Field-Based Measurement of Cardiorespiratory Fitness for Children and the Youth in Low and Middle Income Settings

Abstract

Cardiorespiratory fitness (CRF) has declined worldwide among the youth with 81% of adolescents not being physically active. Promotion of cardiorespiratory fitness among the youth is an important goal with the global challenge of reducing physical inactivity by 15% in the next 7 years. Assessment and monitoring though of cardiorespiratory fitness have not been standardized for children worldwide. Moreover, the gold standard of gas-analyzed cardiopulmonary exercise testing is too expensive for use in low to middle-income countries (LMICs). Submaximal exercise tests that need minimal inexpensive equipment and training for the administering staff are most appropriate for use in population studies and individual CRF assessment in LMICs. Three office-based and two field-based tests (e.g. 20 m shuttle run test) are recommended for use among children in LMICs. Validated CRF questionnaires can be used in epidemiologic studies. Health-related fitness test batteries for children have also been conducted in LMICs. There is, therefore, an urgent need to develop a standardized set of measures for CRF among children that can be implemented internationally. These should be scalable, simple, valid, reliable, easily implementable and can surmount the large geo-regional variations in assessing and reporting CRF among the youth.

1. Introduction

Cardiorespiratory fitness (CRF) is a major component of physical fitness and is the capacity of the cardiovascular and respiratory systems to provide oxygen to skeletal muscle mitochondria for energy production during physical activity [1]. Physical fitness is the more general term that pertains to the ability of the person to perform physical activities or specific exercises [2]. Physical activity (PA), on the other hand, is classically defined as “any bodily movement produced by skeletal muscles that results in energy expenditure” and exercise is a type of physical activity that is “planned to improve or maintain physical fitness [3].” Cardiorespiratory fitness and physical fitness are attributes of a person while physical activity and exercise are behaviors of a person [1]. Cardiorespiratory fitness, physical fitness and regular physical activity have all been essential components for promoting both mental and physical health from youth to adulthood among all individuals [1, 2]. Low levels of CRF among the youth predict poor cardiometabolic health, premature cardiovascular disease, lower academic achievement and lower mental health status [1]. CRF has declined among the youth throughout the world and the 2022 Global Action Plan on Physical Activity (GAPPA) likewise reported that 81% of adolescents did not meet the World Health Organization’s (WHO) recommended levels of physical activity [4, 5, 6]. All countries, therefore, have been challenged to reduce physical inactivity by 15% and to reach this goal by Year 2030 [4].

Promotion of cardiorespiratory fitness and physical activity among the youth have become important goals to improve health and prevent future cardiovascular diseases globally so that assessment and monitoring of these health indices have also been recently given more attention [1]. The American Heart Association even proposes cardiorespiratory fitness being the fifth vital sign [7]. Half of the participating communities, especially among the low- to middle-income countries (LMICs), have not been able to assign a grade for Physical Fitness in the Global Matrix 4.0 Report on Physical Activity due to lack of data [6]. Recent economic challenges and the COVID-19 pandemic have further reduced the access and opportunities to provide and assess physical activity and fitness among the children and youth in LMICs [6]. These gaps highlight the need to provide evaluation tools to assess cardiorespiratory fitness and physical activity in a simple, cost-effective and adaptable manner while maintaining their validity, reliability and reproducibility. This chapter will discuss the different cardiorespiratory fitness and physical activity tests for children and adolescents that can be employed in low-resource settings.

2. Methodology

The following online databases were used as sources for published articles: PubMed, BioMed Central, American Heart Association, ResearchGate, Public Library of Science (PLOS), and Multidisciplinary Digital Publishing Institute (MDPI). Individual terms or a combination of the following keywords were utilized to select relevant literature from the years 2019–2023: fitness, exercise, physical activity, children, adolescents, youth, cardiopulmonary, cardiorespiratory, field test, battery, evaluation, assessment, low-income countries, and middle-income countries. A few related studies older than 2019 that supported findings from the relevant articles were also reviewed (2004, 2006, 2013, 2017). Systematic reviews, studies conducted on populations of low resource countries, and updated clinical guidelines were included as the main source of content for this chapter.

3. Physiology of cardiorespiratory fitness (CRF), CRF testing and correlation with physical activity

The cardiovascular and respiratory systems are inextricably linked by a chain of processes from pulmonary ventilation and diffusion to cardiac contractility, vascular circulation and muscle cell oxygen and nutrient extraction to perform physical work and activity [7]. The best indicator of this integrated cardiorespiratory fitness to perform physical work is the person’s maximal oxygen consumption or aerobic capacity measured through their VO2Max. It is the oxygen consumed in liters per minute, or milliliters per kilogram per minute. Since oxygen consumption (VO2) increases with a given workload, the VO2Max is determined when VO2 reaches a plateau despite increasing work intensity [8, 9]. However, considerations for children not demonstrating a plateau during incremental exercise has prompted the use of the highest VO2 measured in the youth (VO2Peak) instead. This is analogous to the VO2Max in adults [1].

As per Fick’s Principle (cardiac output = oxygen consumption ÷ arteriovenous oxygen content difference), VO2Max is the product of the maximal arteriovenous oxygen difference, and the cardiac output [9]. This explains how the body ensures adequate oxygen delivery from our lungs, to the blood, through our heart, and eventually to our tissues and working muscles during times of physical stress such as exercise [10]. Any cardiac or pulmonary condition impairing gas exchange may serve as a barrier to achieving this. To further illustrate the physiologic changes that take place in a child and adolescent, we examine cardiac output, respiration, and arteriovenous oxygen difference.

During the early phase of physical exercise, the increase in cardiac output comes from the increase both in the left ventricular stroke volume and the end-diastolic volume. As the intensity of exercise progresses, the heart rate rises. Compared to adults, children and adolescents have a smaller left ventricular stroke volume during exercise, which causes a compensatory higher heart rate response [1, 10].

In terms of respiration, tidal volume physiologically increases with progressive physical exercise. Once the tidal volume reaches approximately half the vital capacity of the lungs, minute ventilation increases through increased respiratory rate. Developmental aspects affecting the efficiency of ventilation may be observed throughout childhood and adolescence, and is said to be due to decreasing respiratory rate, increasing tidal volume, and depth of breathing [1, 10].

The arteriovenous oxygen difference pertains to the concentration between arterial and venous blood, and indicates the amount of oxygen consumed by the muscles and organ systems during exercise. During maximal exercise, the arteriovenous oxygen difference between prepubertal boys and girls is zero. Post-puberty, boys would have a higher arteriovenous oxygen difference than girls. During submaximal exercise, arteriovenous oxygen difference is higher in children versus adults. This is said to be a compensatory mechanism by children for the lower cardiac output so as to extract more oxygen from the blood [10].

4. Cardiorespiratory fitness testing through the gas-analyzed graded cardiopulmonary exercise test (CPET)

All these parameters reflecting cardiorespiratory fitness are obtained during a cardiopulmonary exercise test (CPET). CPET is non-invasive, comprehensive, and is regarded as the gold standard method for evaluating physical fitness both in children and adults [11]. It provides a breath-by-breath monitoring of oxygen and carbon dioxide during exercise and the physiologic values discussed above. It is conducted in a laboratory and expertise is needed in its performance and interpretation. The equipment is expensive and calibration and maintenance are meticulous processes requiring expertise as well [1] Reference values for the pediatric population are available, however, caution is still needed in using these values since these can vary according to the type of protocol (whether the treadmill or cycle ergometer was used for exercise) and the correction factors applied to the reference population are sometimes not applicable in certain situations [12]. Lastly, walking or cycling as the activities used in CPET protocols may not reflect the actual types of physical activities that the youth commonly do on a regular basis [12].

Due to these limitations of the CPET, other tests have been utilized to assess cardiorespiratory fitness and these shall be discussed in the following sections. Cardiorespiratory fitness, moreover, is only one of the four major components of health-related fitness and the others are muscular fitness, flexibility and body composition [1]. Muscular fitness is the integrated function of muscle strength, endurance and power to exert maximal force against an external resistance or repeatedly under submaximal loads [1, 13]. Flexibility is the ability of an individual’s joint or series of joints to move through an unrestricted, pain free range of motion. Body composition, the fourth component, is the percentages of fat, muscle or fat-free tissue and total body water in the total body mass of an individual [1]. Specific tests are recommended for these other components of health-related physical fitness.

5. Cardiorespiratory fitness and physical activity correlation

Physical activity (PA) is the voluntary skeletal muscle movement producing energy expenditure and exercise is a subset of PA that is structured, planned and purposefully performed to improve or maintain one or more components of physical fitness [14]. PA is a complex behavior but is usually classified according to the four dimensions of (1) mode or type of activity, (2) frequency, (3) duration and (4) intensity of performing the activity [3, 14]. Regular physical activity, through exercise and training, improves CRF and greater CRF results to more capacity for physical activity, thus, creating a facilitative cycle of an “active-fit lifestyle [1].” The overall effect of improving both CRF and PA is the promotion of health and its related outcomes (Figure 1) [1, 3, 15].

Figure 1.

“Active-fit Lifestyle” facilitative cycle of cardiorespiratory fitness, physical activity and improvement in health-related outcomes among children. CRF: cardiorespiratory fitness.

The recommended physical activity to maintain general cardiopulmonary fitness among adults is well-known [4]. For children and adolescents, differences in the recommended duration, types of activities, and even hours of sleep, may be observed. The WHO recommends physical activities for different youth age groups as follows [4]:

For children under 5 years of age, specifically for those less than 1 year old, interactive floor-based play, several times a day is encouraged. For children under 1 year old who have not achieved mobility yet, at least 30 minutes in a prone position may be done multiple times throughout the day, during waking hours. Reading and storytelling should be included when sedentary, and exposure to screens and gadgets at this age is not recommended. 14–17 hours and 12–16 hours of good quality sleep is recommended for 0–3 months and 4–11 months of age, respectively. This is inclusive of naps during the day.

For children 1–4 years old, at least 180 minutes of varied physical activities at any intensity, distributed throughout the day, is encouraged. No more than 1 hour of sedentary screen time should be allowed for this age group. Good quality sleep should be around 11–14 hours for children 1–2 years old, and 10–13 hours for children 3–4 years old.

Strollers, high chairs, and other similar restraints (including being strapped on to the caregiver) should not go beyond 1 hour at a time within a 24-hour period.

For children and adolescents aged 5–17 years, it is recommended that they do at least an average of 60 minutes per day of moderate-to-vigorous intensity (mostly aerobic) activities distributed across the week. These activities, as well as strengthening exercises, should be done at least 3 days a week.

As in children younger than 5 years, screen time must be minimized or limited in this age group especially when sedentary.

6. Measuring physical activity

There is a variety of measures for PA that are generally divided into subjective or objective categories [3]. Questionnaires, activity logs, and diaries are utilized as self-report or staff-administered methods under the subjective category while objective tests include direct observation, heart rate monitoring, using doubly labeled water, accelerometers and pedometers [2].

Since PA involves a complex behavior with multiple dimensions and domains to consider, there is not one precise, biological marker nor is there one single test to reliably encompass all its facets and dimensions. It also becomes more difficult to quantify physical activity behavior in free-living children and adolescents because only a small segment of the population performs physical activity for the sole purpose of improving fitness or health [14]. Vigorous physical activity among children is also very intermittent with 96% of activity bouts shorter than 10 seconds [14]. Estimation of energy expenditures of childhood PA often uses adult-derived standard energy costs as there no comprehensive reference values available yet for the youth [2].

Nevertheless, PA measures are often utilized, especially the self-report methods, in population surveys and epidemiologic studies since these are the most practical and remain to be valuable in formulating recommendations and policies [2].

7. How to assess cardiorespiratory fitness (CRF) in LMIC’s

Gas-analyzed graded cardiopulmonary exercise test (CPET) is considered the gold standard for assessing cardiorespiratory fitness, however, the significant logistic limitations in procuring, operating and maintaining the metabolic cart and its accessories for CPET hinder its common use in low-middle income countries [1].

Non-gas analyzed tests that may be conducted in an office or on the field are the other tests recommended to assess CRF among children [1]. These are often submaximal exercise tests that estimate the energy expenditure and the CRF of the individual using equations or nomograms that have been validated with the gas-analyzed CPET [9, 16]. These submaximal exercise activities are often easy to perform with varying logistic requirement. Large measurement errors may occur and standardization of the predictive equations and nomograms for children for the different tests are not yet complete [1, 13, 16]. Nevertheless, the submaximal exercise tests that have been validated and that are easy to perform with the least needed equipment are the most appropriate to use in LMICs to assess CRF among children.

Three office-based submaximal exercise tests and two field tests are considered to be most feasible for use in low-resource settings.

7.1 Office-based exercise tests

7.1.1 45-second squat test

The 45-Second Squat Test is also known as the Ruffier-Dickson Test and has been used to classify cardiorespiratory fitness among adults and children [17]. It is simple and does not require any equipment but a timer and a device to check the heart rate.

The heart rate is measured for a full minute either with a pulse oximeter, or manually through auscultation with a stethoscope of heart sounds at the point of maximal impulse (PMI) prior to starting the test (P0). Afterwards, the subject is asked to perform 30 squats within 45 seconds. Proper positioning consists of knee flexion during squats being at approximately 90 degrees, with the subject’s back as straight as possible, and the shoulders flexed forward. Heart rates are recorded at 1–2 minutes into the test, immediately once completed (P1), and after 1 minute of rest (P2) [1, 17].

The Ruffier-Dickson Index (RDI) is then calculated as follows [18]:

$$RDI=\frac{\left(P1-70\right)+2\left(P2-P0\right)}{10}$$E1

Where:

P0 = heart rate prior to starting the test.

P1 = heart rate at 1–2 minutes into the test.

P2 = heart rate after 1 minute of rest.

The RDI indices would correspond to certain levels of exercise endurance capacity from very poor to excellent endurance [18].

The test of 30 squats in 45 seconds is considered a moderate to vigorous impact type of activity that corresponds to ~6 peak MET in oxygen uptake and is associated with a cardiorespiratory load of 128 beats/min peak heart rate [17]. Recent studies though in the pediatric population demonstrated a low validity using the initial RDI formula [19, 20]. A revised Ruffier-Dickson index for children has been validated and commonly used in Ukraine [19].

Ruffier-Dickson index adapted for children (Ic) [19]:

$$Ic=\frac{[P1+2\left(P2-P0\right)\frac{Pa}{Pp}-70}{10}$$E2

Where:

P0 = heart rate prior to starting the test.

P1 = heart rate at 1–2 minutes into the test.

P2 = heart rate after 1 minute of rest.

Pa = normal population heart rate at rest for adults (or 70/minute).

Pp = normal population heart rate at rest for young patients aged 6, 7 and 8 years, respectively (or 102, 98, 93/minute, respectively).

The derived pediatric RDI (Ic), according to the age, would correspond to a certain health level assessment from low to high levels (1–5) [19].

7.1.2 3-minute step test

As with the 45-Second Squat Test, the heart rate is measured for a full minute with a pulse oximeter, or manually through auscultation with a stethoscope of heart sounds at the point of maximal impulse (PMI) prior to starting the test (T0) [1]. A metronome is set at an established cadence (in the Bruggeman study, 96 steps per minute was used) and the subjects are asked to step up and down a step of 12 inches in height for three minutes at a rhythm of “up, up” and “down, down [20].” Heart rates are likewise measured at the first, second and third minutes (T1, T2, and T3) into the test, and again after 1 minute of recovery upon completing the test (T4). The Jacks et al. VO2max predictive formula is used to calculate the subject’s score using the values obtained [20].

The 3-Minute Step and 45-Second Squat Tests showed statistically significant correlations with VO2max [1, 20]. Furthermore, the Step Test shows a high Area Under the Receiver Operator Curve, illustrating its usefulness. Both tests have been found to be conducted safely at the clinic/office setting [20].

7.1.3 6-minute walk test

The 6-Minute Walk Test (6MWT) was deemed to have a relatively poor correlation with VO2Max in healthy children and adolescents to estimate CRF [1]. However, it is said to be a better indicator of the ability of patients to perform their daily activities n some clinical scenarios, and as such would correlate with quality of life measures [21].

In a systematic review by Mylius et al. of 22 studies about healthy participants aged ≤18 years, it sought to determine the reference values for the 6MWT [22]. The prediction equation found to be valid and that yielded the highest R2 value of 0.6 was as follows [22]:

$$\begin{array}{l}6\text{Minute Walking Distance}\\ =\left(4.63\times \text{height in}cm\right)-\left(3.53\times \text{weight in}kg\right)+\left(10.42\times age\right)+56.32\end{array}$$E3

The systematic review by Cacau and colleagues of reference values for the 6 MWT in healthy children and adolescents also supported that current existing reference equations accounted for well-established variables in the context of exercise performance (e.g. height, heart rate, age and weight) [23, 24]. Countries without established reference values for the 6MWT should be encouraged to conduct studies to establish these because it would provide their own clinicians and researchers with a precise interpretation of the test [23].

To date, a growing number of countries have been developing their own reference values and formulas to determine the maximum distance of each age group and sex, and deemed the 6MWT a safe, well-tolerated, and understandable test to conduct both in healthy children, as well as in those with cardiopulmonary and neuromuscular disorders [23].

For this particular test, its suggestion for use may be applied in children between 4 and 12 years old, for as long as they can follow instructions and are motivated in accomplishing the test [23].

The 6MWT requires a stopwatch, a measuring tape or stick, 30 meters of a walkway with obstacles removed, two markers (cones, tape, or any available weighted or stationary object) to determine the turn-around points or end-distances that need to be covered as well as markers on the wall 3 meters apart, a print out of the Borg’s Rate of Perceived Exertion scale in a size 20 font, a sphygmomanometer with an appropriately-sized cuff (must cover at least two-thirds or 70% of the upper arm), and a pulse oximeter to measure oxygen saturation and heart rate [21].

Before the test is started, the participant may be asked to sit on a chair for at least 10 minutes. At this time, the blood pressure may be measured with an appropriately-sized pediatric cuff on a sphygmomanometer, and the heart rate and oxygen saturation on a pediatric pulse oximeter. The Borg Rate of Perceived Exertion is asked while the patient is at rest [1, 24].

The participant is instructed to walk, not run or jog, continuously between the 2 endpoints marked with the stationary object, for a duration of 6 minutes. If they get tired, they may rest (lean against a wall, stop, or slow down), but are advised to continue walking until the 6 minutes are up. As soon as the participant starts walking, the timer must be started. Each time the participant returns to the starting line, mark the lap on the record sheet and the stopwatch. The additional distance covered (meters in the final partial lap) using markers on the wall 3 meters apart as distance guides, must also be recorded. Once the total distance is calculated, round it off to the nearest meter, and record it on the worksheet [21].

Standardized encouragement is provided as follows [21]:

• At the end of 1 minute: “You are doing well. You have 5 minutes to go.”

• At 4 minutes remaining: “Keep up the good work. You have 4 minutes to go.”

• At 3 minutes remaining: “You are doing well. You are halfway done.

• At 2 minutes remaining: “Keep up the good work. You have only 2 minutes left.

• With 1 minute remaining: “You are doing well. You only have 1 minute to go.

• With 15 seconds to go: “In a moment I’m going to tell you to stop. When I do, just stop right where you are and I will come to you.”

• Finally, at 6 minutes: “Stop.”

The examiner must not walk with the patient and must use an even tone in delivering the instructions. After the test, the Borg Rate of Perceived Exertion is asked and recorded again. If the participant reports any chest pain, intolerable dyspnea, leg cramps, staggering, diaphoresis, and pallor or ashen appearance, the test must be discontinued [1, 21].

7.2 Field-based exercise tests

Raghuveer and colleagues discussed different types of run tests that may easily be administered in the school setting, given a large cohort of children and adolescents, and may be used to guide population-based interventions for CRF improvement [1, 13]. Among these are the 20-meter Shuttle Run Test, the 1.5-meter Run Test, and the 12-Minute Run Test [1].

7.2.1 20-meter shuttle run test

The 20-meter shuttle run test (20mSRT) is also known as the beep test, bleep test, PACER (Progressive Aerobic Cardiovascular Endurance Run) test, and Multi-stage fitness test (MSFT) [1]. In this test, the subject is asked to run at increasing standardized paces (beginning with 5 miles/hour or 8 kilometers/hour, progressing with 0.3 miles/hour or 0.5 kilometers/hour increments each minute), between two points that are 20 meters apart. The test is stopped when the subject is no longer able to follow the set pace. The last announced stage number with its equivalent maximal aerobic speed is used to estimate the child’s VO2max using a predictive equation or it can be compared to normative values from a reference population [1, 25].

The 20mSRT is able to elicit a maximal effort in most children and induces the highest peak VO2 compared to other exercises [26]. It simulates the typical youth activities of running, sprinting, jogging and agility in changes in speed and direction. It has been widely studied in both sexes and across a wide range of ethnicities and ages. The criterion validity of the 20mSRT as a measure of CRF is moderate and its reliability is high to very high with a reliability coefficient of 0.78 to 0.93 in children aged 8–18 years [25]. Test-retest differences among children are also not significant. Like other tests for CRF, it must be noted that the maximal effort by the child may be influenced by factors such as physiology (fractional utilization of oxygen, VO2 kinetics, anaerobic capacity, and agility), body mass, motivation, tolerance to discomfort, and self-efficacy [25, 26].

The other advantages of the 20mSRT are its acceptability, feasibility, scalability, the ease in its interpretation and transferability of the results to relevant and health-related situations. The 20mSRT is often included in physical fitness test batteries [26]. It can be administered in group settings such as in schools, thus, large groups of youth can be tested simultaneously. It can be conducted indoors or outdoors and the external pacing can help mitigate pacing variability. The larger time gaps at the initial stages of the test serve as a warm up for most subjects. At the end of the test, children should have a cool-down period with light to moderate intensity aerobic activities lasting for 5–10 minutes [25].

The results from the 20mSRT can either be interpreted using criterion-referenced or normative-referenced standards [25, 26, 27]. Criterion-referenced health-related standards are used to identify pediatric subjects who may need health-related interventions or to screen those at increased risk of future diseases [26]. Screening using criterion-referenced standards are also used for recruitment into military-related or highly physical jobs and for athlete identification. Leger’s original predictive formula for estimating VO2 max from the 20mSRT is still regarded as the most valid equation, however, some authors noted large prediction errors may occur with VO2 max estimation [25, 26, 27]. These authors recommend reporting the number of laps completed as the basis for a child’s fitness assessment [25, 26].

Norm-referenced interpretation, on the other hand, is done when the aim is to compare the subject to his/her peers as is commonly done in physical education and similar situations. Normative data are available for over 1.1 million children and youth from 50 countries for the 20mSRT [25]. Developing countries such as Bosnia, Croatia, Estonia, and Tibet have such pediatric reference values for 20mSRT available [28, 29, 30, 31].

7.2.2 Distance or time-based run tests

In the 1.5-Mile or 2400-Meter Run Test (distance-based) the patient is instructed to complete the run in the shortest amount of time, while in the 12-Minute Run Test (time-based) and the patient is tasked to cover the greatest possible distance, with their best effort [1]. A recent meta-analysis showed moderate to high correlation of the 1.5-Mile (r = 0.79) and 12-Minute Run Test (r = 0.78) with VO2max as measured by CPET and with the 20mSRT as well [32].

7.3 Questionnaires to assess pediatric cardiorespiratory fitness

When submaximal fitness tests are not possible to conduct due to various causes, questionnaires maybe used to assess CRF [1]. Questionnaires are often used for epidemiologic and population-based studies but these methods will not be sufficient to estimate cardiorespiratory fitness in individuals [33].

Questionnaires range from those that assess physical activity to CRF as a single domain [1, 33, 34]. The World Health Organization currently recommends the Global Physical Activity Questionnaire (GPAQ) to standardize the reporting of physical activity for population-based surveys among the different countries [4]. It may not be applicable though to children, especially with the domain on occupational physical activity in the questionnaire [34]. A multi-country study across six continents has been initiated to develop and validate the Global Adolescent and Child Physical Activity Questionnaire (GAC-PAQ) and will be completed in year 2025 [35]. This will help standardize the language of physical activity assessment for children globally.

For the domain of CRF itself, the International Fitness Scale (IFIS) is one of the most validated tool in different countries for the pediatric population [33]. It is composed of five questions on physical fitness, CRF, muscular strength, speed and flexibility that uses a five-point Likert scale [36]. It is concise and can be completed in 5 minutes. IFIS response among children aged 12–18 years significantly correlated with CRF. The analysis of covariance showed adolescents reporting better fitness had higher measured fitness levels for all the variables studied (p < 0.001) regardless of gender, age and weight status [33].

The recent meta-analysis on the use of IFIS to predict CRF among children included two studies from LMICs and showed moderate to substantial reliability with test-retest reliability coefficients ranging from 0.40 to 0.99 [36].

7.4 CRF test batteries

Since health-related physical fitness (PF) is a multi-dimensional attribute, many proponents advocate for a battery of physical fitness tests that will reflect the features of cardiorespiratory fitness, muscle endurance and strength, flexibility, and body composition [26]. A 2021 meta-analysis identified 24 PF batteries for children and the most widely applied batteries were the “Eurofit”, “FitnessGram”, and “Alpha-fit [26].” All of the PF batteries contained cardiorespiratory fitness and upper body strength as components. The most common CRF tests included in the PF batteries, on the other hand, were the PACER and the 1-mile run/walk tests. Estimation of maximum aerobic capacity from these CRF tests have to be balanced with the other factors affecting physical fitness such as sex and adiposity [26].

All of the PF batteries were performed in school settings and some of the studies on PF test batteries in the meta-analysis were conducted in LMICs, hence, denoting the ease of implementation [26]. A PF battery called the Performance and Fitness (PERF-FIT) test battery was designed in South Africa for implementation primarily in low-resource settings [37]. This battery is an instructor-administered skill-related fitness test for children aged 5 to 12 years. The test combines movement skills, agility and power and utilizes task loading or progressive increase in task difficulty to all skill items. Testing equipment should not cost beyond 50 US dollars and these were composed of simple and cheap items such as tape measure, rectangular foam, and soda cans [37]. The content validity indices for the throw and catch items were 0.86 and 1.00, respectively, with the total validity score for the other items reaching 0.99 [37]. The PERF-FIT, therefore, has good validity and is easy to administer in low-resource settings [37].

Overall, the PF batteries utilized throughout the world are too varied and there are only a few studies that have validated these fitness tests in the LMICs. Global collaboration is needed to develop a universal field-based fitness test battery that can easily be implemented with non-specialized staff, low in cost, scalable, easily interpretable while remaining valid and reliable [38].

8. Use of wearable and smartphone technology to measure PA

Objective measurement of physical activity utilizes motion sensors on the subject and the rapid increase in the technology of wearable devices has benefited this field. Wearable technologies are devices with sensors, screen, processor, memory and software that have algorithms to store, filter, interpret and organize real time data of motion and physiologic events happening in the wearer. The main wearable devices to measure PA are the pedometers and accelerometers where the pedometer counts footsteps over a period of time, while the triaxial accelerometer measures acceleration in three planes during body motions [14].

A 2023 meta-analysis showed that varied wearable devices are used to measure PA for children [39]. The pedometers revealed low accuracy at slower speeds and when these were attached to certain clothing items (e.g. skirt part of dresses). Accelerometers provided more data such as the frequency, duration and intensity of PA. The ActiGraph, Actical and ActiTrainer accelerometers had the best measurement properties to assess common movement-related outcomes for school-based activities while, among the pedometers, the Yamax Digi-Walker (SW-200, SW-700 and 701) provided the best measurement for movement-related outcomes [39]. Overall, there are only a few studies on these technologies for school-based activities and most were conducted in high-resource countries [40].

A low-cost commercial smartwatch, the Xiaomi Mi Band (MB), was evaluated in some low-income countries and showed high agreement of MB step counts with subjective reports on physical activity and sleep-related measures [41]. Most of the subjects in this study, though, were adults [41].

Smartphone technology likewise offers a new avenue for physical activity surveillance among children. A 2023 systematic review gathered only cross-sectional and cohort studies that used mobile apps purposely developed for the studies or were downloadable from the Apple App Store or Android Play Store [42]. All studies were conducted in high income countries [42]. The data were not yet sufficient to assess the validity and reliability of smartphone technology for PA surveillance among children and adolescents [42].

9. Conclusion

Improvement in cardiorespiratory fitness among children and adolescents is a global goal so as to promote physical and mental health, improve quality of life and academic achievement and prevent future cardiovascular and chronic illnesses in this population [1, 4]. Assessment and monitoring of CRF are intrinsic processes in this journey of improving physical fitness. While gas-analyzed graded cardiopulmonary exercise testing is the acknowledged gold standard to assess CRF in children, simple office-based and field-based exercise tests are available for use and have been validated for individual and large-scale purposes [1]. Low- to middle-income countries can utilize these low-cost and simple sub-maximal exercise tests while also maximizing the use of readily available physical activity and physical fitness questionnaires. Wearable and smartphone technology are very popular in this age group and its rapid development also helps drive down its cost [39, 40]. More studies and urgent collaboration is needed among all countries to validate, simplify, standardize and implement these fitness tests, whether as individual tools or as a test battery, so that physical inactivity can indeed be significantly decreased among children and the youth by 2030 [38].

Conflict of interest

The authors declare no conflict of interest.