Down syndrome (DS) is one of the most prevalent birth disorders in the United States that arises from a partial or full extra copy of chromosome 21. Individuals with DS experience a variety of cognitive and physical difficulties, including intellectual disabilities, learning and memory issues, defects of the heart and respiratory issues, and vision and hearing problems, among others. The presence of these difficulties could decrease engagement in physical activity, leading to a sedentary lifestyle.
Practical Insight 1: Down syndrome is a prevalent disorder in the United States, affecting 14 of every 10,000 births, with large implications on lifelong health.
People with DS are at a higher risk for developing chronic diseases such as metabolic syndrome, type 2 diabetes mellitus, Leukemia, cancer, and Alzheimer’s disease, all of which have been linked to physical inactivity. In addition, overweight/obesity rates approach 70% in this population (1). Obesity influences physical activity levels as well as having its own health implications. To address the negative health consequences of physical inactivity, the American College of Sports Medicine recommends all adults participate in moderate to vigorous exercise. Although most exercise programs in healthy populations use moderate to vigorous continuous physical activity, there has been increased interest toward incorporating high-intensity interval training (HIIT) into exercise programs (2). For individuals with DS, meeting the time recommendation of 30 minutes, 5 days a week could be challenging, particularly because of transportation needs and dependence on caregivers and/or family members (3). Moreover, issues related to attention span and personal enjoyment of exercise (4) among those with DS could lend themselves to the use of another mode of training other than traditional moderate to vigorous exercise. Therefore, the purpose of this article was to explore the potential benefits of incorporating HIIT as a training modality among individuals with DS.
Issues related to attention span and personal enjoyment of exercise in those with DS could lend themselves to the use of another mode of training other than traditional moderate to vigorous exercise. Therefore, the purpose of this article was to explore the potential benefits of incorporating HIIT as a training modality among individuals with DS.
Practical Insight 2: Reduced physical activity is a preventable risk factor in people with DS, and HITT training could be a possible training solution to reduce morbidity and mortality.
HIGH-INTENSITY INTERVAL TRAINING
HIIT is typically characterized by several high-intensity (work) and low-intensity (rest) bouts that allow individuals to complete similar volume of work, or even less, than a traditional exercise program in minimal time (5). Work and rest bouts can be created using recommended heart rate zones (6), or simply based on the individual’s perceived exertion (6). Although these protocols can be carried out with various modalities, the cycle ergometer seems to be most common (7).
Clinical interest in HIIT grew after the American Heart Association’s 2007 physical activity recommendations, in which vigorous activity became integrated as part of the recommendation. Since that time, HIIT has been promoted as a component of exercise programming and has demonstrated greater improvements in maximal aerobic capacity (V̇O2max) among adults in comparison with continuous submaximal aerobic activity. In addition, improvements of vascular function, insulin sensitivity, and oxidative stress have been observed after long-term HIIT training (8). Moreover, HIIT also has been shown to be as effective as moderate-intensity continuous aerobic training to reduce adiposity of obese individuals, making it a time-efficient mode of exercise (9). Specifically, running-based versions (vs. cycling-based versions) of HIIT have been shown to significantly reduce total, abdominal, and visceral fat mass, regardless of sex (10). Further, there is some evidence to support that higher-intensity interval exercise yields equal or greater enjoyment compared with continuous moderate-intensity exercise (11).
The effects of HIIT are dependent on the duration of the training period. Acutely, HIIT has yielded higher values of postexercise energy expenditure (EE) as well as excess postexercise oxygen consumption, two items that are helpful in determining energy cost of exercise (5,12). Short-term HIIT interventions, defined as less than 12 weeks, have been shown to improve V̇O2max, diastolic blood pressure, and fasting glucose levels in overweight or obese populations (13). Similarly, long-term HIIT interventions of more than 12 weeks have shown improvements in V̇O2max and blood pressure, as well as anthropometric measurements such as waist circumference and body fat percentage in overweight or obese populations (13). Although the effects of HIIT on EE are immediate, the delayed time course of major body composition changes in overweight or obese individuals may become discouraging for some individuals.
DS AND HIIT
Considering the high prevalence of overweight and obesity among individuals with DS (1), the management of body composition is key to maintenance of health. Because EE is typically a focus when managing weight, HIIT could provide a time-efficient way of achieving greater EE in people with DS, when compared with continuous moderate exercise.
Mixed results have been reported regarding body fat measurement after interval training. When measured with bioelectrical impedance spectroscopy, body fat percentage has been demonstrated to decrease after interval training, with subsequent increases in fat-free mass percentage (14). By contrast, Boer and Moss (15) observed no changes in the anthropometric measures of individuals with DS after 12 weeks of interval training, where the higher-intensity interval was described as “all out” (see Table 1). In this study, individuals who performed interval training experienced significantly more weight loss than individuals who performed continuous aerobic training; however, body fat percentage, body fat mass, and waist circumference were not different between groups (15). It is important to note, however, that a combination of increased fat-free mass and decreased body fat percentage might not produce changes in body weight; rather, the combination of these changes indicates a shift in body composition toward a healthier paradigm.
Low levels of aerobic fitness are typically reported in the DS population, which increase the likelihood of developing certain chronic diseases that will affect the long-term quality of life of the individual. Although physical inactivity and deconditioning are contributing factors to lower levels of aerobic fitness, there are other DS-specific factors that could affect an individual’s aerobic fitness. For example, chronotropic incompetence, or a reduced heart rate response to exercise caused by autonomic dysfunction, is commonly seen in people with DS (16). With maximal heart rates that are ~25 to 30 beats lower than adults without DS, people with DS can have reduced work capacity. Moreover, congenital heart disease has been linked to DS, with a higher incidence among female than male children. Although these conditions are common in people with DS, it could be hypothesized that reduced heart rate response to exercise might also be related to reduced effort. Finally, individuals with DS have evidence of mitochondrial dysfunction (17), which can contribute in some of the recognizable aspects of the DS phenotype (low activity levels, poor muscle tone, etc.).
Low levels of aerobic fitness are typically reported in the DS population, which increase the likelihood of developing certain chronic diseases that will affect the long-term quality of life of the individual. While physical inactivity and deconditioning are contributing factors to lower levels of aerobic fitness, there are other DS-specific factors that could affect an individual’s aerobic fitness.
There is evidence to support the use of HIIT for the purpose of increasing aerobic capacity of people with DS. Boer and Moss (15) found significant improvements in V̇O2max after a 12-week “all out” interval training protocol (Table 1), whereas Mendonca and Pereira (14) reported improvement in peak aerobic capacity of over 25%. In addition, improvements in heart rate recovery after a maximal treadmill test after HIIT training have been observed (18). These improvements indicate the potential role of HIIT to improve cardiac autonomic function and vagal activation (18). Moreover, HIIT also has demonstrated greater increases in mitochondrial content biomarkers compared with moderate-intensity continuous training in healthy adults. Although the authors were unable to find evidence of this in people with DS, it does prompt thought about the potential effects of the use of HIIT for people with DS.
Endurance-related factors also can improve in people with DS. More specifically, an increase in time to exhaustion during a V̇O2peak test has been observed in people with DS after HIIT training (15). Further, evidence suggests that peak ventilatory response and submaximal respiratory exchange ratio (RER) also improve after interval training (14). It is possible that the increased aerobic capacity resulted from the high-intensity exercise used in the interval training and combined with the longer exercise intervention period allowed changes in substrate utilization (as indicated by a lower RER) to occur. This finding has ties to the effects of HIIT on body fat, as utilization of fat, instead of carbohydrate, is indicated by a lower RER, which could be part of the mechanism behind the body fat loss demonstrated by people with DS after HIIT training.
Individuals with DS have unique neural pathways as a result of their phenotype (19). As part of this unique phenotype, those with DS have a decrease in attention span and executive function as they age. Executive function is a broad term that envelops regulatory aspects of behavior, including attention, inhibition, and processing speed. Evidence suggests that high-intensity training improves certain aspects of executive function in healthy adults. Inhibitory control has been shown to improve after acute continuous bouts of high-intensity exercise. Although this is important to note, this area of research does have conflicting findings. In individuals with DS, acute bouts of high-intensity exercise have been shown to reduce acute reaction time, another area of executive function. Therefore, it could be hypothesized that this is a result of task-specific fatigue, in which the high-intensity exercise caused fatigue that made the reaction time task more difficult. However, because the specific protocol tested was not interval based (20 minutes of high-intensity exercise), these results can only demonstrate a related effect of HIIT, not an exact effect. It is worth noting that the effects of moderate-intensity acute exercise on executive function are positive. Thus, based on the time commitment, it could be concluded that higher-intensity exercise may be a more favorable option for obtaining the most benefits — both cognitive and cardiovascular — from exercise.
Enjoyment of physical activity is an important factor in participation and choice of physical activity. Enjoyment can be divided qualitatively into areas of interaction, achievement, and process, as defined in a qualitative study of individuals with DS (4). In particular, process can be represented as “enjoyment from performing a particular activity itself” (4). Obtaining information on process can be achieved by asking participants about the activity at hand. Boer and Moss (15) reported that interval training was defined by participants with DS to be “fun and safe” (15). In addition, data related to compliance (defined as training sessions attended) showed that the interval and continuous training interventions had similar compliance (95% and 96%, respectively, n = 13 per group) (15), suggesting that HIIT is a suitable alternative to more traditional continuous training protocols for individuals with DS.
EXERCISE PROGRAMMING FOR DS
Currently, aerobic exercise goals for individuals with DS are similar to those for individuals without a DS diagnosis. Based on the most current recommendations of the American College of Sports Medicine, adults should seek to complete 20 to 30 minutes of moderate (40% to 59% of heart rate reserve [HRR] or V̇O2 reserve [V̇O2R]) or vigorous-intensity (60% to 89% HRR or V̇O2R) aerobic exercise on 3 to 5 days per week in order to achieve aerobic improvements (6). The exercise design for individuals with DS is slightly modified, calling for 3 to 7 days per week of exercise for maximum EE, with 3 to 4 days of that being moderate to vigorous-intensity exercise. Although the reasons for the slight variation in exercise prescription are not clear, there is evidence to suggest the standard recommendation is adequate. Nonetheless, limited data are currently available to variations within the exercise prescription for people with DS. Further research should consider these variations (e.g., frequency, intensity, time, and mode) in the exercise prescription components for people with DS, not only to advance this area of study but also to better serve the DS population.
Resistance training has been shown to be beneficial for people with DS. Although typically performed separately, it also has been used in combination with aerobic and agility training programs (20). Combined training programs have yielded improvements in functional tasks, functional balance, and strength in people with DS. Moreover, decreases in systemic inflammation also have been linked to resistance style training (22). This suggests that popular new programs such as Tabata, circuit training, and the like have the ability to improve function in people with DS. The ability of people with DS to perform functional tasks, such as stair climbing, has been shown to benefit from resistance training. In addition, participation in this type of activity can benefit people with DS as a form of social interaction or partnership in the exercise environment. Although the combination of training with both resistance and aerobic exercise has been shown to be effective, there is room for further research to expand these modalities in people with DS. HIIT with both aerobic and resistance training could be a new and exciting way to incorporate HIIT for people with DS.
Practical Insight 3: Current exercise guidelines only slightly vary between adults with and without DS.
The existing recommendations for exercise for people with DS remain general and somewhat conservative. However, there are emerging data to suggest that incorporating HIIT as part of an exercise program for individuals with DS could be beneficial to achieving critical health outcomes in a time-efficient manner. In addition to physiological and anthropometric improvements, the potential attention deficit experience by individuals with DS might make HIIT more effective and enjoyable compared with more traditional modes of exercise. Specific health issues related to DS, such as cardiovascular abnormalities and joint laxity, need to be addressed to ensure safety. Further, for individuals with DS who have greater balance deficits, HIIT may be safer in larger areas of the gym, where unobstructed exercises may be performed. It is important to note that incorporating HIIT protocols should be based on the individual’s motor abilities and their willingness to participate. Table 2 includes a list of suggestions to follow when incorporating HIIT in the exercise prescription of individuals with DS. Because HIIT can be performed with a variety of equipment, such as a rower, elliptical machine, and others, HIIT workouts can be customized to the desire of the participant, ensuring the participant is well-versed with the equipment. The severity of intellectual disability, health issues, and physiological barriers to exercise may differ from person to person, creating a unique situation for each individual. Therefore, it is important that the exercise program is tailored to meet the specific needs and wants of the individual while also achieving improvements in health outcomes.
Emerging data suggest that incorporating HIIT as part of an exercise program could be beneficial to achieving critical health outcomes in a time-efficient manner for those with DS.
BRIDGING THE GAP
HIIT can be an enjoyable and beneficial mode of exercise in people with Down syndrome. HIIT’s time efficiency makes it a good candidate for exercise prescription in this population because of limited attention span. Aerobically, HIIT has been shown to provide benefit to this population group by increasing capacity and endurance-related factors. HIIT also has been shown to provide cognitive and anthropometric benefits to people with Down syndrome.
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