Globally, one in five people suffer from a mental illness each year, and mood disorders such as depression and anxiety make up the majority of those cases (1). Major depressive disorder is associated with persistent feelings of sadness and a lack of motivation (2). Anxiety disorders are characterized by excessive fear or worry that may develop into overwhelming and unrealistic feelings that interfere with daily life (2). On top of that, individuals with depression and anxiety also suffer from cognitive impairments that include the inability to think decisively or concentrate (2). Physical activity and exercise are known to improve depression and anxiety, and high-intensity interval training (HIIT) is a popular yet unproven mode of physical activity for those struggling with mental illness.
The latest evidence suggests that HIIT may be a useful tool for improving mental health and cognition, but the vigorous bouts may need to be adjusted depending on the individual’s mental status. A key factor to consider when prescribing HIIT for mental health is an individual’s “total stress burden,” which must take into account not only the physical stressors associated with the exercise, but also the psychological stressors associated with their mental illness. Although the term stress has negative connotations, moderate exposure to acute stress is actually favorable and necessary for inducing adaptive change; this is defined as allostasis (3). When the body is pushed outside of its physiological or psychological “comfort zone”, it responds by making adaptive changes that in turn make individuals better equipped to handle future stress (3). However, too much stress — be it physical, psychological, or the combination of the two — causes allostatic load, which weakens the body rather than strengthening it (3). Allostatic load is also a robust predictor of depression and anxiety (4). Therefore, it is critical for health and fitness practitioners to understand how to best train or prescribe HIIT to alleviate psychological and cognitive impairments rather than exacerbate them.
This review synthesizes the current literature on the effects of HIIT for depression, anxiety, and their associated cognitive deficits and provides recommendations for practitioners to adapt HIIT protocols for mental health.
HIIT FOR DEPRESSION AND ANXIETY
The research examining HIIT for mental health subdivides people into two groups: clinical and subclinical. The term clinical is used to describe people with a formal diagnosis, including symptoms that are prevalent nearly every day for at least two consecutive weeks. The term subclinical is used to describe people without a formal diagnosis who are currently experiencing symptoms of depression and anxiety that are not as severe or persistent enough to meet the clinical criteria. Depression and anxiety can affect anyone, even those without a formal diagnosis. In fact, just 6 weeks of chronic stress can induce symptoms of depression and anxiety in people who had never had a mental illness before (5). The next section discusses the benefits of HIIT for clinical and subclinical populations.
HIIT is a promising tool for reducing symptoms of clinical depression and anxiety. In clinically depressed individuals, HIIT reduces depression severity (6). Consistent benefits have been reported for a HIIT protocol that comprises 25 intervals of 30 seconds of work at 80% V˙O2max, followed by 30 seconds of rest, done three times per week for 4 weeks (6). No adverse events have been reported using this protocol. The benefit of HIIT for reducing depressive symptoms is comparable with moderate-intensity continuous training (MICT), which is typically done at an intensity of 50% to 70% HRmax continuously for 30 to 60 minutes. However, HIIT may be more enjoyable over the long term (7) and seems to result in less dropout than MICT, which is particularly important for clinically depressed populations who struggle to adhere to routine exercise programs (6). Training in a group setting that is fun and supportive, may help overcome motivational challenges faced by people with severe mental illness and, therefore, may promote better adherence (6).
HIIT is a promising tool for reducing symptoms of clinical depression and anxiety.
HIIT also is a promising strategy for reducing clinical anxiety. State anxiety refers to the transitory psychological and physiological experience of anxiety, and is reduced by an acute bout of HIIT in clinical populations (8). One study compared HIIT to a lower-intensity stretching protocol and found that HIIT was nearly twice as effective at reducing anxiety symptoms in patients with generalized anxiety disorder after just 12 days of training (9). However, when using HIIT to manage anxiety, it may be important to extend the rest period between high-intensity sets (and workouts) to avoid exacerbating anxiety symptoms, especially because somatic symptoms of anxiety resemble the physical sensations of intense exercise, such as labored breathing and increased heart rate.
People without a clinical diagnosis may feel depressed or anxious from time to time. For example, symptoms of depression can occur in otherwise healthy populations during periods of seasonal change (10). As with clinical depression, HIIT is as effective as MICT at reducing depressive symptoms in subclinical populations (11). However, when considering HIIT for subclinical anxiety, the results are equivocal. HIIT has been reported to have no effect (12), to reduce anxiety (11), and to even worsen it (5). For example, in one study, researchers found protective effects of MICT but not of HIIT on stress-induced anxiety (5). In that same study, the investigators also measured proinflammatory cytokines, which are typically elevated under conditions of chronic stress. Proinflammatory cytokines were reduced by MICT but were elevated by HIIT (5). These results suggest that performing HIIT three times per week could increase the total stress burden, resulting in elevated inflammation and allostatic load.
Furthermore, people with anxiety sensitivity (i.e., those who become more anxious when they experience anxiety symptoms) may be HIIT averse because they fear the increased heart rate and labored breathing from vigorous exercise. Their fear can be so intense that exercising vigorously induces panic, and therefore, it is usually avoided (13). A person can have anxiety sensitivity without having an anxiety disorder; however, it does put them at higher risk of developing an anxiety disorder (14). People with anxiety sensitivity would benefit from HIIT because each intense bout acts as a form of exposure therapy in that it enables the individual to experience the physical symptoms that are feared (e.g., racing heart, labored breathing) but in a safe and self-controlled manner (15). Given their aversion for intense exercise (13), a gradual introduction of HIIT into their regiment is suggested.
People with anxiety sensitivity (i.e., who become more anxious when they experience anxiety symptoms) may be HIIT averse because they fear the increased heart rate and labored breathing from vigorous exercise.
HIIT FOR EXECUTIVE FUNCTIONS
Anxiety and depression are more than just mood disorders. They affect cognition too. Specifically, anxiety and depression are associated with impairments in executive functions, including working memory, mental flexibility, and inhibitory control (16). Working memory allows the brain to briefly retain new information to be used in the short term, allowing a person to keep a mental tab on relevant information while performing a task (e.g., remembering a phone number) (16). Mental flexibility is the ability to effectively switch between thinking about two different mental concepts, allowing the person to adapt and shift their thinking when faced with alternative solutions (16). Inhibitory control is the ability to inhibit impulses and ignore irrelevant information, which is especially important for self-regulation needed to maintain a healthy lifestyle that includes regular physical activity (16).
An acute bout of HIIT improves the executive functions that are impaired with depression and anxiety (17,18). The immediate benefits of HIIT for executive functions stem from increases in oxygenated blood flow to the prefrontal cortex in the brain that can enhance executive functions for up to 2 hours after exercising (19). The long-term benefits of HIIT for executive functions need further investigation.
An acute bout of HIIT improves executive functions and may help overcome deficits in executive functions, especially self-regulation and impulse control, that are common among people suffering from depression and anxiety but needed to stick with an exercise program over the long term.
With respect to the different executive functions, the acute effects of HIIT are strongest for inhibitory control (i.e., ignoring distractions) and have been reported for children, adolescence, and young adults (18). Although there is little evidence examining the acute effects of HIIT for executive functions in older adults, there is some evidence for its chronic benefits (18).
In comparison with MICT protocols, the benefits of HIIT for executive functions are similar (18); however, HIIT may be more enjoyable (7) and result in higher retention (6) compared with MICT.
Overall, an acute bout of HIIT improves executive functions and may help overcome deficits in executive functions, especially self-regulation and impulse control, that are common among people suffering from depression and anxiety but needed to stick with an exercise program over the long term.
HIIT BEST PRACTICES FOR MENTAL HEALTH AND COGNITION
- Work with individuals exhibiting depressive symptoms in a group environment to avoid dropout. Dropout rates for exercise programs tend to be high for individuals struggling with depression. Although HIIT reduces dropout rates (6), research suggests that individuals with severe mental illness would benefit from a group setting that promotes social cohesion and accountability (6).
- Use sufficient rest and recovery between intense sessions for anxious clients to avoid exacerbating symptoms. Space out HIIT sessions by at least 48 hours to ensure full cardiac autonomic recovery (20).
- For beginner exercisers with mental health issues, start with HIIT protocols with lower repetitions and gradually build intensity over time. To avoid triggering the unwanted effects of anxiety sensitivity, it is recommended to prescribe a lower-intensity exercise program for beginner exercisers. Although individuals experiencing depressive symptoms may not be at risk of anxiety sensitivity, many people with depression also suffer from anxiety (3), and therefore, HIIT should be incorporated gradually when first prescribing exercise for clients with mental health issues (clinical or subclinical) until it is clear how much intense exercise the client can tolerate.
- For executive functions, the effects of HIIT are strongest for inhibitory control (i.e., ignoring distractions). The effects are significant across the lifespan and could help to improve the client’s self-regulation and impulse control needed to maintain a healthy lifestyle that includes regular physical activity (Table).
Example HIIT Protocols for Mental Health
|Running or walking
|Running or walking
below 77% HRmax
|Body weight circuit (e.g., jumping jacks, shuttle runs, jump rope, step ups)
|30 sec walk
Repetitions denote the number of times the work period followed by the rest period were repeated.
BRIDGING THE GAP
The evidence for the benefits of HIIT on mental health and cognition is promising. HIIT reduces depression and can help manage symptoms of anxiety; however, nonclinical populations who suffer from anxiety sensitivity may respond better to low- to moderate-intensity exercise protocols. The bottom line: when it comes to exercising for mental health, some is better than none. Even short bursts of vigorous-intensity exercise offer the promise to soothe a depressed and anxious mind and improve concentration.
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