Interval training has been used by athletes and coaches for more than a century as a means to enhance performance. It refers to an intermittent style of exercise in which bouts of more intense effort are separated by periods of recovery within a single training session. The application of interval training for general fitness has also long been appreciated. In their 1974 book, Fox and Matthews (1) wrote, “Pioneered by track and swimming coaches, interval training is the supreme way to condition a person (including) the person who desires to condition himself for health purposes.” An explosion of scientific research over the last decade has advanced our understanding of the physiological adaptations to various types of interval training. This brief review attempts to place the latest findings in context for the fitness and health professional. The focus is on interval training for cardiometabolic and brain health, including application of the method in persons at risk for, or afflicted by, chronic diseases or conditions that affect function.
WHAT IS INTERVAL TRAINING?
A wide range of terms have been used by researchers and fitness professionals to describe various interval training protocols, which has led to a dizzying array of acronyms and general lack of standardization in the scientific literature and general vernacular. It is reasonable to postulate that interval training can be broadly categorized into two main types: aerobic-based and resistance-based (see Sidebar). Aerobic-based interval training generally involves exercises that engage a large mass of muscle (e.g., cycling, running, swimming), performed in a variable intensity manner, which promote the development of cardiorespiratory fitness (CRF). There are three main varieties: (i) sprint interval training (SIT) involves “all out,” “maximal,” or “supra-maximal” efforts in which the absolute workload or speed exceeds that which would elicit maximal oxygen uptake (V˙O2max); (ii) high-intensity interval training (HIIT) involves vigorous but submaximal efforts in which the relative intensity during the intervals elicits ≥80% of maximal heart rate; and (iii) less demanding types of intermittent exercise, typically characterized by alternating periods of light and moderate exercise (e.g., interval walking). Common examples of SIT and HIIT are Tabata-style intervals and fitness studio indoor cycling classes, respectively, whereas interval walking would constitute the third, less demanding form of aerobic-based interval training. Resistance interval training encompasses exercises primarily designed to promote muscular strength and hypertrophy, although it also can be an effective stimulus to promote CRF when sets are performed in a vigorous manner with short periods of recovery. It is typically characterized by bodyweight training exercises (e.g., push-ups, burpees, lunges), plyometrics, and movements that incorporate the use of equipment (e.g., kettlebells) and performed in a circuit-style manner. Relative intensity can vary from light calisthenics to maximal efforts with repetitions performed to muscular failure. Hybrid versions involve both aerobic- and resistance-based interval training combined within a single training session. Interval training can be distinguished from more traditional exercise characterized by moderate-intensity continuous training (MICT) for aerobic adaptations and heavy resistance training for strength/hypertrophy. Both aerobic- and resistance-based interval training have proven immensely popular with fitness enthusiasts, with a common version of each type (i.e., HIIT and body-weight training) ranking among the top fitness trends worldwide for the past several years in an annual survey conducted by the American College of Sports Medicine (2).
Sidebar: Interval Training Terminology
Aerobic-based interval training primarily entails exercises designed to promote cardiorespiratory fitness (e.g., cycling, running, swimming), performed in a variable intensity manner. The two most common variations are “sprint-interval training,” which involves “all out,” “maximal,” or “supra-maximal” efforts, and “high-intensity interval training,” in which efforts elicit at least 80% of maximal heart rate.
Resistance-based interval training encompasses exercises primarily designed to promote muscular strength, typically characterized by bodyweight training exercises (e.g., push-ups, burpees, lunges), plyometrics, and movements that incorporate the use of equipment (e.g., kettlebells, resistance bands) and performed in a circuit-style manner.
INTERVAL TRAINING AND CARDIOMETABOLIC HEALTH
CRF is an important indicator of overall health, with a recent position statement from the American Heart Association making the case to include it as a clinical vital sign (3). This is owing to the fact that increases in CRF equivalent to one or two metabolic equivalents (METs) are associated with considerably (10% to 30%) lower adverse cardiovascular event rates (3). Compared with traditional health markers measured in the doctor's office, the mortality risk reduction associated with a 1-MET higher CRF is comparable with 7-cm, 5-mmHg, and 1-mmol decreases in waist circumference, systolic blood pressure, and plasma glucose, respectively. The relevance of these numbers is highlighted by the fact that just a few weeks of interval training can enhance CRF by ≥1-MET in a time-efficient manner. For example, Phillips et al. (4) recently studied a practical, time-efficient cycling protocol that involved five, 1-minute efforts at an intensity of ~100% to 125% V˙O2max, performed over a ~15-minute period including warm-up, recovery periods, and cool-down. When performed 3 times per week for 6 weeks, the protocol boosted CRF by an average of ~10% (~1-MET) in previously sedentary overweight and obese adults at risk for type 2 diabetes. This finding is consistent with the results of a systematic review and meta-analyses that evaluated 65 intervention studies that involved an interval training intervention eliciting >90% of maximal heart rate. The authors concluded that HIIT may serve as a time-efficient substitute or as a compliment to more commonly recommended MICT for improving cardiometabolic health (5). This finding has been echoed in other systematic reviews and meta-analyses, which have concluded interval training elicits increases in CRF that are superior to traditional continuous exercise despite reduced total time commitment in both healthy individuals (6) and people with cardiometabolic diseases (7).
Interval training is also effective for improving other health indices including those indicative of blood sugar control. The relevant body of literature is smaller than for CRF, but a recent systematic review and meta-analysis concluded that HIIT induces cardiometabolic adaptations similar to those of MICT in prediabetes and type 2 diabetes (8). The effect of resistance compared with aerobic interval training is less well-studied, but a recent report described the effects of a CrossFit program that involved calisthenics, gymnastics, and weightlifting in addition to other exercises such as rowing (9). The sessions ranged from 8 to 20 minutes in duration including a warm-up and cool-down, and the main high-intensity phase elicited a heart rate of >85% of maximum. Six weeks of training, three times per week, improved insulin sensitivity and other indices of metabolic syndrome in overweight and obese adults with type 2 diabetes. The overall message for practitioners and their clients is that interval training can serve as a viable alternative to traditional exercise to enhance cardiometabolic health, and thus broadens the effective exercise options to choose from.
INTERVAL TRAINING AND THE BRAIN
Exercise is one of the most powerful lifestyle interventions to alter the adult brain, and emerging evidence points to interval training as an effective way to enhance various aspects of brain function including motor learning (10,11), cognition (12), and emotional regulation (13). Although the exact mechanisms through which interval exercise impacts brain function remain unclear, one obvious potential mechanism is increased blood flow to the brain. However, during exercise, the enhanced blood flow is primarily directed to skeletal muscle; there are only modest increases in blood flow to the brain, specific to the regions engaged by the exercise (i.e., motor cortex). However, these increases in brain blood flow plateau at moderate intensities of ~60% V˙O2max (14), suggesting that HIIT may provide no additional increases to blood flow over and above MICT.
A more promising mechanism to explain the effects of interval training on brain function is the upregulation of growth factors, such as brain-derived neurotrophic factor (BDNF). Compared with MICT, HIIT produces more BDNF, suggesting that it may be an advantageous mode of exercise for promoting brain function via this mechanism (15). BDNF is critical for the growth, functioning, and survival of brain cells targeting key brain regions including the hippocampus, frontal and motor cortices, and the striatum. In these areas, BDNF enhances cell-to-cell communication. BDNF also promotes growth of new brain cells in the hippocampus. Although this cannot be directly measured in the human brain, magnetic resonance imaging can be used to characterize changes in the structure of the hippocampus (16). Indeed, aerobic exercise has been shown to increase the size of the hippocampus in both young adults and older adults. It also may counteract the typical loss of hippocampal size that occurs with aging and Alzheimer’s disease.
Individuals can learn new movements or re-learn movements that are lost after neurological injury or disease. Motor learning plays an essential role in the rehabilitation of movement. When HIIT, performed on a stationary bike, is combined with motor skill training, the ability to learn a new motor skill is improved. In one study, healthy adults learned to perform precise wrist movements (i.e., match the position of a target cursor displayed on a computer screen) after which they performed a bout of HIIT immediately, 1 hour or 2 hours after the training (10). Only the group receiving HIIT immediately after motor learning demonstrated improved performance in the wrist movement skill when tested 7 days later. These findings are important because they suggest that motor learning is improved when combined with HIIT performed shortly after learning a new skill. Furthermore, motor learning improvements are most evident when interval training is performed at high intensity (11), suggesting that high-intensity exercise yields the greatest benefits to improve motor learning.
Two key brain regions impacted by interval exercise that directly support cognition are the hippocampus and the frontal cortex. The hippocampus is critical for learning and memory. HIIT improves memory function supported by the hippocampus, including high-interference memory (i.e., the ability to distinguish between highly similar events such as finding a familiar face in a crowd). Previously sedentary young adults who performed 6 weeks of HIIT showed a 10% improvement in high-interference memory compared with sedentary control group (12).
The frontal cortex of the brain governs executive functions related to self-regulation and focusing attention. Interval exercises improve executive functions in both the lab and academic settings. In the lab, the ability to selectively attend to a specific task and ignore distractions improves immediately after an acute bout of interval exercise (17) and in response to 2 weeks of SIT (18). In an academic setting, high-intensity exercise breaks improve selective attention in children (19) and young adults (20).
Interval exercises impact emotional responses in a comparable way with continuous protocols, but with the added benefit of being more time efficient and enjoyable (17,21). Furthermore, 6 weeks of HIIT was comparable with MICT at protecting previously sedentary university students against the development of depression; however, HIIT led to greater increases in perceived psychological stress (13), suggesting the associated physiological arousal induced by HIIT may exacerbate symptoms of psychological stress. This may be especially true for newly trained individuals. It follows that the mental health benefits of HIIT may be more effective for trained individuals whose systems have already adapted to the more strenuous exercise.
APPLICATION OF INTERVAL TRAINING TO CLINICAL POPULATIONS
HIIT may enhance rehabilitation strategies that aim to improve motor learning, executive functions, and memory in individuals with neurological impairments.
HIIT has recently been combined with motor skill learning in individuals with chronic stroke symptoms. In a study by Nepveu et al. (22), participants with chronic stroke experienced motor skill training followed by either HIIT performed on a recumbent stepper or no exercise. The group receiving HIIT experienced greater improvements in motor skill learning when tested 1 day later (22). These data indicate that HIIT provides an opportunity to improve motor learning abilities in neurologically injured populations.
HIIT also may benefit individuals living with Parkinson’s disease (PD). After 16 consecutive weeks of high-intensity intervals involving strength and aerobic training in individuals with moderate PD, symptoms improved and there were beneficial changes in muscle (less fatigable myofibers) (23). Furthermore, 16 weeks of HIIT in moderate PD increases brain activity with changes that relate to improvements in quality of life (24). These data support the use of HIIT to improve symptoms in PD. To date, no studies have examined whether HIIT enhances motor skill learning in PD. This avenue of research would have a significant impact on rehabilitation approaches; motor skill training can be made more effective by combining it with HIIT thereby improving the memory of the practiced skill. Combining HIIT with motor skill training may ultimately improve the ability to maintain long-term motor memory in PD and also may enhance motor learning in other neurological conditions.
Attention Deficit Hyperactivity Disorder
Individuals with attention deficit hyperactivity disorder (ADHD) are easily distracted and struggle to focus their attention. Given the benefits of interval exercises for improving executive functions, students with ADHD may benefit from incorporating exercise breaks in the classroom to help re-focus their attention, regulate their classroom behavior, and improve their overall academic performance. Indeed, preliminary evidence finds HIIT to be more effective than MICT at improving subjective ratings of attention in boys with ADHD (25), but more research is needed.
Alzheimer’s and Related Dementia
Alzheimer’s disease is a progressive neural degenerative disorder that damages the hippocampus and causes severe memory loss. Exercise reduces the risk of developing Alzheimer’s disease and may help slow the progression of the disease (26). However, most of this research used lower intensity exercise protocols. To our knowledge, interval exercises have not been used as a therapeutic approach in this population. Given that HIIT increases BDNF and improves hippocampal memory function, it follows that interval exercise may benefit Alzheimer’s patients but more research is needed.
Interval training is a time-efficient strategy to elicit physiological adaptations linked to improved health, especially gains in CRF. Interval training and brain function is an emerging area of research, but preliminary evidence suggests that brief, vigorous, intermittent exercise can enhance memory and attentional focus.
BRIDGING THE GAP
Interval exercise is an infinitely variable form of training that can boost cardiometabolic health in a time-efficient manner, including in people with chronic diseases. An emerging area of research is the impact of interval exercise on brain health and function. Although most of the evidence is preliminary — especially in clinical populations — interval exercise enhances growth factors like BDNF that support the function of brain cells. It can also improve the ability to learn a new motor skill and may augment rehabilitation therapy to improve neurological recovery. More research is needed to understand the benefits of interval exercise in neurological disease and injured populations and to determine whether delaying the progression of disease or improving the opportunity to recover after injury may be facilitated by interval training.
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