1 INTRODUCTION

High-intensity interval training (HIIT), which consists of multiple short and intense exercise bouts interspersed by recovery intervals, is a time-efficient form of health-enhancing exercise¹ that allows strategic variations for potentially maximizing exercisers' enjoyability.^{2, 3} Research shows that HIIT has short-term and long-term positive effects on executive function (EF),⁴ a set of higher-level cognitive processes that encompass inhibitory control, working memory, cognitive flexibility, and the optimization of goal-directed behaviors in adapting to changing environments.⁵ The EF gain during the recovery period following HIIT has been linked with temporal changes in arousal and its related neural mechansims (e.g., locus-coruleous norepinephrine system⁶). Such an acute effect can be observed thorughout the life span,⁷ including young adults who are at their peak of cognitive functioning,^8-10 indicating HIIT as an effective strategy to enhance cognitive performance¹¹ and learning¹² while increasing physical activity and reducing sedentary time. However, the potential of HIIT for enhancing EF may not have been fully realized because most HIIT-based programs only focused on the intense exercise bouts, ignoring the great potential lost in the use of recovery intervals, which are typically passive and do not engage cognitive effort. This knowledge gap has hindered the understanding and the maximization of HIIT-induced EF gains, despite evidence indicating additional cognitive benefits from exercise when combined with cognitively engaging activity.¹³

Mindfulness is a cognitive activity that can be incorporated into the recovery intervals of HIIT. Mindfulness is a state of mind that enables an individual to be aware of the present in a nonjudgmental manner by regulating attention.¹⁴ Chronic mindfulness practices have been associated with superior arousal regulation and enhanced cognitive function.^{15, 16} Systematic reviews have provided evidence that weeks to months of regular engagement in mindfulness can improve attention and EF performance,¹⁵ presumably via modifying the underlying brain structures (e.g., anterior cingulate cortex) and network connectivity (e.g., default mode network).¹⁷ Recent meta-analyses have extended the EF-enhancing effect of mindfulness to the acute level,¹⁸ as intentionally achieving a state of mindfulness through performing acute bouts of attention- and breath-focused mindfulness practice can improve EF performance.^{19, 20}

Although both HIIT and mindfulness have similar acute beneficial effects on EF,^{4, 19-21} the functional brain activation corresponding with these cognition-enhancing effects appear to be differential, as HIIT primarily influences the neural marker of processing speed (i.e., P3 of the event-related potential [ERP])⁶ while mindfulness affects neural activation (i.e., N2-ERP) related to conflict-related processing.²⁰ Because the ability to resolve conflicting information with the presence of distractions and to quickly process behaviorally relevant information are important for the successful and efficient implementation of EF, interventions combining HIIT with mindfulness may enhance both processing speed^{6, 22} and conflict detection^{20, 23} during stimulus evaluation, maximizing the intervention effects on EF performance.

To date, only one study has investigated the feasibility and the speculated efficacy of such an integrated mind–body approach—mindful HIIT—for improving cognition. Through incorporating mindfulness-based breathing exercises and body scans during the recovery intervals of HIIT,²⁴ findings from this study showed enhanced attention performance following mindful HIIT, though the attentional gain was less comprehensive compared with traditional HIIT without involving mindfulness. Specifically, decreases in all error types were observed following the traditional HIIT while the mindful HIIT condition only reduced the commission errors.²⁴ Interestingly, such attenuated attentional benefit following mindful HIIT may be attributable to individual differences in dispositional mindfulness,²⁴ as smaller attentional gains were found in individuals who scored lower on the non-reactivity facet of dispositional mindfulness. In line with the theory of depletion of self-control resources²⁵ in the context of acute exercise,²⁶ individuals possessing the nonreactivity trait may experience less depleted cognitive resources by disengaging the emotionally eliciting responses (i.e., discomfort and exhaustion) associated with physical exertion. As a result, more cognitive resources become available for implementing the mindfulness activities and speculatively achieving a higher mindfulness state during recovery, thus supporting the subsequent attention performance.^{24, 25} However, findings from this study were limited as it lacked the quantification of mindfulness induction, making it difficult to determine whether and to what extent the mindfulness state was achieved, whether the mindfulness state was related to individual differences in dispositional mindfulness and was different between the mindful and traditional HIIT, and whether the manipulation of mindfulness state contributed to cognitive changes following mindful HIIT.²⁴

Accordingly, this study aimed to utilize a novel mindful HIIT protocol to induce mindfulness to determine the effect of mindfulness induction on subsequent EF by comparing EF performance following a mindful HIIT condition versus a non-mindful HIIT condition. It was hypothesized that performing mindfulness-inducing activities incorporated into the recovery intervals of HIIT would lead to a higher level of mindfulness state and that the magnitude of mindfulness state would be correlated with dispositional mindfulness. Given the potential complementary effects of HIIT and mindfulness on arousal regulation and neurocognitive function,^{6, 15, 16, 19, 20, 22} exercise-related changes in EF performance were hypothesized to be more positive following the acute bout of mindful HIIT compared with the HIIT that involves less of a mindful state during its recovery intervals.

2 METHODS

3 RESULTS

3.1 Psychophysiological response outcomes

As shown in Table 1, higher SMSPA scores were reported following the Mindful than Non-mindful condition while no difference in other measures was observed. FFMQ scores were positively associated with SMSPA scores for the Mindful condition (r = 0.38, p = 0.015) but not for the Non-mindful condition (r = 0.18, p = 0.268) (Figure 1A).

TABLE 1. Mean and standard deviation of psychophysiological responses to exercise by condition and statistics of between-condition differences in these psychophysiological responses.

Measure	Mindful	Non-mindful	Paired t-test
RPE	12.2 ± 2.1	12.2 ± 2.4	t = 0.1, p = 0.458, dz = 0.02
HR%ExerciseInterval	81.7 ± 2.1	81.4 ± 2.2	t = 1.2, p = 0.246, dz = 0.19
HR%RecoveryInterval	59.5 ± 6.3	59.2 ± 6.1	t = 0.5, p = 0.600, dz = 0.08
SMSPA	2.7 ± 0.5	2.4 ± 0.6	t = 3.1, p = 0.004, dz = 0.49
PACES	93.5 ± 18.0	91.9 ± 18.6	t = 0.8, p = 0.448, dz = 0.12

4 DISCUSSION

This study comparing two HIIT protocols with and without mindfulness-inducing recovery intervals showed that the Mindful condition induced a greater level of state mindfulness than the Non-mindful condition. Further, dispositional mindfulness was positively correlated with the level of mindfulness state following the Mindful condition but not the Non-mindful condition. Despite the successful manipulation of mindfulness state, time-related changes in performance during the three cognitive tasks did not differ following the Mindful versus Non-mindful conditions, except for the time-related increases in performance accuracy during the cognitive flexibility task selectively following the Non-mindful condition. Contrary to the current hypothesis, these findings suggest that an increased mindfulness state did not correspond with additional modulation in inhibitory control and working memory and may have attenuated HIIT-related positive changes in task performance requiring cognitive flexibility.

The successful manipulation of the mindful state was confirmed by a higher score on SMSPA following the Mindful condition than the Non-mindful condition, indicating the modifiability of state mindfulness and the feasibility of utilizing the recovery intervals to provide additional mindfulness experiences. Importantly, this mindfulness-inducing effect was achieved without compromising the subjective and objective physical exertion and self-reported enjoyment. Though postexercise increases in mindfulness induction were reported in a previous study directing exercisers' attention to somatic sensations without judging them as positive or negative when engaged in aerobic walking,⁴² the current study was the first to replicate this similar effect by strategically delivering mindfulness during the recovery intervals of HIIT. Further, dispositional mindfulness was positively correlated with state-mindfulness after the Mindful condition but not the Non-mindful condition in which the manipulation of mindfulness was absent.³⁵ These novel findings suggest that although the Mindful HIIT protocol accumulates both physical activity and mindfulness experiences, the level of mindfulness state resulting from this intervention may vary as the function of individual differences in the ability to be mindful.

Similar to previously reported acute benefits of HIIT to EF,^{4, 8, 9, 27, 28} performance during the flanker, switch-flanker, and n-back tasks improved from the pretest to posttest for both the Non-mindful and Mindful conditions. Such general time-related improvements in performance across EF domains and intervention conditions were consistent with cognitive enhancements commonly observed during the 1-h recovery period following exercise,^{24, 43} given that all cognitive tasks were completed at about 30 min after exercise ceased. These exercise-induced positive changes in EF outcomes may possibly result from the elevated arousal and associated increases in locus coeruleus norepinephrine activation, cerebral blow flow, circulating brain-derived neurotrophic factor (BDNF), and cerebral lactate metabolism.⁴³

The exception of the general time-related EF change was the increase in ACC during the switch-flanker task for the Non-mindful condition but not the Mindful condition. This finding was similar to prior research showing that HIIT-induced enhancements in attention performance was attenuated when additional mindfulness-inducing instructions were used during the recovery intervals.²⁴ Novel to the current study was the extension of this effect to performance outcomes during tasks requiring cognitive flexibility, suggesting that the positive change in this EF domain following HIIT may be hindered by incorporating mindfulness induction during the recovery intervals.

One explanation of this finding was that the mental state achieved during the Mindful condition may be less optimal for the subsequent EF performance. It is possible that providing explicit instructions to be mindful in the Mindful condition induced higher levels of state mindfulness at a cost of cognitive fatigue. That is, the novelty of the added mindfulness-inducing instructions to HIIT could contribute to a level of cognitive fatigue that was not favorable for subsequent cognitive performance. Indeed, research found impaired cognitive performance following aerobic exercise while performing cognitively demanding tasks compared with aerobic exercise alone.⁴⁴ Interestingly, a state of heightened attention could benefit ideational flexibility and related underlying brain functional connectivity⁴⁵ when combined with properly designed sensory-motor activities (e.g., Quadrato Motor Training). It is possible that the physical demand of the paced walking during the recovery intervals was too intense to be harmonized with the mindful-inducing activities, leading to higher levels of cognitive fatigue. Further, given that the switch-flanker task required cognitive processes not only involving the shift between different stimulus–response mappings, but also inhibition to perceptual interference, performance during this complex task may be particularly susceptible to cognitive fatigue.

The other potential explanation was the additional beneficial effect of instructions associated with the Non-mindful condition on cognitive flexibility. Although instructions used in the Non-mindful condition were designed to minimize mindfulness through directing participants' awareness away from the current experience and attentional focus toward judgmental thoughts, this approach could have engaged participants in reflective, self-assessing, evaluative processes that better prepared them to perform the subsequent EF tasks. This speculation was indirectly supported by research showing the positive associations of cognitive reflection with cognitive abilities⁴⁶ and processes involving inhibition, shifting, and associated prefrontal activation.⁴⁷ Whether the variation of instructions provided during the recovery intervals of HIIT could generate differential effects on subsequent cognition is a research question that has practical significance and requires future research to explore.

This study has several limitations. First, because a true control condition without HIIT and mindfulness induction was not included, it is impossible to eliminate the possibility that the observed time-related changes in EF outcomes may have resulted from a learning effect. Second, the Non-mindful HIIT condition was used as an active control condition to be contrasted with the Mindful condition in order to isolate the EF changes resulting from the manipulation of the mindfulness state. Because both conditions provided instructions during the recovery intervals, the between-condition EF differences observed in this study did not provide direct evidence that incorporating mindfulness-based recovery intervals into HIIT can generate differential cognitive effects compared with traditional HIIT protocols adopted in the existing literature. Lastly, the selective between-condition difference in time-related changes in ACC during the cognitive flexibility task could result from the fixed sequence of task administration. Taken together, future research is needed to include both passive (i.e., non-mindful sedentary) and active control conditions (i.e., HIIT-only, mindfulness-only) control to further isolate the unique contribution of HIIT and mindfulness to a specific EF domain.

5 CONCLUSION

The novel mindful HIIT intervention successfully resulted in a higher mindful state, but similar levels of exercise-related physical exertion and enjoyment, compared to HIIT without mindfulness-based recovery intervals. The observed increases in mindfulness induction may have an attenuating effect selectively on performance during subsequent tasks requiring cognitive flexibility but not tasks primarily involving inhibitory control and working memory. Despite the evidence that incorporating mindfulness into the recovery intervals of HIIT did not lead to additional positive changes in EF, these findings suggest the feasibility and time efficiency of the integrated mindful HIIT approach for increasing physical activity while providing opportunities for mindfulness experiences.

6 PERSPECTIVE

High-intensity interval training (HIIT) has become increasingly popular and is recognized as effective for improving various health outcomes and feasible for various populations and settings. Although HIIT is not necessarily the most preferred exercise type due to its high-intensity nature, there has been an ongoing effort in the field of sport and exercise science to systematically evaluate the use of HIIT for health promotion and to utilize data-driven and theory-based approaches to refine the existing HIIT protocols for optimal feasibility and efficacy. In line with such an effort, the current study took the first step to establish an innovative—yet feasible and effective—HIIT paradigm that delivers potentially health-enhancing mindfulness experiences. Through providing the first evidence that additional mindfulness induction following mindful HIIT can generate similar positive changes in inhibitory control and working memory but a smaller degree of such changes on cognitive flexibility compared with non-mindful HIIT, the current study opens a series of new research questions including, but not limited to, identifying strategies that maximize, individual difference factors that moderate, and neural mechanisms that cause the cognitive benefits of mindful HIIT as well as the long-term effects of engagements in mindful HIIT on cognition.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.