The greatest public health problem of the 21st century is physical inactivity which is usually the consequence of modern sedentary lifestyles (Booth et al., 2000; Trost et al., 2014). Most international guidelines for physical activity recommend at least 150 min of moderate-intensity physical activity (3–5.9 metabolic equivalents; METs) or 75 min of vigorous-intensity aerobic physical activity (≥6 METs) per week for adults (Ainsworth et al., 2011; Hallal et al., 2012; Brinsley et al., 2021). However, estimates based on self-reported data show that 40–60% of the general adult population are not sufficiently active (Hallal et al., 2012). This may lead to non-communicable diseases, including cardiovascular, coronary heart disease, diabetes, and cancer, which account for seven of the ten most common worldwide reasons for premature death (Hallal et al., 2012; Brinsley et al., 2021).

The modern workplace has recently been recognized as an alternative setting for physical activity or exercise for people who may not have time, e.g., during lunch break, to participate in more formal exercise sessions (Kuoppala et al., 2008; Dalager et al., 2016). In this regard, hatha yoga (HY) lends itself to forming part of a general health regimen to prevent physical inactivity (Larson-Meyer, 2016). Additionally, HY aims to improve the body, breath, and mind and prepare self-realization as an alternative form of exercise (Schmalzl et al., 2015; Papp et al., 2019). Previous review and meta-analytic findings have shown that HY decreases blood pressure, blood lipids, glycosylated hemoglobin, low-density lipoprotein, and increases high-density lipoprotein cholesterol (Hagins et al., 2013; Cramer et al., 2014).

However, the common HY program lasts for approximately an hour, which is unsuitable for most people in the workplace. Therefore, a program of high-intensity interval training (HIIT) is an alternative with preferred physical exercises which are also ranked in the top 10 fitness trends of the American College of Sports Medicine (Thompson, 2021). HIIT improves cardiovascular fitness as measured by maximal oxygen uptake (VO_2max) and includes repeated rounds of exercise that achieve >90% of maximal heart rate (HR_max), the second ventilatory threshold (>VT₂), over second lactate threshold (>4 mmol·L⁻¹; zone 3: high-intensity exercise), and >85% of peak and maximal oxygen uptake (VO_2peak and VO_2max) (Billat, 2001; Treff et al., 2019; Jamnick et al., 2020).

Hatha yoga is considered as a low-to-moderate-intensity physical activity based on MET values and percentages of HR_max and VO_2max (Hagins et al., 2007; Ainsworth et al., 2011; Ray et al., 2011). Furthermore, HY can be a form of high-intensity exercise (HIE) (Papp et al., 2019). High-intensity hatha yoga (HIHY) includes vigorous sun salutation (SS) physical exercises (asanas) at rapid speed. The most common exercise sequence of HY programs consists of SS (Pascoe and Bauer, 2015; Larson-Meyer, 2016; Papp et al., 2019).

Typically, meditative relaxation (savasana) such as passive recovery is conducted after HY and HIHY (Sharma et al., 2007; Papp et al., 2019). However, this recovery method is not suitable after HIHY in the workplace. In light of this, active recovery helps regenerate metabolic pathways which provide greater oxygen uptake (VO₂) and O₂ transfer into muscle cells, both of which are necessary for the resynthesis of adenosine triphosphate (ATP) (Menzies et al., 2010; Cupeiro et al., 2016; Yang et al., 2020). The lactate shuttle mechanism plays a crucial role in lactate clearance (Brooks, 2018). Lactate links glycolytic and oxidative energy systems. During active recovery, the accumulated lactate is predominantly re-metabolized by the cell-cell lactate shuttle, and by the Cori cycle and gluconeogenesis. These mechanisms are supported by increased hepatic blood flow during the active recovery phase (Nielsen et al., 1999; Yang et al., 2020). Furthermore, active recovery (low-intensity) activates key enzymes and hormonal regulators of gluconeogenesis such as phosphofructokinase, pyruvate carboxylase, phosphoenolpyruvate carboxykinase, glucagon, cortisol, and other related regulators (Yang et al., 2020).

At present, it is unclear how different energy systems contribute during HIHY. In general, yoga studies have focused on the psychological aspects and benefits, and during HIHY only physiological parameters such as VO_2peak, peak lactate concentration (peak La⁻), and peak heart rate (HR_peak) have been analyzed. Following HIHY, the traditional passive recovery process of savasana has commonly been utilized although an active recovery causes faster physiological regeneration. Therefore, this study aimed to define the different energetic contributions during HIHY and to compare the magnitude of changes in physiological parameters between passive and active recovery after HIHY.

The metabolic energy contributions during HIHY are currently unclear and it is somewhat controversial which recovery methods are physiologically more efficient after HIHY. To the best of our knowledge, this study is the first to evaluate how different energy systems contribute during HIHY and how physiological parameters are influenced by different recovery methods (active vs. passive) afterward. The major findings indicated that oxidative energy predominates over anaerobic energy contributions (W_PCR and W_Gly).

Randomly assigned participants in recovery groups showed no significant differences in HR_peak, HR_mean, VO_2peak, VO_2mean, and peak La⁻ during HIHY. These indicated that all participants conducted the same HIHY workout. Additionally, values of HR_peak (95.6% of HR_max), HR_mean (88.7% of HR_max), METs (10.5 and 8.5), and peak La⁻ (8.3 mmol·L⁻¹) exhibited parameters consistent with HIE (>90%, ≥6 METs; vigorous/heavy, >4 mmol·L⁻¹; zone 3: HIE, respectively) (Jetté et al., 1990; Billat, 2001; Ainsworth et al., 2011; Treff et al., 2019; Jamnick et al., 2020). Furthermore, VO_2peak (36.9 mL·kg⁻¹·min⁻¹) indicated a result similar to a previous study in which a VO_2max of 37.5 mL·kg⁻¹·min⁻¹ during HIHY was reported (Table 2) (Papp et al., 2016). Regarding the energetic contribution, a predominant utilization of W_Oxi in kJ and % (81.9%) was found and was dominant over W_PCR, W_Gly, and the entire anaerobic system (W_PCR + W_Gly). These results were influenced by the duration of HIHY (10 min) and decreased the contribution provided by the glycolytic energy system (Heck et al., 2003; Yang et al., 2020; Park et al., 2021). To obtain a maximal lactate production rate, 10 s exercise duration was suggested because the contribution of the glycolytic system (accumulated lactate rate) decreases with increasing duration of maximal exercise, due to inhibition of phosphofructokinase activity (Heck et al., 2003). Consistent with this, previous studies have shown that the contribution of the oxidative energy system was increased as a consequence of increased VO₂ uptake during taekwondo (62–70%) and 2,000 m rowing (83–85%), activities which lasted ~6 and 8.5 min, respectively, while the glycolytic system was reduced (de Campos Mello et al., 2009; Campos et al., 2012).

After 10 min recovery phases, the active recovery group (40–45% of HR_max) had faster lactate clearance (resynthesis) than the passive recovery group (Figures 4A,C). Consequently, higher VO_2post and lower ΔVO₂ were found in the active recovery group compared with the passive (Table 3). This is consistent with a previous study that reported blood lactate concentration after intense running (VO_2max and lactate threshold test) was reduced more by active rather than passive recovery regimens with intensities of 25–63% of VO_2max and 40–80% of lactate threshold (Menzies et al., 2010). For a low-intensity activity or exercise, such as walking or jogging ATP resynthesis is affected more by substrate-level and oxidative phosphorylation reactions than by accumulated lactate concentration (Rodríguez and Mader, 2011; Yang et al., 2020). In particular, this mechanism is affected by more skeletal muscle activation, including more O₂ uptake into skeletal muscle cells (Cupeiro et al., 2016; Brooks, 2018; Yang et al., 2020). It mostly occurs in type 1 muscle fibers which predominantly express monocarboxylic transport 1 (MCT1) while MCT2 is prominently expressed in the liver. MCT1 is the most important protein for lactate transport into or out of red blood cells (Menzies et al., 2010; Brooks, 2018; Yang et al., 2020). The study of Yang et al. suggested that accumulated lactate is predominantly eliminated by the Cori cycle during the low-intensity exercise/recovery phase (Yang et al., 2020). As evidence of the mechanism in the liver, hepatic blood flow is increased and muscle lactate output and hepatic lactate uptake are similar during recovery, while a two-third decrease in hepatic blood flow is among the most distinct alterations during HIE in humans (Nielsen et al., 2007). With regard to this aspect, when exercise intensity is higher than 50% of VO_2max, gluconeogenesis is decreased because of reduced hepatic blood flow (Nielsen et al., 2007; Yang et al., 2020). After active recovery in this study, VO_2post was 33% of VO_2peak during HIHY (Table 3). Therefore, lactate accumulated during HIHY might be resynthesized in large part by gluconeogenesis during active recovery in the liver. According to the findings of this study, active recovery is more effective at regenerating the metabolic system after 10 min HIHY.

This study identified the energetic contribution during HIHY and physiological differences between active and passive recovery. However, this study has some limitations. The baseline VO_2max, which can enable the determination of VO₂ levels during HIHY as percentages of VO_2max, was not measured in this study. Furthermore, the pure contribution of the calculated glycolytic energy system was limited. This was underestimated because lactate elimination and production rate could not be analyzed between HIHY. Therefore, further studies are expected to investigate different durations of exercise, e.g., 2, 4, 6, 8, and 10 min, which should be randomly determined and blinded to the duration. Furthermore, 10 min HIHY should be used and considered in interventional studies of how cardiovascular/cardiorespiratory fitness such as VO_2max and VO_2peak will be changed in different populations, such as the unskilled general public.

Our findings indicated that 10 min HIHY is suitable for an HIE session based on the high levels of physiological parameters and energetic contributions. Because of the high level of exercise intensity, this form of exercise is appropriate for relatively healthy employees in the workplace who may have HY experience, but do not have time for physical exercise. However, for safety, HIHY should be preceded by appropriate warm-up exercises, such as the classical sun salutation, which is performed slowly. Finally, this study found that active recovery is a more helpful method compared with traditional passive (savasana) recovery after 10 min HIHY. After active recovery, people can participate in further HIHY sessions during short breaks such as lunchtime. Consequently, a quicker return to the workplace can be supported by metabolic regeneration.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

The studies involving human participants were reviewed and approved by the Institutional Ethics Committee of CHA University (No. 1044308-202007-HR-026-02). The patients/participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

K-HL and W-HY were involved in study conception and design and wrote the first draft of the manuscript. K-HL, H-MJ, and W-HY collected the data and analyzed the data. All authors revised, edited, and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.