Cardiorespiratory Fitness in Youth – An Important Marker of Health: A Scientific Statement From the American Heart Association

Abstract

Cardiorespiratory fitness (CRF) refers to the capacity of the circulatory and respiratory systems to supply oxygen to skeletal muscle mitochondria for energy production needed during physical activity. CRF is an important marker of physical and mental health and academic achievement in youth. However, only 40 percent of United States youth are currently believed to have healthy CRF. In this statement, we review the physiologic principles that determine CRF, tools that are available to assess CRF, modifiable and non-modifiable factors influencing CRF, the association of CRF with markers of health in otherwise healthy youth, and the temporal trends in CRF, both in the United States and internationally. Development of a cost-effective CRF measurement process that could readily be incoproated into office visits and in field settings to screen all youth periodically could help identify those at increased risk.

Introduction

Cardiorespiratory fitness (CRF) refers to the capacity of the circulatory and respiratory systems to supply oxygen to skeletal muscle mitochondria for energy production needed during physical activity.^{1, 2} Low or unhealthy CRF is a strong, independent predictor of cardiovascular disease (CVD) and all-cause mortality in adults.¹ In youth, CRF is a predictor of a number of health indicators including cardiometabolic health,^{3, 4} premature cardiovascular disease,⁵ academic achievement,⁶ and mental health.^{4, 7} Unfortunately, currently only 40 percent of 12–15 year olds in United States (US) are believed to have healthy CRF.⁸ In addition, over the past six decades, CRF has declined, both in the US and internationally.^9–11 Although the reasons for this decline are not well-understood, an increase in obesity, increased sedentary time, decreased levels of moderate-to-vigorous physical activity, and social and economic changes may have contributed.^{9, 11}

Although CRF is at times assessed in certain youth, such as those with congenital heart disease, asthma, and cystic fibrosis, assessment of CRF has a broader range of applications. CRF is an objective measure of health that can be tracked over time and compared across populations.^{1, 9} Whereas self-reported physical activity levels can be unreliable¹² and only provide a snapshot of behavior, assessments of CRF provides a more robust measure of cardiovascular health. Consistent with this sentiment, a recent American Heart Association Statement suggests that CRF be assigned as a “vital sign” as it has the power to predict mortality in adults similar to traditionally assessed risk factors, such as tobacco use, type 2 diabetes mellitus, hypertension, and hypercholesterolemia.¹

The central focus of this statement is to raise awareness of clinicians regarding the importance of CRF in predicting current and future health in otherwise healthy youth, knowing that CRF measurements provide an objective measure of health as opposed to physical activity recall which is the current practice. An explicit purpose of this statement is to explore valid, low-cost alternatives to traditional Cardiopulmonary Exercise Tests (CPET) to assess CRF in otherwise healthy youth in office settings with limited space, that can be performed by personnel not formally trained in exercise physiology. This statement will review current knowledge related to the association between CRF and health outcomes in youth, describe the added value of CRF to improve risk prediction, and highlight gaps for future research with the following areas addressed:

• Physiologic considerations

• Various tests that can be used for assessing CRF in the field and office settings

• Key modifiable and non-modifiable factors influencing CRF including effect of interventions

• CRF’s impact on cardiovascular, cerebrovascular, cognitive and mental health

• Temporal trends in CRF in youth nationally and internationally

• Knowledge gaps and suggestions for future research

This statement will not discuss special risk groups of youth such as those with unpalliated/palliated congenital heart disease.¹³ Physical activity guidelines for youth are covered in detail in other documents¹⁴ and its discussion will be limited. The focus of this statement is to primarily examine CRF in otherwise able and healthy, disease free youth.

Health-Related Fitness And Associated Physiological Changes

Although this Statement focuses on CRF, this is only one of four distinct health-related fitness components. CRF, also known as cardiorespiratory endurance, cardiovascular fitness, aerobic capacity, and aerobic fitness among others, refers to the capacity of the body’s circulatory and respiratory systems to supply oxygen to skeletal muscle mitochondria for energy production during physical activity.¹⁵ A second component, muscular fitness, is the body’s ability to exert maximal force against an external resistance (i.e., muscular strength), or repeatedly under sub-maximal loads (i.e., local muscular endurance). Third, flexibility refers to an individual’s range of motion around a joint, or group of joints. Flexibility is important for preventing musculoskeletal injury, maintaining functional independence, and for performing sports and activities of daily living. The fourth component, body composition, is the relative proportion of total body mass composed of fat, fat-free tissue, and total-body water.

Table 1 includes a summary of physical activity intensity categories for youth aged 8–18 years based on heart rate, maximum oxygen uptake, perceived exertion and metabolic equivalent (MET). Energy expenditure often is quantified as MET, with one MET equal to 3.5 mL O₂/kg/min (oxygen consumed).¹⁶ Energy expenditure ranges from low levels used during sedentary activities (1 to 2 METs) to the considerable levels required during sprint interval training (9 to 20 METs).¹⁶ Compared to adults, energy expended is typically higher in youth, leading to the underestimation of energy expenditure if adult reference values are used. As such, Table 1 includes age- and sex-appropriate MET values associated with activity of varying intensity.^{17, 18}

Table 1.

Categories of Physical Activity For Youth Aged 8–18 Years

Intensity category	Description	Example activities	Measures (mean values for age 8–18 years)
Sedentary	Waking behavior typically performed in a sitting, reclining or lying posture.	Sitting or reclining while watching television, playing video games, driving, reading and fishing.	< 1.5 METs
			<40% HRmax
			<20% HRR
			<20% V̇O2max
			RPE: < 8
Light	Light aerobic activity that does not cause a noticeable increase in breathing and can be sustained for atleast 60 minutes.	Domestic or occupational tasks such as washing dishes, ironing, working at a desk or performing office duties	1.5 to 4 METs
			40 to 63% HRmax
			20 to 39% HRR
			20 to 45% V̇O2max
			RPE: 8 to 11
Moderate	Aerobic activity that can be sustained while maintaining a conversation uninterrupted.	Gentle swimming, social tennis and golf.	4–6 METs
			64 to 76% HRmax
			40 to 59% HRR
			46 to 63% V̇O2max
			RPE: 12 to 13
Vigorous	Aerobic activity during which a conversation cannot be maintained. An intensity that may last up to 30 minutes.	Jogging, aerobics, fast bicycling, resistance training, competitive sports	6–9 METs
			77 to 95% HRmax
			60 to 89% HRR
			64 to 90% V̇O2max
			RPE: 14 to 17
Near-maximal to maximal	Activity that typically cannot be sustained for longer than 10 minutes.	Sprinting, periods of competitive team sport activity.	≥ 9 METs
			≥ 96% HRmax
			≥ 90% HRR
			≥ 91% V̇O2max
			RPE: ≥ 18

Notes: Table adapted from Norton and colleagues,¹⁶ ACSM,¹⁶³ Butte and colleagues,¹⁷ and Eather and colleagues.¹⁸ The reported MET values in this table were derived from the Youth Compendium of Physical Activities for specific activities and adapted by Eather and colleagues.¹⁸ Children undergo systematic changes in body composition as a result of growth and maturation, which has implications for activity intensity classifications. As such, MET cut-points should be adjusted for differences in resting energy expenditure. Youth METs have been adjusted to account for the unique physiological characteristics of children and adolescents; % HRmax, percentage of heart rate maximum (heart rate maximum = 220 – age); %HRR, percentage of heart rate reserve (heart rate reserve = HR_max − resting HR); %V̇O_2max, percentage of maximum oxygen uptake. Borg’s Rating of Perceived Exertion (RPE) scale, ranging from 6–20.^{45, 164–166}

Physiologic Changes

At any given baseline level, CRF potentially may increase or decrease, depending on one’s ability to be physically active. Physical activity-induced improvements in CRF may be explained by structural and functional adaptations leading to a better oxygen transport system,¹⁹ such as increased blood volume, myocardial contractility, ventricular compliance, and angiogenesis,²⁰ which all lead to an increased cardiac output.^{21, 22} This was illustrated by Rowland and colleagues²² who found that the cardiac index (cardiac output ÷ body surface area) was significantly greater in trained youth cyclists compared to their non-trained peers. On the other hand, there appears to be little difference in maximal oxygen extraction between trained and untrained youth^{23, 24} and findings have been equivocal as to whether exercise-induced improvements in stroke volume are due to increases in cardiac dimensions.²⁵

Although the two are often conflated, physical activity and CRF are distinct but related concepts. Physical activity is voluntary movement produced by skeletal muscles that results in energy expenditure.² “Exercise” and “training” refer to a subset of physical activity where the goal is to improve performance, health or both.² CRF can reflect an individual’s past physical activity as well as reflect the ability to be physically active (an individual with greater CRF has more capacity for physical activity), forming a virtuous cycle of an active-fit lifestyle. Thus, physical activity is a behavior (“will do”), while CRF represents an individual’s capacity (“can do”) to perform certain types of physical activity.

Key points

• Exercise induced improvements in CRF are due to structural and functional adaptations in the oxygen transport system.

• Physical activity, exercise and CRF are associated but distinct concepts.

How to Measure Cardiorespiratory Fitness in Youth

CRF can be measured, or estimated, using a variety of tests and protocols. The tests used to measure CRF that require maximal effort are referred to as maximal exercise tests. Maximal exercise tests often but not always are performed in the office setting and usually measure cardiometabolic parameters such as inspiratory and expiratory gases, blood pressure, heart rate, and the heart’s electrical activity. Tests that do not require maximal effort are referred to as submaximal exercise tests. Submaximal exercise tests often estimate CRF using equations or nomograms that have been validated against CRF measurements obtained directly during a maximal exercise test. Submaximal tests can be used when a maximal test cannot be performed due to safety, setting or cost. Although submaximal tests are easier to perform, there are often large measurement errors and thus estimated CRF comparisons are fraught with inaccuracies. However, these tests may be useful for identifying and following those with low/unhealthy CRF. Table 2 summarizes key information regarding some of the commonly used tests to measure or estimate CRF.

Table 2:

Comparison Of Selected Tests Used To Measure Cardiorespiratory Fitness¹

	Description	Ability to Assess CRF2	Limitations	Suggestions for Clinical Practice
Cardiopulmonary Exercise Test (Gas-analyzed)	Participants exercise with incrementally increasing difficulty/workload with V̇O2 measured via respiratory gases	+++	Sophisticated equipment needed	Gold standard for measurement of V̇O2
20mSRT3 (Non-gas-analyzed) (Field based)	Participants run/walk between two points on a floor in sync with audio signals with incrementally increasing frequency	++	Need 20 meters open space	Modified protocols are available for office populations
Run tests (e.g., 1.5 mile / 2400 meters) (Field based)	Participants run a given distance as quickly as possible	++	Very dependent on motivation and body size.	Often used in school settings
Step Test (Office or Field based)	Participants step up and down on a block of a given height. Each stage is associated with an increased step rate.	+	Validity not well-established	Portable, test can be performed in small spaces
Walk tests (Office based) (e.g. 6-minute walk test)	Participants instructed to walk as far as possible in 6 minutes	+/−	Poor validity in healthy populations	Useful for populations with low exercise capacity
Questionnaires	Questionnaire to assess fitness level	+/−	Large error in estimation of V̇O2	Used for population research mainly

¹Tests presented were collated to give examples of various testing categories or explanations of protocols and is not meant to be exhaustive.

²Scale ranges from +/− (least) to +++ (most) and reflects writing groups’ overall assessment of test’s usefulness in reflecting CRF.

³20mSRT – 20-meter Shuttle Run Test

The measurement and reporting of CRF depends on various factors such as: 1) the test used and protocol; 2) whether CRF is measured or estimated; 3) whether CRF measures are reported as absolute values versus indexed to body size; and 4) participant motivation.²⁶ The reader is referred to CRF normative measures that are test and protocol-specific.^27–29

For each test described below, it is assumed that participants are able-bodied youth without impairment and that maximal effort is given. Although modifications can certainly be made for many of these tests for youth with physical and/or cognitive impairments, we will not be discussing them in that context as the primary purpose of this statement is to address CRF in healthy youth.

Commonly used terms to describe CRF measures are listed here:

	Unit
V̇O2peak (peak oxygen uptake)	L/min
V̇O2max (maximal oxygen uptake)	L/min
V̇O2peak (scaled to body weight)	mL/kg/min
V̇O2max (scaled to body weight)	mL/kg/min
# of 20 meter Shuttle Run Test laps or stages completed	N
Work	Watts (absolute or scaled)

Gas-Analyzed Testing

Graded Cardiopulmonary Exercise Test:

Based on the Fick principle, oxygen uptake (V̇O₂) is the product of cardiac output (heart rate and stroke volume) and the arterio-venous oxygen difference.³⁰ Thus, V̇O₂ is dependent on cardiac function, the ability of the lungs to act as gas exchange organs, the binding of oxygen to the blood that is primarily dependent on hemoglobin content, and the ability of the muscles to extract oxygen from the circulation for energy transfer. The gold standard for determining V̇O₂ is by measuring O₂ and CO₂ partial pressures in expired air at regular intervals during graded exercise to exhaustion, typically on a treadmill or cycle ergometer. Testing CRF in this way is known by various terms, such as cardiopulmonary exercise test (CPET), cardiorespiratory exercise test, or a graded exercise test.

The highest oxygen uptake attained during graded exercise to volitional exhaustion (V̇O_2max) is considered the best indicator of CRF by the World Health Organization.³¹ V̇O_2max is the reflection of the maximal oxygen flux through the lungs, transported by the circulation to the mitochondria of the exercising muscle. V̇O_2max remains the only index that integrates pulmonary, circulatory, and muscular function into a single number. However, the utility of V̇O_2max measurements in youth has been questioned. Traditionally, for V̇O_2max to be determined, there must be a plateau in the oxygen uptake curve. Even the earliest pioneers appreciated that youth do not often demonstrate a plateau during incremental exercise³² and that the greatest V̇O₂ measured in youth, termed V̇O_2peak, is likely analogous to V̇O_2max measured in adults.^{33, 34} We will use both terms (V̇O_2peak and V̇O_2max), reflecting as closely as possible the measures used in the cited studies.

Reporting norms for V̇O_2peak or V̇O_2max in youth is further complicated by the wide range of body sizes even at a given age. Although CRF values often are indexed to body size, it is not clear that this is always appropriate as it may not fully account for the residual effects of body size. In a systematic review and meta-analysis, it was found that adolescent participants with obesity had comparable CRF to participants without obesity when expressed in absolute values (V̇O_2peak L/min), but lower values when scaled for weight (V̇O_2peak mL/kg/min), and different still when scaled to lean mass.³⁵ On the other hand, if allometric scaling is undertaken, it remains sample-specific and cannot necessarily be extrapolated to all populations.³⁶ At this time, there is no accepted standard in regard to scaling in reporting CRF; hence attention should be paid to units when comparing CRF between participants and studies.

Even as CPET provides a wealth of data, clinicians should be aware of limitations including the limited ability to perform this test in settings other than the office or hospital. The test requires expensive equipment and well-trained staff, which are not always available. The metabolic cart requires meticulous maintenance and calibration. Another limitation is that most CPET parameters are measured breath-by-breath, with a range of options to analyze the data and filter the “noise” in the data. This can introduce differences between laboratories and equipment, which makes comparisons among participants and studies difficult. Finally, the pattern of activity performed during CPET may not reflect the types of physical activities youth are commonly engaged in.

Non-Gas-Analyzed Tests

Field-Based Tests:

1. 20 Meter Shuttle Run Test

For reasons stated above, alternative tests for measuring CRF in youth have been developed.^{37, 38} One such test is the 20-meter shuttle run test (20mSRT) developed by Leger et al.^{39, 40} The 20mSRT, and its variants, is now the most widely used test to estimate CRF in youth in the world.⁴¹

There are several different names used for the 20mSRT - Beep test, PACER (Progressive Aerobic Cardiovascular Endurance Run) test, and Multi-stage fitness test; however, the protocols are very similar. Typically, youth are instructed to run at an increasing standardized pace (starting around 5 miles/hour [8 kilometers/hour], increasing in 0.3 miles/hour [0.5 kilometers/hour] increments each minute), noting the number of laps or stages they can keep up with the pace as the result, which can then be compared to a reference population.⁹

The 20mSRT has been studied in both sexes and in a range of ethnicities and ages.^{27, 42} According to a systematic review, the 20mSRT is a valid estimate of CRF when compared to CPET measured CRF.⁴³ In this review of 73 studies addressing the criterion-related validity of field-based fitness tests in children and adolescents, there was strong evidence that the 20mSRT had moderate to high validity against CPET to estimate CRF. As the 20mSRT can be administered in group settings such as in schools, it is efficient for testing large cohorts of youth simultaneously, and is thus feasible for population-based CRF surveillance. However, as is true for all CRF tests, it is influenced by motivation and performance. If estimated V̇O_2peak is used as an endpoint for comparison, large prediction errors can influence results.⁴⁴ Thus number of laps completed or stages reached may be better endpoints to report. Clinicians should be aware of the specific 20mSRT protocol used when comparing to reference values.

In the U.S., the 20mSRT is commonly used as a component in the “FitnessGram.”⁴⁵ The FitnessGram is a group of tests used to assess various forms of fitness in school-aged youth. In addition to 20mSRT, the FitnessGram measures body mass index (BMI), abdominal strength, trunk extensor strength, upper-body strength, and flexibility. The results are classified into various “Fitness Zones.”⁴⁵ All 50 states in the US currently use the FitnessGram to assess over 22 million students each year.²⁹

2. Run Tests

In run tests the participant is given a set distance (e.g., 1.5 miles or 2400 meters) or time (e.g., 12 minutes) and instructed to complete the run in as short amount of time as possible or cover the greatest possible distance, respectively. Mayorga-Vega et al⁴⁶ recently performed a meta-analysis to determine which distance and/or time was most appropriate to use in youth. Of the various distances and times used, they found that the highest correlation to CPET measured V̇O_2max, was with the 1.5 mile (2400 meters) distance (r =0.79) and 12 minutes time (r=0.78), showing moderate-to-high correlation.⁴⁶ Regarding the validity of the 1.5 mile and 12-minute run tests when compared to the similarly reliable 20mSRT, data from two large meta-analyses^{42, 44} indicate that run tests are equally valid compared to the 20mSRT.

Office-Based Tests:

The text below describes some of the commonly used tests that are suitable for use in office settings, but is by no means exhaustive.

1. Ebbeling test (single stage treadmill walking test)

This test is performed on a treadmill with a 5 percent grade incline. The heart rate is measured after 4 minutes and is combined with speed, age, and sex to estimate CRF.⁴⁷ Nemeth et al evaluated the Ebbeling test in 130, 11–14 year-olds who were overweight and concluded that the CRF estimate was within 10 percent of the V̇O_2max (mL/min) measured from CPET.⁴⁸

2. Åstrand-Rhyming test

This test is performed using a cycle ergometer and is often used in Europe. This test is typically performed over six minutes with a constant load (or single stage) aimed at producing a heart rate between 125–170 beats per minute. The heart rate and the workload is used to estimate V̇O_2max from a nomogram.⁴⁹ The Åstrand-Rhyming test has been evaluated in 11–12-years-olds and found to have a strong correlation of 0.82 in girls and moderate correlation of 0.52 in boys when compared to CPET-measured V̇O_2peak (L/min).⁵⁰ The authors did not explore the reasons for the differences in correlation coefficients between boys and girls.

3. PWC170 (Physical Work Capacity Corresponding to a Heart Rate of 170 beats per minute)

The PWC170 test has been used since the 1960’s. It is administered using a cycle ergometer and typically conducted with 3 × 3-minute stages or 3 × 4-minute stages of increasing workload. Work (Watts) is measured once the heart rate reaches 170 beats per minute. PWC170 was moderately well-correlated with measured V̇O_2peak (mL/kg/min) in 11–16 years-olds, with the correlation depending on the stage length (i.e., 0.70 for two minutes, 0.56 for three minutes, and 0.61 for six minutes).⁵¹

4. 6-minute walk test (6MWT)

This is the most commonly administered walk test and measures the distance walked in 6 minutes.⁵² The 6MWT is easy to administer and international guidelines have been established,^{53, 54} along with test-specific reference standards.⁵⁵ However, the use of the 6MWT is less useful in healthy youth to estimate CRF. The 6-minute walk test shows a relatively poor correlation with V̇O_2max, except in populations with moderate-to-severe limitations in CRF⁵⁴ or reduced walking capacity of less than 300 meters.⁵⁶ Therefore, their use should be considered only when there is reason to suspect low CRF.^{54, 57}

5. Step tests (Queen’s College or Harvard step tests)

Step tests are another category of tests, using stepping up and down a bench in an effort to engage larger muscle mass. One of the first such protocols described in children (Harvard Step Test) involved stepping up to a 12-inch bench at a rate of 24 steps per minute for a duration of 3 minutes with heart rates collected post-exercise.⁵⁸ Recently, Hayes et al reported the validity of a step test in elementary school children and showed that the step test along with sex, and BMI significantly predicted V̇o_2max (R² = 0.51).⁵⁹ Heart rates in youth during step tests have been reported to be strongly associated with V̇O_2max (r=0.8, p<0.01)⁶⁰ regarldess of stepping frequency.⁶⁰ Step tests require minimal equipment, are easy to administer in limited indoor spaces and can be administered by personnel with little or no formal training in exercise physiology, which make them a suitable alternative to CPET to estimate CRF in office settings. The step test can also be performed on the bleachers at schools and is suitable for testing in group settings simultaneously. It is important to monitor consistency with step cadence and foot strike pattern as repeated breaches may affect results.

Questionnaires:

Some youth are unable to complete fitness testing for various reasons (body size, maturity limitations, etc.) so methods to estimate CRF without objective testing have been evaluated. Questionnaires may offer the least burdensome method for examining CRF in youth. However, questionnaires are currently used for epidemiologic studies and not for estimating CRF in individuals.

The International Fitness Scale (IFIS) is one option.^{61, 62} It consists of five questions using a five-point Likert scale on general physical fitness, CRF, muscular strength, speed/agility, and flexibility. IFIS is designed to measure CRF in populations and can be completed in about 5 minutes. Ortega et al reported that in 3,059 youth aged 12–18 years, the IFIS was linearly related to CRF (mL/kg/min) as estimated by the 20mSRT with an odds for having a healthy CRF based on FitnessGram thresholds of 7.3 (95% CI 4.0–13.5) for those reporting very good CRF on the IFIS questionnaire.⁶¹ However, its usefulness at the individual level is not established.⁶¹ It should also be noted that correlation between IFIS and the FitnessGram compares surrogates to surrogates and does not use measured V̇O₂ as a reference.

Key points

• The most accurate measure of CRF in youth is gas-analyzed (measured) V̇O_2peak obtained during a graded CPET but this testing cannot be universally performed.

• Graded tests such as the 20mSRT provide the best alternative to CPET in a field setting.

• Step tests may be a good alternative to CPET when space and resources are limited.

• In general, tests that require more effort are preferred to tests that primarily measure function, such as walk tests.

• Estimated V̇O_2peak can be misleading and needs to be reconciled with other factors such as the protocol and testing used and participant motivation/effort.

• Questionnaires may provide insightful information for epidemiological purposes but are considered the least accurate method for assessing CRF.

Factors Affecting CRF in Youth

Studies have investigated the relationship between CRF and various non-modifiable and modifiable factors including genetics,⁶³ age, sex,⁶⁴ race/ethnicity,⁶⁵ physical activity and dietary patterns,^{66, 67} obesity,^{68, 69} sedentary time,⁷⁰ built environment,⁷¹ and socioeconomics.^{72, 73}

Below is a discussion of these topics.

Non-Modifiable:

Genetics

In adults, it has been noted that an individual’s response to physical training varies widely, with some people markedly increasing their CRF (“responders”), while others only have a minimal increase in CRF (“non-responders”).^{74, 75} One study suggested that nearly 50 percent of an individual’s response to training is inherited.^{63, 76} Further, the variance in response to aerobic training was 2.5 times higher between families than within families.⁷⁶ But, not one of the nearly 300,000 single nucleotide polymorphisms studied have been found to be associated with exercise-induced changes in V̇O_2max (mL/min).⁷⁷ Thus, evidence supporting specific genetic polymorphisms influencing CRF remains weak⁷⁸ and the mechanisms by which genes affect CRF are still unclear.⁷⁵ There is no evidence for genetic variations impacting CRF (mL/kg/min) among elite athletes.⁷⁹ Studies in youth examining genetic differences in CRF are lacking.

Age and Sex

As youth age, there is an increase in CRF as measured by V̇O_2max (mL/min) for both boys and girls.⁸⁰ While CRF increases in both boys and girls as they age, the increase in girls occurs at a slower rate.^{81, 82} Regardless of age, boys have a higher V̇O_2max than girls across,^{9, 83} even after controlling for lean body mass and cardiac size.⁸³ Potential explanations for this difference include sex-related differences in muscle fiber type, oxygen extraction, or the lipid content of myofibrils.^{83, 84}

Race/Ethnicity

In adults, V̇O_2max has been noted to be higher in Caucasians compared to African Americans⁸⁵ and to those of Chinese ethnicity.⁸⁶ However, the relationship between race/ethnicity and CRF (mL/kg/min) in adults weakens after adjustment for BMI, lifestyle factors, socioeconomic status, and other CVD risk factors.⁸⁷ Similarly, racial/ethnic differences in CRF in youth are unclear. Studies using data from the 1999–2004 and 2012 cohorts from the National Health and Nutrition Examination Survey did not find differences in CRF in youth across race/ethnicity groups (V̇O_{2 max -} mL/kg/min was measured from a submaximal, gas-analyzed test).⁸ But, Shaibi et al found that Hispanic youth had lower V̇O_2peak (mL/kg/min) than non-Hispanic white and non-Hispanic black youth.⁸⁸ This is consistent with international comparisons in which youth in South America had lower CRF compared to youth from Europe and Africa.⁸⁹ Similarly, Bansal et al, found that African-American children have lower CRF versus Caucasian children (V̇O_2max - mL/kg/min). However, these differences in CRF were not adjusted for environmental and psychosocial factors or habitual physical activity.⁹⁰

Prematurity

Using data from Northern Ireland Young Hearts Study, investigators found that compared to those born at full-term, even those born slightly early, between 37 and 38 weeks gestation, had a 57 percent higher risk of having low CRF (mL/kg/min) at ages 12, 15 and 22 years.⁹¹ These effects were not related to decreased physical activity.^{91, 92} In a meta-analysis, participants born prematurely had approximately 13 percent lower CRF than those born at term.⁹³ The mechanism is not clear, but may be related to smaller lung volumes.

Modifiable:

Habitual Physical Activity And Exercise Training

It is generally assumed that physically active youth have higher CRF. However, the strength of the association between habitual physical activity and CRF in youth is small-to-moderate,⁹⁴ with most of the benefits accruing only with sustained vigorous physical activity.^94–96 A number of factors may explain the lack of a strong association between physical activity and CRF in youth. First, CRF has an incompletely defined, but clear hereditary component. Second, habitual physical activity levels in youth rarely achieve the vigorous intensity or duration necessary for the improvement of CRF. Finally, challenges in the accurate assessment of both physical activity and CRF may mask the relationship.

Using an objective measure of physical activity, Gutin et al found that CRF (mL/kg/min) in youth had a stronger relationship with the time spent in vigorous physical activity than with the time spent in moderate- or light-intensity physical activities.⁹⁷ In general, training programs of various intensities can improve V̇O_2max or V̇O_2peak in pre-pubertal youth, but engaging in increased amounts of intense physical activity can lead to up to a 10 percent improvement in these parameters.^{98, 99}

The importance of high levels of moderate-to-vigorous physical activity is illustrated best by studies of high intensity interval training (HIIT). Evidence is growing that HIIT may be effective in improving youths’ CRF. HIIT is typically considered to be exercise that is characterized by alternating intermittent bursts of vigorous activity with periods of rest or low-intensity activity. Studies have demonstrated that small amounts of vigorous, maximal-to-near-maximal activity can induce improvements in youths’ V̇O_2peak. For example, Costigan and colleagues¹⁰⁰ conducted a systematic review of the effects of HIIT on youth’s CRF. In this review, the adjusted difference between groups in V̇O_2max, was 2.6 mL/kg/min (95% CI 1.8 to 3.3, p<0.001) in favour of adolescents participating in HIIT. Interventions ranged from 4 weeks to 8 months in duration and the majority of studies involved three sessions per week of maximal sprint running. These studies however provide less evidence for the exact dose (i.e., frequency, intensity, time, and type) of physical activity that is needed to improve CRF.

Although the impact of physical activity on CRF is variable, even small improvements in CRF with increases in physical activity resulted in major health benefits in adults.¹ In fact, it is well established that moving from the lowest quintile CRF to the next-lowest quintile group is associated with the most striking health benefits in adults.¹ No studies to date have measured the impact of physical activity in youth with low baseline CRF, but this is a critical health question to answer as they potentially stand to benefit most from intervention.

Sedentary Time

The amount of time spent sedentary comprises up to 75 percent of a 15 year old’s waking hours and has increased from 7 hours to 8.2 hours per day from 2003 to 2016 in adolescents in England and the US.^{101, 102} A recent American Heart Association Statement on sedentary time in adults¹⁰³ noted several meta-analyses suggesting a strong relationship between sedentary time and all-cause death. In a recent meta-analysis in adults, the negative effects of high levels of sedentary time were reduced with high levels of moderate-to-vigorous physical activity, but not eliminated.¹⁰⁴

The relationship between sedentary time and CRF in youth is unclear. Studies have demonstrated both the presence^{70, 105, 106} and absence^{107, 108} of a relationship. In a large study of 11–13 year-old females, objectively-measured physical activity improved CRF (mL/fat-free mass/min), but there was no relationship between CRF and objectively-measured sedentary time.¹⁰⁹ In another study, CRF was associated with objectively-measured sedentary time, independent of time spent in moderate-to-vigorous physical activity.⁷⁰ The authors of a recent meta-analysis examining the cross-sectional association between total sedentary time and CRF in children and adolescents (n = 4499 participants) found conflicting results. In children, there was a significant association (r = −0.06, p = 0.037), whereas there was no association was found in adolescents (r = 0.02, p = 0.7).¹¹⁰

Obesity

Youth with obesity who are less physically active exhibit lower V̇O_2max (mL/kg/min) than their normal weight peers.¹¹¹ Byrd-Williams et al, in a longitudinal study evaluating risk factors for the development of type 2 diabetes among Hispanic youth, found that high CRF (mL/min) is associated with less subsequent weight gain over time in boys, but not in girls. Specifically, this study found that for each 15 percent increase in V̇O_2max from baseline there was an associated 1.4 kg lower fat mass over 4 years.¹¹² Therefore, optimal CRF could modify BMI suggesting a bidirectional relationship between obesity and CRF. There are reports evaluating the relationship between genes associated with obesity and V̇O_2max /trainability. Such studies have suggested that there is a shared genetic thread between obesity and CRF, regardless of whether V̇O_2max is indexed to fat-free mass or total body weight.¹¹³ Lifestyle interventions, regardless of whether youth gain or lose weight, may have a beneficial effect on CRF. In a study of 11–18 year-old females enrolled in a six-month program of dietary counseling combined with supervised aerobic and resistance exercise training, CRF improved in those who lost weight more than in those who did not, but V̇O_2max improved with intervention in both groups as a function of the increase in fat free mass.¹¹⁴ The authors however do not report on potential interplay between the dietary and exercise training aspects of this lifestyle intervention.

Diet

An overall healthy dietary quality score was associated with better CRF in the “Coronary Artery Risk Development in Young Adults” study in all race-sex groups of youth studied, except African Americans.⁶⁷ A dietary pattern specifically rich in fruits and vegetables was associated with healthy CRF in New Zealand and European youth.^{115, 116} The nutritional contributions to CRF is rooted in mitochondrial energetics that are fundamental to skeletal muscle oxidative capacity and efficiency, and therefore to CRF.¹¹⁷

As defined at the opening of this statement, CRF reflects the integrated ability to transport oxygen from the atmosphere to the mitochondria to perform physical work. A signature feature of mitochondria is their ability to proliferate, or conversely to be degraded in response to nutritional and extracellular environmental stimuli. Exercise training and dietary patterns rich in omega 3 fatty acids and polyphenols are the principle external influences known to promote mitochondrial bioenergetic pathways.¹¹⁸ And several specific essential fatty acids and polyphenolics, including from cocoa, apples, beets, pomegranates, grapes, olives and cruciferous vegetables have been shown to increase mitochondrial biogenesis, and improve mitochondrial function.¹¹⁹ Nitrate, an inorganic ion abundant in fruits and vegetables, can also be converted in the mammalian mouth and gut to bioactive nitric oxide, further reducing the oxygen cost of exercise.¹²⁰

Social, Economic, and Environmental Factors

Disparities in CRF may be socioeconomically driven with both rates of poor nutrition and physical inactivity greatest among urban youth.¹²¹ Also, the environment’s effects on lifestyle and CRF may be mediated through varying levels of physical activity due to the built environment. Gahche et al⁸ did not find a difference in socioeconomic status and CRF (submaximal, gas-analyzed, measured CRF - mL/kg/min), but other studies have found that poor socioeconomic status is associated with low CRF (measured using the 20mSRT) in youth.⁷³

A recent study identified a strong negative association between country-level CRF and income inequality. In countries with a wide income gap between rich and poor residents, youth had poorer CRF.⁸⁹ In a review, the same authors reported that countries with a widening economic gap between rich and poor residents had less favorable CRF trends (i.e., large declines).⁹ While these assessments of income inequality may not be stringent, these data provide some proof of concept that there are social and economic determinants of CRF.¹²²

Figure 1 summarizes key influencers of CRF and outcomes influenced by CRF.

Figure 1 -.

Cardiorespiratory Fitness In Youth – Key Influencers And Effects

Key points

• Hereditary factors are known to influence CRF but specific genes that explain these differences have not yet been elucidated.

• Racial/ethnic differences in CRF seem to be related to extrinsic factors such as lifestyle, other CVD risk factors, and socioeconomic status.

• Age, sex, and vigorous physical activity are the most influential determinants of CRF in youth. Influence of modifiable factors on CRF is likely mediated by duration, frequency, and intensity of physical activity.

• There is little evidence to suggest that sedentary behavior is related to CRF in youth, once adjusted for objectively measured physical activity.

• Nutrient modulation of CRF may be mediated by mitochondria number and function.

Implications Of CRF For Health Outcomes

CRF And Health Outcomes In Adults

Numerous large studies have established that in adults, low CRF is associated with greater risk for all-cause mortality, CVD events, and cancer mortality, independent of, and perhaps more strongly than, traditional risk factors.^{1, 123, 124} A nonlinear pattern, whereby the largest benefit occurs between the least fit and next-least fit groups, underscores the potential benefits of even modestly increasing CRF in the most sedentary individuals,¹ but there are no studies in youth in this regard. Apart from mortality, low CRF in adults is also associated with greater risks for congestive heart failure, stroke, type 2 diabetes mellitus, some cancers, and neuropsychological disturbances (e.g., dementia, anxiety, and depression).^{1, 123, 125} Most importantly, improvements in CRF over time are associated with reduced mortality and morbidity.^{125, 126}

CRF Tracking

In light of these well-documented benefits of optimal CRF in adults, the degree of CRF tracking from childhood to adulthood is of interest. Several studies have found that the degree to which CRF tracks into adulthood varies by methodology (e.g., measured or estimated V̇O₂), sex, and length of follow up. In general, studies found that tracking was low-to-moderate for spans up to 40 years.^127–130

Childhood CRF And Health Outcomes

Longitudinal data on the relationship between CRF in youth and CVD endpoints have primarily come from studies following male military recruits. These studies have collectively demonstrated inverse associations between CRF (Watts/kg) in youth and all-cause mortality (hazard ratio [HR] 0.49, 95% CI, 0.47–0.51 for highest versus lowest quintile of CRF);¹³¹ CRF and myocardial infarction (HR 0.82, 95% CI 0.80–0.85 per 1 SD higher CRF);⁵ CRF (Watts) and stroke (HR 0.84, 95% CI 0.81–0.88 per 1 SD higher CRF);¹³² CRF (Watts/kg) and heart failure (HR 1.60, 95% CI 1.44–1.77 for low vs high CRF);¹³³ and CRF (Watts) and disability (HR 1.85, 95% CI 1.71–2.00 for low vs high CRF).¹³⁴

Childhood CRF has also been associated with cardiometabolic risks and a variety of more proximal health outcomes.¹³⁵ In a study of 154 youth followed for 24 years, improvement in CRF was associated with lower arterial stiffness (for each unit increase in measured CRF adjusted for body weight, carotid compliance was higher [p=0.04], even after adjustment for several risk factors).¹³⁶ Cross-sectional and short-term longitudinal studies have also shown an inverse relationship of childhood CRF with adiposity,^{112, 137, 138} waist circumference,¹³⁹ blood pressure,¹⁴⁰ insulin resistance, nonalcoholic fatty liver disease,^{141, 142} and a clustered cardiometabolic risk score.¹³⁵ Further, in a systematic review and meta-analysis, low CRF was significantly associated with the development of pediatric metabolic syndrome.¹⁴³ In the only prospective study included in this meta-analysis, youth with metabolic syndrome had an odds ratio of 6.1 (95% CI, 1.2–60.3) for having had low CRF (mL/kg/min) seven years earlier.¹⁴⁴

Given these associations, several studies have developed criterion-referenced CRF cut points to help identify youth with high cardiometabolic risk.¹⁴⁵ These studies attempt to define CRF thresholds in youth to help providers identify those with the highest risk of cardiometabolic disease. In a meta-analysis combining seven published criterion-referenced standards on 9280 youth aged 8–19 years from 14 countries, CRF below 35 mL/kg/min for girls and 42 mL/kg/min for boys identified youth with higher likelihood of adverse cardiometabolic risk factors (e.g., insulin resistance, dyslipidemia, adiposity, high blood pressure) with odds ratios of 5.7 (95% CI, 4.8–6.7) for girls and 3.6 (3.0–4.3) for boys.¹⁴⁶ For ease of interpretation, 20mSRT stages that achieve these CRF cut points for boys and girls of different ages have also been published.¹⁴⁶

CRF and Lung Function

In a population-based study with cross-sectional and longitudinal components, each standard deviation higher CRF was associated with 2–3 percent greater predicted value of both forced expiratory volume in the first second and forced vital capacity among individuals aged 9 through 38 years. Moreover, improvements in CRF during youth were associated with better lung volumes.¹⁴⁷ However, these improvements were not necessarily related to any measures of change in physical activity or interventions undertaken during the course of the longitudinal follow up.

Childhood CRF – Cognitive And Mental Health Outcomes

CRF has been associated with a range of cognitive and academic outcomes in youth. Academic achievement generally has been found to be positively associated with CRF, though most studies have used a cross-sectional design.^{3, 6, 148} Among longitudinal studies, maintaining a healthy CRF, or improving CRF over time, has been associated with better academic achievement.^148–150 For example, in a recent large longitudinal study of ~400,000 Taiwanese junior high school students followed for three years, there was a dose-dependent, positive association between number of years with high CRF (top age- and sex-specific quartile vs. bottom three quartiles of CRF for all three years) and standardized test scores in the third year with between-group differences up to 0.3 standard deviations for math and science after adjustment for sex, BMI, and urbanization.¹⁵¹ Although effect sizes have varied across studies, even small effect sizes could be impactful at the population level.

High CRF may improve school achievement through improving cognitive abilities or psychological factors.⁶ Higher CRF has been associated with better attention allocation, cognition modulation (as assessed by task performance and event-related brain potentials), as well as more efficient neural activation in the prefrontal and parietal cortices (as assessed by functional magnetic resonance imaging).¹⁵² In a randomized trial involving a physical activity intervention in 8-year-olds, neural efficiency increased in direct proportion to the increase in CRF.¹⁵³ In another intervention, youth receiving structured physical activity had both an increase in performance on cognitive tests and in V̇O_2max (mL/kg/min), although the relationship between the change in V̇O_2max and cognitive performance was not assessed.¹⁵⁴ Higher CRF has also been associated with better relational memory (learning about the relationship between two stimuli), potentially mediated by larger bilateral hippocampal volume.¹⁵² Indeed, a variety of structural brain changes (e.g., altered cortical grey matter thickness and integrity of white matter tracts) have been observed in association with CRF, potentially related to effects of CRF on angiogenesis, neurogenesis, and neuroplasticity via increases in brain-derived neurotrophic factor.^{6, 7, 148}

Further, better childhood CRF has also been associated with a lower incidence of mental disorders (mood disorders, psychosis, or suicidality)¹⁵⁵ as well as improved self-worth^{156, 157} and life satisfaction.^{158, 159} In fact, in an exercise intervention study in children, effects on mental health outcomes were more strongly related to improvements in CRF than to changes in body composition.¹⁵⁷ These mental health effects are thought to be related to structural brain changes and changes in brain signalling (e.g., serotonin).

Key points

• A linear inverse relationship exists between CRF during youth years and all-cause mortality, as well as cardiovascular disease across the lifespan.

• In youth, a protective inverse association has been demonstrated between CRF and multiple conditions that compound cardiovascular risk, including but not limited to metabolic syndrome, type 2 diabetes-mellitus, nonalcoholic fatty liver disease, and mental health disorders.

• CRF is also positively associated with cognitive function, self-worth, and life satisfaction in youth.

Epidemiology Of CRF In Youth - Temporal Trends

Both in the US and internationally, CRF in youth is thought to have declined over the past 40 years.^8–10 Globally, a decline in CRF in youth has been noted since the 1960s.¹¹ Armstrong et al. reported a small but downward trend in the gas-analyzed V̇O_2peak (mL/kg/min) in approximately 4000 youth from five countries between 1962 and 1994. While this represents the best available data on trends in gas-analyzed V̇O_2peak, the study is dated.⁹⁹ No study has examined trends in allometrically-scaled V̇O_2peak for youth.

United States:

Using a nationally representative sample in the US, only 42 percent of 12–15 year olds had healthy CRF (mL/k/min) in 2012 (Figure 2).⁸ The percentage of boys who had healthy CRF decreased significantly from 65 percent in 1999–2000 to 50 percent in 2012. For girls, the percentage decreased over the same time period, though not as substantially, from 41 to 34 percent.⁸ Additionally, 54 percent of normal-weight youth had healthy CRF, whereas only 30 percent of youth who were overweight (BMI ≥85^th percentile for age and sex) and 20 percent of youth with obesity (BMI ≥95^th percentile for age and sex) had healthy CRF. This percentage did not differ by race and Hispanic origin or family income-to-poverty ratio.⁸ Others have reported declines in mean CRF of 0.9 mL/kg/min, per decade, between 1995 and 2013 in 166,900 US youth aged 9–17 years.⁹

Figure 2 -.

Percentage Of Youth Aged 12–15 Years Who Had Healthy Cardiorespiratory Fitness, By Sex And Survey Period: United States, 1999–2004 and 2012.⁸

International:

CRF declined by over seven percent from 1981 to 2014 in a recent analysis of 137 studies that reported 20mSRT data on youth aged 9–17 years.⁹ Temporal trends were estimated at the country-sex-age level for 19 high-income and upper middle-income countries. CRF (mL/kg/min) trends varied over time and across countries. Moderate CRF declines were seen in earlier years, these declines have then slowed and stabilized since 2000.⁹ However, not all data suggest that there has been a decrease. In Greece, using a measure of CRF based on the 20mSRT, there was an increase in CRF in both genders from the cohorts evaluated in 1992–93 and 2006–07.¹⁶⁰ It should be noted that these CRF estimates based on the 20mSRT, which are only moderately correlated with V̇O_2peak, remain imperfect.

A significant percent of the reported decline in CRF (mL/kg/min) may be attributable to the increasing prevalence of obesity.⁹ Caution should be used while interpreting associations between V̇O_2peak when indexed to body weight, as indexed values may systematically underestimate V̇O_2peak in youth with obesity. Thus, a weight scaled CRF may underestimate fitness in this population. For example, in a study of Norwegian military volunteers over a 22-year period, CRF declined by 8 percent, but body weight increased by 7 percent, suggesting only a minimal change in absolute V̇O_2max (mL/min) over this period.¹⁶¹ Similarly, Andersen and colleagues¹⁶² found that there was no difference in absolute V̇O_2max (mL/min) between cohorts tested in 1983, 1997, and 2003 in both boys and girls. The authors noted that there were changes in BMI and that maximal performance decreased with time, suggesting that these trends need to be validated in rigorous studies before determining if there have been secular decreases in CRF over the past several decades.¹⁶² While it is difficult to know whether declines in field tested CRF reflects a true decline in underlying cardiovascular function, or an increase in body size, or both, tests such as the 20mSRT, 1.5 mile run, and 12-minute run tests are suggestive of a decline in underlying V̇O_2peak (mL/kg/min). Trends in these weight-bearing CRF tests better reflect trends in typical youth aerobic activities of daily living.

Key points

• One-half of boys and two-thirds of girls aged 12–15 years do not have healthy CRF.

• Only 1 in 5 youth with obesity have healthy CRF.

Gaps and Limitations

1. Although several tests beyond CPET are currently available to measure CRF in office and field settings in youth, there is a pressing need for standardization of testing protocols, uniform interpretation of tests and, data harmonization. Tests such as the step test may be a suitable alternative to CPET in the office setting but needs further study.

2. Stronger clinic-community partnerships to share results or easily access CRF assessments performed at different settings would be meaningful in providing a customized counseling and intervention.

3. Research is needed to further determine which interventions improve CRF in youth including youth with obesity and or low CRF. We need more research to determine thresholds at which intervention is needed.

4. There is need for continued collection of data to assess impact of CRF in youth on CVD outcomes as currently longitudinal data are limited.

5. Furthermore, research should aim to determine the reasons for the reported decline in CRF in youth in order to develop strategies to reverse this trend.

Conclusion

Healthy CRF is positively associated with cardiovascular health, academic achievement and mental wellbeing in youth. Accurate and reliably measured CRF may identify youth who would benefit from lifestyle interventions but may be missed by subjective physical activity recall, anthropometric measures, or CVD risk factor testing which are current standards of care.

Although accurate assessment of CRF in youth has traditionally relied on CPET, less resource-intensive tests, in particular, the 20mSRT in the field setting, are useful. Office-based CRF testing that can be performed by providers with little or no formal training in exercise physiology and using low-cost equipment is also superior to physical activity recall. With future research, a practical, widely applicable test to estimate CRF in office settings may become a reality and an essential part of health assessment in all youth during office visits.

Every child will benefit from a CRF estimate as part of a yearly physical. Repeated bursts of vigorous physical activity, including HIIT, improve youth CRF. Public health measures and school policies that support lifestyle improvements to improve CRF in individuals and populations are expected to result in substantial health and cognitive benefits.