Clinical Context

It is well-proven that cardiorespiratory fitness and physical activity can improve cardiometabolic risk factors, but what is the trend in these values across early adulthood? A previous study Camhi and colleagues addressed this issue, and their results were published in the June 2013 issue of Metabolic Syndrome and Related Disorders.^[1]

A total of 4161 adults between ages 18 and 30 years reported their levels of physical activity over the course of 20 years in this study, and they underwent a graded exercise treadmill test to assess cardiorespiratory fitness at years 7 and 20. Not surprisingly, participants in the top 10% of cardiometabolic risk factors had lower rates of physical activity compared with their peers among Black men, White women, and White men. More surprisingly, physical activity was unrelated to cardiometabolic risk among Black women.

The other main finding from this study is that physical activity declined with age, regardless of sex or race. Establishing healthy physical activity habits and maintaining them are critically important public health goals that members of the health care team can collaborate to improve cardiorespiratory fitness and health among adolescents. The current study evaluates how the intensity of physical activity relates to cardiorespiratory fitness among adolescents.

Study Synopsis and Perspective

Vigorous physical activity for 20 minutes a day was enough to maximize cardiorespiratory benefits in adolescents, based on data from more than 300 individuals.

Current recommendations for physical activity in children and adolescents from the World Health Organization call for moderate to vigorous physical activity (MVPA) for an average of 60 minutes a day for physical and mental health; however, guidance on how much physical activity teenagers need to maximize cardiorespiratory fitness (CRF) has not been determined, Samuel Joseph Burden, BMedSci, from John Radcliffe Hospital Oxford (England), and colleagues write.

"Although data in young people are limited, adult studies have shown that regular, brief VPA is highly effective at improving health markers, including CRF, which is also an important marker of health in youth," the researchers write.

In a study published in Pediatrics, the researchers examined the associations between physical activity intensity and maximal CRF.^[2] The study population included 339 adolescents aged 13 to 14 years who were evaluated during the 2018 to 2019 and 2019 to 2020 school years. Participants wore wrist accelerometers to measure the intensity of physical activity and participated in 20-meter shuttle runs to demonstrate CRF. The researchers used partial multivariable linear regression to assess variables at different intensities, including moderate physical activity (MPA), light physical activity (LPA), and sedentary time, as well as VPA.

The wrist monitors measured the intensities of physical activity based on the bandpass-filtered followed by Euclidean norm metric, a validated metric. "Previously validated thresholds for [bandpass-filtered followed by Euclidean norm] were used to determine the average duration of daily [physical activity] at each intensity: 0.1 g for [LPA], 0.314 g for MPA, and 0.998 g for VPA," the researchers write. "[Physical activity] below the threshold for LPA was categorized as sedentary time."

Participants wore the accelerometers for 1 week; value recording included at least 3 weekdays and 1 weekend day, and each valid day required more than 6 hours of awake time.

Overall, VPA for up to 20 minutes was significantly associated with improved CRF. However, the benefits on CRF plateaued after that time, and longer duration of VPA was not associated with significantly greater improvements in CRF. Neither MPA nor LPA was associated with any improvements in CRF.

Participants who engaged in an average of 14 minutes (range, 12-17 minutes) of VPA per day met the median CRF.

The researchers also conducted independent t tests to assess differences in VPA at different CRF thresholds.

Those in the highest quartile of VPA had CRF z scores 1.03 higher than those in the lower quartiles.

Given that current PA guidelines involve a combination of moderate and vigorous PA that could be met by MPA with no VPA, the current findings have public health implications for improving CRF in adolescents, the researchers write.

Even with MPA as an option, most adolescents fail to meet the recommendations of at least 60 minutes of MVPA, they add. "One possible reason is that this duration is quite long, requiring a daily time commitment that some may find difficult to maintain. A shorter target of 20 minutes might be easier to schedule daily and a focus on VPA would simplify messages about the intensity of activity that is likely to improve CRF."

The study findings were limited by several factors, including the use of data from only 2 schools in the United Kingdom, which may limit generalizability. In addition, future research would ideally include a more direct assessment of VO₂max, the researchers note. However, the results were strengthened by the large and diverse study population, including teenagers with a wide range of body mass indexes as well as CRF.

Future research is needed to test whether interventions based on a target of 20 minutes of VPA creates significant improvements in adolescent cardiometabolic health, the researchers conclude.

The current study suggests that counseling teenagers about physical activity may be less challenging for members of the health care team if optimal cardiorespiratory benefits can be reached with shorter bouts of activity, Michele LaBotz, MD, from Intermed Sports Medicine, South Portland, Maine, and Sarah Hoffman, DO, from Tufts University, Boston, Massachusetts, write in an accompanying editorial.^[3]

The results have 2 key implications for pediatricians, the authors note. First, "optimal CRF can be achieved with much shorter periods of activity than previously recommended." Second, "current 'moderate to vigorous' [physical activity] recommendations may not be sufficient to improve CRF in adolescents, if achieved through moderate activity only."

However, although shorter periods of higher-intensity exercise reduce the time burden for teenagers and families, specific education is needed to explain the extra effort involved in exercising vigorously enough for cardiorespiratory benefits.

Patients can be counseled that activity is vigorous when they start to sweat, their face gets red, and they feel short of breath and unable to talk during activity," they explain. These sensations may be new and uncomfortable for children and teenagers who have been quite sedentary or used to low-intensity activity. Dr LaBotz and Dr Hoffman advise pediatricians to counsel patients to build intensity gradually, with "exercise snacks" that involve several minutes of activity that become more challenging over time.

"Exercise snacks can include anything that elevates the heart rate for a minute or more, such as running up and down the stairs a few times; chasing the dog around the backyard; or just putting on some music and dancing hard," the editorialists write.

"Some exercise is better than none, and extrapolating from adult data, the biggest benefit likely occurs when we can help our most sedentary and least fit patients become a bit more active, even if it falls short of currently recommended levels," they concluded.

The study was supported by grants to various researchers from the British Heart Foundation, the Elizabeth Casson Trust, the U.K. National Institute of Health Research, the Professor Nigel Groome Studentship scheme (Oxford Brookes University), and the U.K. Department of Health.

Clinical Implications

• A previous study found that participants in the top 10% of cardiometabolic risk factors had lower rates of physical activity compared with their peers among Black men, White women, and White men. Surprisingly, physical activity was unrelated to cardiometabolic risk among Black women. The other main finding from this study is that physical activity declined with age, regardless of sex or race.
• The current study suggests that 20 minutes per day of VPA can promote higher levels of cardiorespiratory fitness. LPA and MPA were not associated with fitness, and levels of VPA beyond 20 minutes per day did not create significantly higher levels of fitness compared with 20 minutes per day.
• Implications for the healthcare team: The healthcare team should promote VPA for at least 20 minutes per day among adolescents. The current findings have public health implications that health care team members can use to improve cardiorespiratory fitness in adolescents.

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