The PRISMA recommendation was followed for this study, available at http://prisma-statement.org/. This systematic review was registered with the International Prospective Registry of Systematic Reviews (PROSPERO), registration number: CRD42018082050.

Electronic searches were conducted until October 2023, regardless of the year of publication, in the following databases: ScienceDirect, Medical Literature Analysis and Retrieval System Online- MEDLINE (via PubMed), Cumulative Index to Nursing, and Allied Health Literature - CINAHL, Scientific Electronic Library Online – SciELO, and Web of Science. The search strategy was performed with the terms "respiratory muscles" combined with "maximal respiratory pressure" or "maximal inspiratory pressure" or "maximal expiratory pressure" and "reference values" or "reference equations" or "normal values" and "child" or "adolescent." These terms were adapted according to the requirements of the different databases. To illustrate for the SciELO database the following search strategy was used: (respiratory muscles) AND (maximal respiratory pressure) OR (maximal inspiratory pressure) OR (maximal expiratory pressure) AND (reference values) OR (reference equations) OR (normal values) AND (child) OR (adolescent). In addition, through the EndNote software (www.endnote.com), the database studies were grouped, and all duplicates were removed. A manual search was performed from the reference list of all included articles, adding relevant studies.

Inclusion criteria consisted of original studies that determined normative values and/or reference equations for MIP and/or MEP in healthy children and/or adolescents; regardless of the year of publication; published in English, Portuguese, or Spanish. Studies involving obese individuals or athletes, as well as theses and dissertations, or studies presenting the same sample, were excluded.

Two independent reviewers (NP and OR) selected titles and abstracts to identify possible relevant studies. The full-text of the selected studies were analyzed to determine whether they met the inclusion criteria. A third reviewer (DV) was asked to resolve disagreement between the two reviewers. Each selected study was independently reviewed using a data extraction form.

Data were extracted using a standardized form that included the following sections (1) study characteristics (author, journal, year, number, volume, location, authors contact, title, and purpose); (2) participant characteristics (number, age, sex, height, weight, body mass index (BMI), lung function, level of physical activity, and ethnicity); (3) technical aspects (type of mouthpiece, air leak orifice size, pressure recorded, type of manometer, initial lung volume to perform the test, nose clip, the duration of maneuver maintenance, interval time between MIP tests, interval time between MEP tests, interval time between MIP and MEP tests, number of attempts, prior training, interruption criteria, verbal instructions and demonstrations, and equipment calibration). For the meta-analysis, the studies were stratified into two groups: 4–11 years and 12–19 years.

The methodological quality of each study was assessed by relevant items from QUADA-2 scale (Quality Assessment of Diagnostic Accuracy Studies), which is an evidence-based quality assessment tool used in systematic reviews of diagnostic accuracy. QUADAS-2 verifies the risk of bias through four domains: patient selection, index test, reference standard, and flow and time, also verifying the applicability with the same domains, except flow and time. The tool consists of questions to guide the classification and must be answered with "yes," "no," or "unclear". If there is a "no" answer, it is considered a high risk of bias. If all the answers are "yes”, then low risk. Finally, the risk of bias is considered "high," "low," or "unclear," and there is no need to use numbers to score. The same steps are used to classify applicability.39 For this systematic review, the reference standard domain was not considered.39

Characteristics of the participants, equipment, and maneuver, as well as pulmonary function values, study quality, normative values, and reference equations were synthesized in tables. Moreover, pooled estimates of MIP and MEP, according to age group and sex, were obtained using random effect models to account for the heterogeneity across the individual effects of the studies and generated values of mean [95% confidence interval (95% CI)]. Heterogeneity between studies was investigated using I2 statistics and the chi-squared test. The Comprehensive Meta-Analysis software (BioStat, Englewood, New Jersey, USA) and SPSS IBM version 19 were used for data analysis.

As shown in Fig. 1, 252 studies were identified through a database search and a manual search of reference lists. Duplicate studies were excluded, and 246 titles and abstracts were analyzed. There were 203 exclusions by eligibility criteria, resulting in 43 full-text studies. Finally, 15 studies were excluded because the purpose of the study was not to determine normal values and/or reference equations for healthy children and adolescents, or they included obese individuals or athletes, or they presented the same data of previous studies. Therefore, 28 studies were included in this systematic review, and 19 in the meta-analysis.

Fig. 1.

PRISMA flow diagram.

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Table 1 shows the characteristics of the participants. The total sample consisted of 5798 individuals, 52% were female. The age range was from 4 to 19 years old. The studies were performed in the United States,32,33 Korea,34 Japan,24 Netherlands,20 Switzerland,23 England,10 Germany,27 Spain,9 Greece,30 Poland,25 France,38 Australia,26,35 Canada,8,14,15 India,40 and Brazil.16,17,19,21,22,28,29,31,36,37 Only 15 studies provided BMI values,16,17,19-23,28,30-33,36,37,40 of these, the sample was classified as underweight or eutrophic. Thirteen studies reported spirometry variables, 10 of which presented forced expiratory volume in one second (FEV1) and forced vital capacity (FVC).14,19-22,25,26,27,31,37 The studies by Da Rosa et al.17,36 showed only FEV1, and Smyth et al.8 presented only FVC data.

The characteristics of the equipment used in the studies to measure MIP and MEP and their specifications are shown in the supplementary material (S1 and S2). A rigid, flat, and plastic mouthpiece was used to perform the tests in five studies,10,21,22,28,29 while nine studies did not describe this item.15,20,24,27,31,32,34-36,38 Seventeen studies described the width of the air leakage orifice,8,9,14,16,17,19,22,25,26,28-30,32,33,36,37,38 which ranged from 0.5 to 2 mm, one study reported the length (3.1 cm),8 and another reported only the presence of the orifice, but did not specify the dimension.21 A nose clip was used in 19 studies,9,14-17,19-23,26,28,29,31,33,36,37,38,40 in two studies the authors chose not to use it,10,24 and seven studies did not report if a nose clip was used.8,25,27,30,32,34,35 Twenty-three studies chose to record the plateau pressure of the maneuvers,8-10,14-17,19,21-23,25,26,29-38,40 while one recorded the plateau and the peak pressures.28

The sustained maneuver time was reported by 19 studies,8-10,14,16,17,19,21-23,25,26,29-31,36-38,40 which ranged from one to three seconds. For initial volume, the residual volume (RV) and the total lung capacity (TLC) were chosen for the MIP and MEP maneuvers in 21 studies.8-10,14,16,17,19-22,25,26,28-30,32,33,35,36,37,40 Before performing the tests, participants were verbally instructed on how to perform the maneuver in 20 studies8,9,16,17,19,21-26,28,29,31-34,36,37,40 and in 14 studies a demonstration of the test was also performed.9,16,17,19,21,22,24,25,31-33,36,37,40 The number of maneuver attempts ranged from one to nine times, with most studies using between three9,10,17,19,20,25,27,29,31-33,35-37,40 and seven17,29,36 attempts.

The methodological quality of the studies was analyzed using QUADAS-2 (Table 2). For the risk of bias in Patient selection, 21 studies had a high risk of bias8-10,14-16,22-30,32-35,38,40 for not reporting information on spirometry and/or BMI. In the domain of the Index test, 10 studies were considered unclear,20,24,26,27,30-32,34,35,40 because they did not provide enough information to be accurately assessed, resulting in concern about the risk of bias. Only one study20 had a high risk of bias in the Flow and time domain due to the different numbers of participants for each test. Regarding the domain of Applicability, 21 studies were considered at high risk of bias in the Patient selection,8-10,14-16,22-30,32-35,38,40 and only one study was considered unclear in the index test,26 bringing a concern with the risk of bias. The following studies were considered as low risk of bias in all domains: Da Rosa et al.,36 Da Rosa et al.,17 Heinzmann-Filho et al.,19 Lanza et al.,21 Marcelino et al.37

Figures 2 and 3 provide the MIP and MEP values analyzed separately by sex and the following age groups: 4–11 and 12–19 years old. Nineteen studies8,15-17,19-22,24,27-31,33-35,37,40 were included in the meta-analysis. The other nine9,10,14,23,25,26,32,36,38 studies were not included due to lack of data in the original articles, repeated data, or inability to contact the main authors. The mean MIP for female participants aged 4 to 11 years was 65.8 [95% CI (59.3, 72.3)] cmH2O and the MEP was 72.8 [95% CI (64.9, 80.8)] cmH2O. The mean MIP for male participants aged 4 to 11 years was 75.4 [95% CI (67.0, 83.9)] cmH2O and the MEP was 84.0 [95% CI (73.6, 94.3)] cmH2O. In the age group from 12 to 19 years, the MIP for the females was 82.1 [95% CI (73.0, 91.2)] cmH2O and the MEP was 90.0 [95% CI (78.8, 101.2)] cmH2O. For males aged 12 to 19 years, the mean MIP was 95.0 [95% CI (86.1, 104.0)] cmH2O and the MEP was 105.7 [95% CI (90.9, 120.5)] cmH2O. Additional information about the normal values of MIP and MEP can be found in supplementary material Tables S3, S4, and S5.

Fig. 2.

MIP and MEP for Boys/Girls, 4–11y.

MIP, maximal inspiratory pressure; MEP, maximal expiratory pressure; y, years; SD, standard deviation; CI, confidence interval; n, sample size; NR, not reported.

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Fig. 3.

MIP and MEP for Boys/Girls, 12–19y.

MIP, maximal inspiratory pressure; MEP, maximal expiratory pressure; y, years; SD, standard deviation; CI, confidence interval; n, sample size; NR, not reported.

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The coefficient of determination (R2) values of the 15 studies that presented reference equations of MIP and MEP ranged from 6.8% to 63%9,10,15,17,19,21,22,24,25,27,31-33,37,40: Arnall et al.32 from 8% to 26%; Arnall et al.33 from 15% to 33%; Da Rosa et al.17 from 13.9% to 21.6%; Gomes et al.31 from 25% to 63%; Domenech-Clar et al.9 from 21% to 51%; Gaultier and Zinman15 from 10% to 33%; Heinzmann-Filho et al.19 from 41% to 59%; Lanza et al.21 from 27% to 34%; Mendes et al.22 was 27%; Tagami et al.24 from 13% to 21%; Tomalak et al.25 from 9.1% to 28%; Wilson et al.10 from 10.8% to 35.6%; and Marcelino et al.37 from 6.8% to 18.9%. The studies from Mellies et al.27 and Pawar et al.40 did not present the R2 values.

The studies by Lanza et al.,21 Mendes et al.,22 and Pawar et al.40 formulated a unique equation in which dichotomous data were used to differentiate between sexes, while other studies9,10,15,17,19,24,25,27,31-33 formulated one equation for females and another for males. In contrast, Marcelino et al.37 proposed an equation for MIP in which dichotomous data were used to differentiate sexes, and for MEP, they provided one equation for females and another for males. The studies by Arnall et al.,32 Arnall et al.,33 and Marcelino et al.37 reported the lower limit of normal for MIP and MEP. In the study by Tomalak et al.,25 specific equations were developed for the sitting and standing position tests. All details are described in the supplementary material (S5).