24-h movement guidelines and its association with health-related physical fitness in Brazilian adolescents

Abstract

Purpose

To examine the link between adherence to 24-h movement guidelines (isolated and combined) and muscular strength, cardiorespiratory fitness, and obesity indicators in Brazilian adolescents.

Methods

Data from 980 adolescents (14–19 years) in Florianópolis, Brazil, were analyzed. The dependent variables were body mass index (BMI), body fat, handgrip strength (HGS), maximal oxygen consumption ($\dot{\mathrm{V}}$O_2max). The independent variables were physical activity (PA, IPAQ), screen time (ST), sleep (bedtime, wake-up time). Compliance was calculated for each behavior and combinations. Multiple linear regression models were employed.

Results

Overall compliance: 4.1% (boys), 4.9% (girls). Positive associations were found between PA adherence and HGS/ $\dot{\mathrm{V}}$O_2max in both sexes, ST adherence and $\dot{\mathrm{V}}$O_2max, and adherence to all three guidelines and $\dot{\mathrm{V}}$O_2max. Girls showed positive associations between combined PA ​+ ​sleep adherence and HGS, ST ​+ ​sleep and $\dot{\mathrm{V}}$O_2max, and negative associations between adherence to two guidelines and BMI. Boys exhibited a negative association between PA and body fat, positive between ST and HGS, and positive/negative between combined PA ​+ ​ST adherence and HGS/body fat. Moreover, adherence to all three guidelines associated positively with HGS.

Conclusion

Adhering to 24-h movement guidelines, alone or in combination, benefits muscular strength and cardiorespiratory fitness in Brazilian adolescents. However, simultaneous adherence did not correlate with obesity indicators.

Graphical abstract

List of abbreviations

PA

Physical Activity

ST

Screen time

PA ​+ ​ST

Physical Activity ​+ ​Screen time

PA ​+ ​Sleep

Physical Activity ​+ ​Sleep

ST ​+ ​Sleep

Screen time ​+ ​Sleep

BMI

Body Mass Index

HGS

Handgrip Strength

BM

Body Mass

mCAFT

modified Canadian Aerobic Fitness Test

$\dot{\mathrm{V}}$O_2max

Maximal Oxygen Consumption

1. Introduction

Maintaining muscle strength, cardiorespiratory fitness and body composition, components of health-related physical fitness, contribute to better carrying out daily activities and building an autonomous and long-lasting life.¹ Conversely, low levels of muscular strength² cardiorespiratory fitness,³ and high levels of body fat⁴ are associated with an increased risk of all-cause mortality.

Among the correlates for promoting health-related physical fitness indicators are movement behaviors that can be performed 24 ​hour (h) a day, including physical activity (PA), sleep duration and sedentary behavior measured by screen time (ST).^5,6 Regular PA is associated with the maintaining and improving muscular strength,² improving cardiorespiratory fitness,³ and controlling body fat.⁷ Short sleep duration has been associated with an increased risk of obesity,⁸ decreased cardiorespiratory fitness⁹ and muscular strength.¹⁰ Longer periods of ST have been associated with a greater risk of obesity,^11,12 decline in muscular strength and cardiorespiratory fitness.^11,13

The 24-h movement guidelines, as advocated by the Canadian Society for Exercise Physiology, recognizes their interconnectedness, suggesting that changes in one behavior can influence the others' duration.⁵ Studies indicate associations between these behaviors and cardiorespiratory fitness,¹⁴ although the relationship with muscular strength^14,15 and body fat^16,17 presents divergent results. However, even though the adoption of 24-h movement guidelines contributes positively to overall health,^5,6 the prevalence of adolescents meeting all three guidelines for these behaviors was 3.54% for boys and 1.86% for girls.¹⁸ In Brazil, the prevalence of meeting guidelines in children and adolescents was 11.7% and fell to 7.5% after COVID-19 isolation.¹⁹ These numbers underscore the scarcity of individuals who comprehensively integrate such healthy practices into their routines.

According to the literature, some 24-h movement guidelines are more likely to impact health-related physical fitness indicators. Although combinations of 24-h movement guidelines may coexist within the same individual, their combined effect on these indicators has been scarcely investigated.^14,16,17 The available evidence suggests that the effect of 24-h movement guidelines depends on the specific combination adopted by an individual. However, the combination of 24-h movement guidelines that has a higher or lower impact on health-related fitness indicators remains unknown. Therefore, investigating combinations of 24-h movement guidelines and their associations with health-related physical fitness may contribute to the development of more targeted preventive strategies to improve adolescents' health.

Therefore, this study aimed to investigate the association between adherence to 24-h movement guidelines individually (PA, sleep, ST), in combinations (PA ​+ ​sleep, PA ​+ ​ST, ST ​+ ​sleep) and simultaneous adherence to the three guidelines recommendations (PA ​+ ​ST ​+ ​sleep) with muscle strength, cardiorespiratory fitness, and obesity indicators according to sex. Based on this, we stated the following hypotheses: (a) adherence to the 24-h movement guidelines would be associated with better body composition, muscle strength, and cardiorespiratory fitness; and (b) adherence to a greater number of guidelines (3 ​> ​2 ​> ​1 ​> ​0) would correlate more strongly with these health outcomes.

2. Methods

This is a cross-sectional study written according to STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) recommendations. Data for the present study were collected in 2017 and 2018 in the city of Florianópolis, Santa Catarina, Brazil. They came from a macro-project entitled “Levels of physical activity, physical fitness and health-related social behavior in adolescents: A secular trend study”.

2.1. Ethical approval

The macroproject of the present study was approved in July 2017 by the Research Ethics Committee with Human Beings of the Santa Catarina State University (protocol No. 2,172,699). Written informed consent was obtained from parents/guardians and assent from adolescents under 18 years of age. All ethical principles were followed throughout the study, in accordance with the Declaration of Helsinki.

2.2. Data

The study population comprised adolescents aged 14–19 years enrolled in public schools in Florianópolis. The sample size calculation was based on the high school population (10 119 students), considering a 95% confidence level, a 4% margin of error, an estimated 50% prevalence for an unknown outcome, and a design effect of 1.0.²⁰ An additional 10% was added to the sample number to mitigate potential sample losses, reaching a minimum sample of 624 adolescents.

For data collection, authorization was initially sought from the Florianópolis Education Management (GERED) to conduct the research within the public school system. After receiving authorization, the study aimed to better represent the city demographically by considering the five regions defined by the Municipal Health Department (Continent, Center, East, North, and South). The school with the largest number of high school students in each region was invited to participate. In each participating school, classes were randomly selected, and all adolescents were invited to participate voluntarily. This process continued until the target number of students was reached in each region. All data were collected on school premises, either in the classroom or in an appropriate environment when necessary. Adolescents who voluntarily agreed to participate and did not have any physical or mental conditions preventing them from completing the questionnaire or participating in the physical test battery were considered eligible.

2.3. Dependent variables

Height measurements were taken using a portable stadiometer from Sanny® (São Paulo, Brazil) (CSEP, 2021), with a resolution of 0.1 ​cm. Body mass was measured using a digital scale from Tanita® (São Paulo, Brazil), with a resolution of 100 ​g and values recorded in kilograms (kg).²¹ Body Mass Index (BMI) was estimated as a continuous variable (kg·m⁻²). For the estimation of relative body fat,²² two non-consecutive measures of triceps and subscapular skinfolds were taken (a third measure was taken if there was a difference of 5% or more between the first two measures)²¹ using a caliper from Cescorf® (Porto Alegre, Brazil), with a resolution of 0.1 ​mm. To ensure assessment quality, intra-class correlation (ICC 3, k) and Coefficient of Variation (CV) mean was analyzed between triceps (ICC: 0.99; CV: 2.5%) and subscapular measurements (ICC: 0.99; CV: 2.6%), ICC values greater than 0.90 indicate excellent reliability and the CV is within the tolerable 5%.^23,24

Muscular strength was assessed through Handgrip Strength (HGS) using a digital dynamometer from JAMAR® model Plus+, with a resolution of 0.1 ​kg (Chicago, USA). The test application protocol was detailed in a previous study,²⁵ according to battery guidelines.²¹

To normalize HGS values based on adolescents' body size, the following allometric equation was adopted^25,26:

$$Y=a{x}^{k_1}{z}^{k_2}$$ (1)

In the equation described above, “Y” refers to the variable aimed at eliminating the effect of body size (i.e., HGS), and “x” and “z” are the independent variables (body mass and stature). Additionally, the equation has a proportionality coefficient “a” and allometric exponents “k₁” and “k₂”. The allometric exponents are determined through linear regression after logarithmic transformation of the values of “Y,” “x,” and “z”:

$$\log \left(Y\right)=\log \left(a\right)+k_1\log \left(x\right)+k_2\log \left(z\right)$$ (2)

For analysis purposes, k₁ and k₂ were determined according to gender. The following gender-specific exponents were used to normalize the HGS results for body mass (BM) and height (h):

boys, HGS ​= ​aBM^0.329 × h^1.771; girls, HGS ​= ​aBM^0.265 × h^1.519.

Cardiorespiratory fitness was assessed using the modified Canadian Aerobic Fitness Test - mCAFT, measured through the step test.²¹ The test involves eight stages of ascents and descents on two steps, 20.3 ​cm high (totaling 40.6 ​cm), rhythmically through a musical sequence. Each stage lasts 3 ​min and has a 20-second (s) break. The stage and initial speed are determined by age and sex. The maximum heart rate (HRmax) was estimated using the formula: 220 - age, measured with a Polar® heart rate monitor at the end of each stage. The test is stopped when the participant reaches 85% of their HRmax, recording the last stage completed if this occurs in the middle of the stage.²¹

For the estimation of $\dot{\mathrm{V}}$O_2max (Maximal oxygen consumption), the test battery provides a reference value for oxygen expenditure during the test according to the completed final stage calculated with the following formula:

$$Estimated{O}_{2\mathit{max}}:[17.2+(1.29\times O_{2}cost)–(0.09\times wt.inkg)–(0.18\times ageinyears)]$$

2.4. Independent variables

The PA level was assessed using the International Physical Activity Questionnaire (IPAQ - short version) validated for Brazilian adolescents.²⁷ Adolescents aged 14–17 years old were considered physically active if they practiced moderate to vigorous PA on average 60 ​minute (min) per day during the week. For those aged 18 to 19, physically active adolescents were considered those engaging in at least 150 ​min of moderate to vigorous PA per week.^5,6 Participants whose activity levels fell below these criteria were classified as insufficiently active.

The ST was evaluated considering time spent on electronic devices (television, computer, and video games) on weekdays and weekends.²⁸ Daily screen time for each device was calculated using the equation [(weekday time ∗ 5 ​+ ​weekend time ∗ 2)/7]. Total daily screen time was then determined by summing the values for television, computer, and video games. For adolescents aged 14 to 17, up to 2 ​h of daily screen time was considered adequate, while for those aged 18 to 19, up to 3 ​h was deemed appropriate.^5,6 Adolescents exceeding these limits were classified as having excessive ST.

Sleep duration was analyzed using bedtime and wake time on weekdays and weekends. The average sleep duration was calculated with the formula:

$$[(\text{weekday sleep hours}\ast 5+\text{weekend sleep hours}\ast 2)/7].$$

For adolescents aged 14–17 years, 8–10 ​h of sleep per day was considered adequate, while 7–9 ​h per day was deemed adequate for those aged 18 to 19.^5,6 Sleep durations outside these ranges were classified as inadequate.

In addition to investigating the relationship between each movement behavior individually (PA, ST, sleep), different combinations of adherence to the guidelines (PA ​+ ​sleep; PA ​+ ​ST; ST ​+ ​sleep), were evaluated, as well as the number of 24-h movement guidelines simultaneously met by each individual (0, 1, 2, and 3 guidelines) were assessed.

2.5. Covariates

Adolescents completed a questionnaire to gather sociodemographic data, including sex, age, and socioeconomic level, estimated by the Economic Classification Criterion of Brazil.²⁹ This economic level questionnaire generates a score from 0 to 100, evaluating the education level of the head of the family, material assets and basic sanitation. The higher the score, the higher the economic level.

In addition to the sociodemographic information, sexual maturation was assessed using the development of pubic hair, based on adapted Tanner stages, as proposed by Adami and Vasconcelos.³⁰ The instrument includes five illustrations representing different stages of sexual maturation: Illustration 1 represents the infantile state (pre-pubescent), Illustration 2, to 4 represent the maturation process (pubescent), and Illustration 5 represents the mature adult state (post-pubescent). Adolescents selected the illustration that best matched their current stage of development. For analysis purposes, the stages of sexual maturation were used in their original structure, with responses ranging from 1 to 5.

2.6. Data analysis

All analyzes were carried out in IBM SPSS Statistics 20.0, adopting a significance level of p ​< ​0.05. Descriptive statistics were used using frequencies, means and 95% confidence intervals (CI). Independent samples t-tests assessed differences between sexes in quantitative variables, while the χ² test identified differences between categorical variables.

To investigate the association between adherence to 24-h movement guidelines (analyzed as individual variables, combinations of two guidelines, and the number of simultaneous guidelines adopted), non-compliance with any movement guidelines was used as the reference category. Multiple linear regression models, adjusted for covariates, were employed to analyze the outcomes: muscular strength, cardiorespiratory fitness, body fat, and BMI. The results were presented as regression coefficients (β) with standard errors (SE), 95% confidence intervals (CIs), and coefficients of determination (R²). Considering the potential influence of sex in adopting 24-h movement guidelines (Tapia-Serrano et al., 2022), interactions between these variables in association with the outcome variables were tested in regression models. A p-value < 0.10 for the interaction term indicated heterogeneity in associations by sex.³¹ Therefore, the results of the analyses were stratified by sex.

3. Results

In this study, the final sample comprised 1 026 adolescents. Thirty-two adolescents were excluded due to missing behavioral information (PA, ST, and sleep), seven were older than 19 years, six did not complete the cardiorespiratory fitness test, and one did not report sexual maturation. This resulted in a final sample of 980 adolescents, with the majority being male (51.3%).

Analyzing sex differences, boys were older, had higher economic status, greater height and body mass, lower body fat percentage, and higher $\dot{\mathrm{V}}$O_2max values compared to girls. Additionally, a majority of boys (56.7%) were post-pubertal, while most girls were pubertal (65.8%) (Table 1).

Table 1.

General sample information according to sex.

Variables	Male n ​= ​503 Mean (95%CI)	Female n ​= ​477 Mean (95%CI)	p-value	ES
Age (years)	16.6 (16.5–16.7)	16.4 (16.3–16.5)	0.040	0.07
Economic level (score)	40.0 (39.0–40.9)	37.5 (36.6–38.5)	< 0.001	0.11
Height (m)	1.77 (1.76–1.78)	1.66 (1.65–1.66)	< 0.001	0.66
Body mass (kg)	66.8 (65.7–68.0)	58.1 (57.0–59.1)	< 0.001	0.33
BMI (kg·m−2)	21.2 (20.9–21.6)	21.2 (20.8–21.5)	0.716	0.01
Body fat (%)	17.8 (17.2–18.4)	26.7 (26.2–27.2)	< 0.001	0.58
Normalized HGS (kg/BMk1·hk2)∗	3.8 (3.8–3.9)	4.3 (4.2–4.4)	< 0.001	0.31
$\dot{\mathbf{V}}$O2max(ml·kg−1·min−1)	46.7 (46.2–47.2)	39.7 (39.4–40.0)	< 0.001	0.59
	n (%)	n (%)	p-value
Sexual maturation			< 0.001
Pre-pubescent	2 (0.4)	10 (2.1)
Pubescent	216 (42.9)	314 (65.8)
Post-pubescent	285 (56.7)	153 (32.1)

95% CI: 95% confidence interval; ES: Effect Size; BMI: Body Mass Index; HGS: Handgrip strength; BM: body mass; h: height; ml·kg⁻¹·min⁻¹: Milliliters of oxygen absorbed per kilogram of body mass within a 1-min timeframe; $\dot{\mathrm{V}}$O_2max: Maximum oxygen consumption; kg·m⁻²: kilograms per square meter; m: meters; kg: kilograms. ∗Despite the observed differences, HGS values were normalized according to sex, resulting in different allometric exponents.

Differences in the adoption of 24-h movement guidelines were observed between sexes (p ​< ​0.05). Boys exhibited a higher prevalence of meeting PA and the combination of PA and sleep recommendations, whereas girls showed a higher prevalence of meeting ST and the combination of ST and sleep recommendations. Only 4.2% of boys and 5.0% of girls simultaneously met all three guideline recommendations. There were no significant differences between sexes in simultaneous adherence to all three behaviors (Fig. 1).

Fig. 1.

Prevalence of adolescents who meet the guidelines for physical activity, screen time and sleep and their combinations according to sex (male – a; female – b).

Table 2 displays the findings from the isolated and combined associations of 24-h movement guidelines with BMI and body fat percentage. Regarding body fat percentage, meeting PA recommendations and simultaneous adherence to PA and ST recommendations showed a negative association. However, among boys, no isolated or combined associations were found between guideline adherence and BMI. Among girls, meeting at least two guidelines was negatively associated with BMI, marking the sole significant association observed.

Table 2.

Isolated and combined associations of 24-h movement behaviors with BMI and fat percentage according to sex.

		BMI			Body Fat (%)
Behaviors		β (SE)	95% CI	R2	β (SE)	95% CI	R2
Male	Physical activity	0.13 (0,33)	−0.52 to 0.77	0.033	−1.55 (0.60)	−2.72 to −0.38∗	0.027
	Screen time	0.29 (0.41)	−0.50 to 1.09	0.034	−0.56 (0.75)	−2.03 to 0.92	0.014
	Sleep	0.38 (0.33)	−0.28 to 1.04	0.036	0.91 (0.62)	−0.31 to 2.12	0.017
	PA ​+ ​ST	−0.05 (0.51)	−1.06 to 0.96	0.033	−2.69 (0.94)	−4.54 to −0.85∗	0.029
	PA ​+ ​Sleep	0.53 (0.39)	−0.24 to 1.30	0.037	−0.26 (0.73)	−1.69 to 1.16	0.013
	ST ​+ ​Sleep	0.57 (0.60)	−0.61 to 1.76	0.035	0.59 (1.12)	−1.61 to 2.78	0.014
	24-h movement recommendations met			0.025			0.018
	Not recommendations	Reference	Reference		Reference	Reference
	One recommendation	0.29 (0.42)	−0.54 to 1.11		0.31 (0.78)	−1.22 to 1.84
	Two recommendations	0.45 (0.45)	−0.44 to 1.34		−0.74 (0.83)	−2.38 to 0.89
	Three recommendations	1.01 (0.88)	−0.72 to 2.73		−0.98 (1.62)	−4.17 to 2.21
Female	Physical activity	−0.57 (0.39)	−1.33 to 0.19	0.013	−0.85 (0.58)	−1.99 to 0.29	0.007
	Screen Time	−0.24 (0.38)	−0.98 to 0.50	0.010	−0.12 (0.56)	−1.23 to 0.99	0.003
	Sleep	−0.15 (0.35)	−0.83 to 0.54	0.009	0.25 (0.53)	−0.78 to 1.28	0.003
	PA ​+ ​ST	0.26 (0.60)	−0.93 to 1.45	0.009	0.75 (0.91)	−1.03 to 2.52	0.004
	PA ​+ ​Sleep	−0.41 (0.49)	−1.36 to 0.55	0.010	−0.24 (0.73)	−1.67 to 1.19	0.003
	ST ​+ ​Sleep	0.08 (0.48)	−0.86 to 1.02	0.009	0.94 (0.72)	−0.47 to 2.5	0.006
	24-h movement recommendations met			0.022			0.020
	Not recommendations	Reference	Reference		Reference	Reference
	One recommendation	−0.66 (0.43)	−1.51 to 0.19		−0.90 (0.65)	−2.17 to 0.37
	Two recommendations	−1.09 (0.49)	−2.04 to −0.14∗		−1.42 (0.73)	−2.84 to 0.01
	Three recommendations	0.21 (0.86)	−1.47 to 1.90		1.66 (1.28)	−0.85 to 4.18

Analysis adjusted for age, sexual maturation, and economic level.

∗: p ​< ​0.05; ∗∗: p ​< ​0.001; β: Regression coefficients; SE: Standard Error; 95% CI: 95% confidence interval; R²: Determination coefficient; BMI: Body Mass Index; PA: Physical activity; ST: Screen time.

Table 3 presents the results regarding the association between individual movement guidelines, combinations of two guidelines, and the simultaneous adoption of all three 24-h movement guidelines with normalized muscle strength and $\dot{\mathrm{V}}$O_2max. In boys, meeting PA and ST recommendations showed positive associations with both muscle strength and $\dot{\mathrm{V}}$O_2max. Additionally, combinations such as PA and ST, and adherence to all three guidelines simultaneously, were positively associated with these outcomes.

Table 3.

Isolated and combined associations of 24-h movement behaviors with normalized HGS and $\dot{\mathrm{V}}$O_2max according to gender.

		Normalized HGS			$\dot{\mathrm{V}}$O2max
Behaviors		β (SE)	95% CI	R2	β (SE)	95% CI	R2
Male	Physical activity	0.24 (0.06)	0.12 to 0.36∗∗	0.050	2.52 (0.51)	1.52 to 3.51∗∗	0.059
	Screen time	0.21 (0.08)	0.06 to 0.35∗	0.035	1.52 (0.63)	0.28 to 2.77∗	0.023
	Sleep	0.02 (0.06)	−0.10 to 0.14	0.020	−0.30 (0.53)	−1.44 to 0.73	0.013
	PA ​+ ​ST	0.37 (0.09)	0.18 to 0.55∗∗	0.049	3.11 (0.80)	1.54 to 4.67∗∗	0.041
	PA ​+ ​Sleep	0.09 (0.07)	−0.06 to 0.23	0.023	0.92 (0.62)	−0.29 to 2.13	0.016
	ST ​+ ​Sleep	0.15 (0.11)	−0.07 to 0.37	0.024	0.54 (0.95)	−1.33 to 2.41	0.013
	24-h movement recommendations met			0.051			0.042
	Not recommendations	Reference	Reference		Reference	Reference
	One recommendation	0.16 (0.08)	0.01 to 0.31∗		1.19 (0.65)	−0.10 to 2.48
	Two recommendations	0.32 (0.08)	0.15 to 0.48∗∗		2.66 (0.70)	1.28 to 4.04
	Three recommendations	0.37 (0.16)	0.06 to 0.69∗		2.72 (1.37)	0.04 to 5.41∗
Female	Physical activity	0.18 (0.08)	0.02 to 0,34∗	0.019	0.73 (0.33)	0.08 to 1.37∗	0.022
	Screen time	0.11 (0.08)	−0.05 to 0.26	0.013	0.63 (0.32)	0.01 to 1.26∗	0.020
	Sleep	0.2 (0.07)	−0.13 to 0.16	0.009	0.01 (0.30)	−0.58 to 0.59	0.012
	PA ​+ ​ST	0.20 (0.13)	0.05 to 0.45	0.015	0.85 (0.51)	0.16 to 1.85	0.018
	PA ​+ ​Sleep	0.24 (0.10)	0.04 to 0,44∗	0.021	0.61 (0.41)	−0.20 to 1.42	0.016
	ST ​+ ​Sleep	0.04 (0.10)	−0.16 to 0.23	0.010	0.88 (0.40)	0.09 to 1.68∗	0.022
	24-h movement recommendations met			0.019			0.025
	Not recommendations	Reference	Reference		Reference	Reference
	One recommendation	0.05 (0.09)	−0.16 to 0.23		0.18 (0.37)	−0.54 to 0.90
	Two recommendations	0.17 (0.10)	−0.03 to 0.37		0.63 (0.41)	−0.18 to 1.44
	Three recommendations	0.29 (0.18)	−0.07 to 0.64		1.62 (0.73)	0.19 to 3.05∗

Analysis adjusted for age, sexual maturation, and economic level.

∗: p ​< ​0.05; ∗∗: p ​< ​0.001; β: Regression coefficients; SE: Standard Error; 95% CI: 95% confidence interval; R²: Determination coefficient; HGS: Handgrip strength; $\dot{\mathrm{V}}$O_2max: Maximum oxygen consumption; PA: Physical activity; ST: Screen time.

For girls, meeting PA recommendations and simultaneous adherence to PA and sleep recommendations were positively associated with normalized muscle strength. Regarding $\dot{\mathrm{V}}$O_2max, meeting recommendations for PA and ST individually showed positive associations. Simultaneous adherence to ST and sleep recommendations, as well as meeting all three guidelines, were positively associated with $\dot{\mathrm{V}}$O_2max in combined analyses.

4. Discussion

The results of this study revealed that adherence to all three 24-h movement guidelines was linked to higher cardiorespiratory fitness levels in both boys and girls. However, only boys showed a positive association between adherence to all three recommendations and muscular strength. These findings align with existing literature on cardiorespiratory fitness,¹⁴ but contradict previous studies that found no association between muscular strength and adherence to 24-h movement guidelines.^14,15 This difference can possibly be attributed to the method of expressing muscular strength; the present study used allometry, a procedure recommended to minimize the influence of body size on muscular strength results. In contrast, previous studies used absolute data or normalization using simple ratios, which may compromise the accuracy of the results.³²

It is hypothesized that the positive association between the number of 24-h movement guidelines and improved muscular strength and cardiorespiratory fitness could be attributed to the cumulative effects of each behavior. For example, regular PA provides mechanical stimuli to the muscles, causing neural adaptations and increasing metabolic capacity in muscle cells, thus increasing muscle strength levels.^33,34 In relation to cardiorespiratory fitness, PA improves the transport and use of oxygen by mitochondrial metabolism, as well as improving the diffusion and use of oxygen by skeletal muscle, promoting improvements in $\dot{\mathrm{V}}$O_2max.³⁵

Insufficient sleep duration has been linked to disruptions in endocrine secretions, including elevated nocturnal cortisol levels and reduced concentrations of anabolic hormones like growth hormone (GH). In contrast, adequate sleep is associated with increased GH release and suppression of the hypothalamic-pituitary-adrenal axis and sympathetic nervous system.³⁶ Therefore, it is theorized that adequate sleep duration directly impacts muscular strength and aerobic fitness by supporting regular GH secretion, which aids in increasing muscle mass without affecting contractile strength or fiber composition, and by regulating metabolic changes and oxygen uptake.^37,38

Furthermore, it is believed that the control of sedentary behavior (in general) can prevent decreases in FFM and an increase in fat mass, considering that high ST can contribute to the decrease in skeletal muscle function.^11,39 Therefore, it is hypothesized that adopting a greater number of movement behaviors over 24 ​h could minimize metabolic disturbances induced by sedentary behavior, which negatively impact lean mass,¹¹ and consequently muscular strength.^11,33 As for cardiorespiratory fitness, reducing sedentary behavior can lead to increases in peak $\dot{\mathrm{V}}$O_2max of 2.18 ​mL·kg⁻¹·min⁻¹ in adults, without the addition of physical activity.¹³ This may occur due to better cardiovascular responses provided by increased muscle activity and metabolic demands, as well as promoting better dilation of blood vessels, lower sympathetic nervous system activity, lower-grade systemic inflammation, blood pressure, and oxidative and vascular stress.^11,13

Unlike previous studies,^16,17 this study did not find associations between meeting all guidelines and obesity indicators. However, among girls, adhering to at least two guidelines showed a negative association with BMI. This could be attributed to a healthier lifestyle promoting improved dietary habits,⁴⁰ which are crucial in managing weight as diet plays a significant role in overweight and obesity.^17,41

It is notable that PA, whether alone or combined with other behaviors, was consistently linked to health-related physical fitness indicators, except for the “ST ​+ ​Sleep” association with cardiorespiratory fitness in girls. This aligns with Tapia-Serrano et al.¹⁴ who found PA to be the primary determinant of physical fitness. Thus, increasing physical activity can lead to beneficial adjustments in other 24-h movement guidelines, promoting overall health-related physical fitness.

In this sense, the different associations observed between boys and girls may stem from girls' lower adherence to physical activity recommendations, as previously observed in studies.^18,42,43 This lower adherence could be due to socio-cultural barriers that hinder girls' participation in PA,^42,43 leading to reduced enjoyment and confidence in their physical abilities, compounded by social stereotypes and prejudices.⁴⁴ Therefore, addressing these barriers inclusively and sensitively in policy and strategy development is crucial to promoting equitable participation in physical activities among adolescents, especially enhancing female engagement.

The study has several limitations that merit acknowledgment. Its cross-sectional design prevents establishing causality for the observed associations. The use of questionnaires to assess 24-h movement guidelines introduces recall bias and requires dichotomizing continuous data. Using accelerometers could offer more precise data and allow for Compositional Data Analysis. However, the questionnaires used were validated, similar to instruments used in global research. Additionally, the equation used to estimate maximum heart rate is a study limitation; validated equations or maximal effort tests are recommended for accurate maximum heart rate determination.

Despite the acknowledged limitations, the present study demonstrates several strengths. It provides a holistic view of how 24-h movement guidelines contribute to physical performance (muscle strength and $\dot{\mathrm{V}}$O_2max) and obesity indicators in adolescents, as these aspects are commonly investigated separately. Furthermore, by considering the behavioral and physical fitness differences between the sexes, the study offers the opportunity to provide appropriate guidance to mitigate the health inequalities observed between boys and girls. Another important factor is the use of allometry to adjust for the influence of body size on handgrip strength measurements, enhancing the accuracy of the results and further bolstering the study’s robustness. Furthermore, we accounted for the sexual maturation in our analysis, a factor often neglected in similar studies. Lastly, investigating Brazilian adolescents enable a deeper interpretation and geographical comparison of the benefits associated with adherence to the 24-h movement guidelines.

In conclusion, adherence to 24-h movement guidelines improves muscle strength and cardiorespiratory fitness in adolescents, but does not significantly impact indicators of obesity. Different combinations of adherence affect physical fitness differently between boys and girls, highlighting the importance of PA in promoting general health. Highlighting the need for integrated and holistic approaches to adolescent health, especially given the low rates of adherence to these guidelines.

These results can support school programs and health policies that promote the adoption and improvement of healthy movement behaviors in adolescents, grounded in local data. Adhering to movement guidelines contributes positively to adolescents' physical fitness and overall health, which in turn aids in disease prevention and reduces the future economic burden on public health systems.

CRediT authorship contribution statement

Jean Carlos Parmigiani De Marco: Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Formal analysis, Data curation, Conceptualization. Tiago Rodrigues de Lima: Writing – review & editing, Visualization, Validation, Supervision, Methodology, Formal analysis. André de Araújo Pinto: Writing – review & editing, Supervision, Project administration, Investigation. Javier Brazo-Sayavera: Writing – review & editing, Visualization. Andreia Pelegrini: Writing – review & editing, Visualization, Validation, Supervision, Project administration, Methodology, Investigation, Conceptualization.

Ethical approval statement

Ethical approval was obtained from the Ethics Committee of the Santa Catarina State University in July 2017 (protocol No. 2,172,699). Informed consent was obtained from each participant. All ethical principles were followed throughout the study, in accordance with the Declaration of Helsinki.

Funding information

This work was supported by the [Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES)] Finance Code 001; and [Fundação de Amparo à Pesquisa e Inovação do Estado de Santa Catarina (FAPESC)] under Grant [2017TR646].

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.