-by-breath gas exchange automatic system (Metalyzei[R] 3B, Cortex Biophysik GmbH, Leipzig, Germany). [V.sub.max] was defined as the velocity reached during the last full stage added with the proportional time in the following incomeplete stage before the volitional exhaustion (Midgley et al., 2009). For example, if the participant completed the 13 km/h stage and reached the volitional exhaustion in the next 30 s during the 14 km/h stage the [V.sub.max] was defined as 13.5 km/h. V[O.sub.2peak] was considered as the higher value of the last 30 s of oxygen uptake before volitional exhaustion (Midgley et al., 2009).
30/30 s and 60/60 s LV-HIIT sessions
Participants performed both LV-HIIT sessions on a motorized treadmill (RT350, Movement[R], Sao Paulo, Brazil). The 60/60 s LV-HIIT protocol consisted of 10 x 60 s intervals at 100% of [V.sub.max] interspersed with 60 s of passive recovery. The 30/30 s LV-HIIT protocol consisted of 20 x 30 s intervals at 100% of [V.sub.max] interspersed with 30 s of passive recovery. Both LV-HIIT sessions lasted 30 minutes, including 5 minutes of warm-up and 5 minutes of cooldown at 4 km/h. Both exercise sessions were performed at the same mean load ([V.sub.mean], 8.2 [+ or -] 0.9 km/h) according to equation [V.sub.mean] = ([V.sub.peak] X [t.sub.peak] + [V.sub.rec] x [t.sub.rec]) / ([t.sub.peak] + [t.sub.rec]) (Tschakert and Hofmann, 2013). The HR (bpm) was recorded every 1 minute during LV-HIIT sessions. The V[O.sub.2] was continuously recorded (breath-by-breath) using a gas exchange automatic system (Metalyzer 3B, Cortex Biophysik GmbH[R], Leipzig, Germany). Energy expenditure in the sessions were calculated including warm-up and cool-down periods.
During LV-HIIT sessions, oxygen uptake (V[O.sub.2]), carbon dioxide output (VC[O.sub.2]), and ventilation (VE) were continuously recorded (Metalyzer[R] 3B, Cortex Biophysik GmbH, Leipzig, Germany) and the mean values of every 30 s were considered for analysis. HR (bpm) was continuously recorded throughout the LV-HIIT sessions using a HR monitor (RS800cx, Polar Electro[R], Oy, Kempele, Finland) and the HR of the last 5 s of interval and recovery periods were considered for analysis. For comparison between the two LV-HIIT protocols the equivalent times for interval and recovery periods (i.e. 20%, 40%, 60%, 80%, and 100%) were considered; i.e. the 2nd, 4th, 6th, 8th, and 10th interval...