The beneficial effect of physical exercise or physical activity (PA) on the brain has been well established. Both acute and regular PA can boost a range of cognitive functions and enhance mood and mental health [1
]. For instance, a single bout of aerobic exercise such as treadmill running enhances working memory [6
], inhibitory control capacity [7
], attentional orienting [8
], creativity [9
], and positive moods [10
]. Regular PA conducted several times a week, such as running and popular sports, can promote cognitive development [11
], slow cognitive aging [13
], buffer stress response [14
], and prevent [15
] and treat [16
Furthermore, the beneficial effect of PA is generally considered dose-dependent, such that greater amount and higher intensity of exercise is associated with more enhanced outcomes, for instance, cognitive functions [17
]. However, a closer look at the literature suggests this dose-dependent effect in terms of both amount and intensity has only been formally tested in acute but not regular PA (see [17
] on cognitive functions; see [19
] on adult neurogenesis in animals, i.e., mice). In the case of regular PA, the literature has primarily focused on the amount (or frequency and duration) of exercise (see [20
] on cognitive functions, i.e., dementia; see [14
] on stress resilience; see [15
] on depression or general mental health), and only limited studies have assessed the influence of exercise intensity (see [23
] on depression).
Since PA performed in higher intensity causes more extensive, more powerful, and longer-lasting neurobiological changes, it may prove more beneficial to brain functions. Indeed, a single bout of vigorous- rather than low-intensity cycling has been reported to increase the peripheral level of brain-derived neurotrophic factor (BDNF) [25
]. BDNF is a member of the neurotrophin family of growth factors that support the production, growth, differentiation, and survival of neurons. Peripheral BDNF can pass the brain–blood barrier and benefits the brain through, for instance, enhancing neurogenesis in the dentate gyrus of the hippocampus [5
]. Similarly, an episode of moderate to vigorous running but not low-intensity walking increases circulating endocannabinoids [26
], which pass the brain–blood barrier, act as a neurotransmitter believed to contribute to “runner’s high”, and have analgesic and anti-anxiolytic effects ([26
]; see [27
] for a study conducted in mice). Lastly, PA at vigorous intensity is more effective at increasing aerobic capacity or fitness [28
], the latter being linked to higher cognitive functions [29
] and lower stress response [30
] (for a review, see [4
Therefore, we hypothesized that frequent PA at moderate and vigorous rather than low intensity exerts greater benefit to cognitive functions and mental health. In the present study, we set out to test this hypothesis in a sample of young adults using a battery of questionnaires and laboratory tests. Investigating this hypothesis also allows us to propose more specific recommendations or guidelines for public health promotion in young adults.
In the present study, we employed two different methods to evaluate PA, PA levels and intensity-specific frequency. Our results showed that compared with those at Low PA level, subjects at High PA level used active coping more often and behavioral disengagement less often, and demonstrated greater drive for rewards and more advanced personal growth. Most of these results, except active coping, remained significant after controlling gender and age. These results suggest that a greater amount of PA is associated with more matured psychological coping strategies in the face of negative situations [55
], enhanced appetitive motivation, which is often compromised in psychiatric disorders [57
], and superior psychological development and wellbeing [59
However, the categorization of PA levels based on the proposed criteria is not without its limitations. As we have introduced, High PA level is defined as meeting either of two criteria: (a) vigorous-intensity activity on >3 days/week and accumulating at least 1500 MET-minutes/week; or (b) >7 days of any combination of walking, moderate-intensity, or vigorous-intensity PA achieving at least 3000 MET-minutes/week. From the categorization criteria, we could infer that vigorous-intensity PA in particular may be responsible for the above benefits, although one cannot be sure if that is the case. Individuals may be categorized as High PA level simply due to their greater amount of moderate-intensity PA and walking, with few vigorous-intensity PA. In other words, we cannot tease apart the contribution of PA at different intensities. This limitation is further emphasized by our observation that the frequency of walking is negatively associated with that of vigorous-intensity PA. Therefore, the sensitivity of PA levels in capturing the true amount of PA may have been compromised and confounded by combining the measure of walking with that of vigorous-intensity PA. In order to investigate what intensity PA contributes greater benefits, it is necessary to employ intensity-specific measures.
For this purpose, in the present study, we employed intensity-specific frequencies, that is, days conducting one specific intensity of PA per week. After controlling gender and age, we found it was vigorous-intensity PA that brought various psychological benefits. More frequent vigorous-intensity PA was associated with more frequent use of active coping and fewer use of behavioral disengagement for coping with challenging situations. It was also associated with greater self-perceived autonomy and personal growth. Notably, in predicting personal growth, the contribution of the frequency of vigorous-intensity PA was much bigger than that of age (standardized coefficient 0.404 vs. 0.270). More frequent moderate-intensity PA was also associated with fewer use of behavioral disengagement. In contrast, the frequency of walking was not associated with any of the outcome variables we investigated.
These results confirm our hypothesis and are consistent with the neurobiological literature that moderate- to vigorous- rather than low-intensity PA causes extensive, powerful, and long-lasting physiological changes, which account for the cognitive and psychological enhancing effects of PA [25
]. Therefore, our results go beyond previous reports that regular PA enhances mental health [14
] by specifying that it is moderate- to vigorous-intensity PA, rather than low- to moderate-intensity walking, that is responsible for this benefit.
To give the general public specific recommendations on the frequency of PA, we combined the frequency of moderate and vigorous intensity PA and created a new measure, the frequency of MVPA. With MVPA, we observed significant associations with cognitive functions and state anxiety. Thus, individuals with more frequent MVPA performed better on a 2-back working memory task. They could better differentiate targets from non-targets. Meanwhile, they also demonstrated fewer symptoms of state anxiety. These results are in line with a large amount of reports that PA conducted regularly enhances cognitive functions and mental health [11
]. Rather than low- to moderate-intensity walking, our results indicate that it is MVPA that exerts these benefits.
We next compared subjects who conducted 1~2 days per week (lasting at least 10 minutes each time) of MVPA with those with no MVPA to investigate whether this minimum frequency of MVPA can bring any cognitive and/or mental health benefits. Compared with those with no MVPA, we found that subjects conducting 1~2 days of MVPA were more easily affected by happiness, more likely to use reappraisal for emotion regulation, more likely to use active coping, positive reframing, and religion, and less likely to use behavioral disengagement for dealing with challenging situations. In other words, mere 1~2 days of MVPA per week may bring people more positive emotions and more mature coping strategies [55
Note that subjects conducting 1~2 days of MVPA did not differ from those conducting no MVPA in terms of their total PA (MET-minutes/week) or proportion of different PA levels. This again supports the argument that rather than the total amount, more intense PA is preferred. In our study, on average, subjects conducting 1~2 days of MVPA did 0.5 days × 45.63 min/day = 22.82 min of vigorous- and 1.06 days × 66.25 min/day = 70.23 min of moderate-intensity PA per week. This amount of PA is below the level recommended by the World Health Organization for adults [61
] of doing at least 150 min of moderate-intensity, or 75 min of vigorous-intensity PA per week, or any equivalent combination of the two.
A limitation of our study is the employment of the short- instead of the full-form IPAQ to evaluate PA. Consequently, we could not make differentiation of various PA domains, that is, PA conducted during work versus during transportation, for housework, or for recreation. Previous research suggests that different domains of PA may have distinct effects on mental health [62
]. A second limitation of our study is our sample size is rather small and we conducted multiple tests for some forty dependent variables. Given the explorative nature of our study, we did not perform corrections of the alpha level to control the false discovery rate, and this may have increased the number of false positives. Future research with bigger sample size is required to validate our results by further correcting the alpha level based on established procedures.
A third limitation is the cross-sectional design of this study. The cross-sectional design does not allow us to make firm causal inferences on the associations we identified. It is possible that subjects with higher cognitive functions, more mature coping strategies, and better mental health may be more likely to engage in MVPA. Furthermore, our sample was primarily university students, which does not allow us to generalize our findings to older people or people from other settings. Future research should investigate whether our findings hold in a prospective context and other settings and whether interventions with as few as 1~2 days of MVPA cause meaningful cognitive and mental health changes.