This study featured a cross-sectional design and was reported following the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement . The research complied with all the relevant national regulations and institutional policies, including the Declaration of Helsinki, and was approved by the scientific ethics committee of the Universidad Santo Tomás, Chile (Folio ID-98-19).
Before participating in the study, all children and their parents were fully informed about the protocol and written informed parental consent and participant assent were obtained. The sample size was calculated based on the largest width of the confidence interval (W) and the largest standard deviation (S) of maximum isometric strength reported in a previous study :
Where zα′ = 1.96, S = 0.27, and W = 0.38. The required sample size was a minimum of seven participants per sex per age group. Children between 7–15 years of age from a school located in the central area of Chile were recruited. All participants presented with typical development, Latino ethnicity, and were Spanish language speakers. Ethnicity was reported following the Standards for the Classification of Federal Data on Race and Ethnicity . Participants were included if they had intact cognitive function to understand the orders given by the evaluator. Participants were excluded if they presented with (i) pain during investigation procedures (ii) a history of medical, neurological, or musculoskeletal impairments that could affect strength measurements, (iii) use of medications for pain or musculoskeletal disorders, (iv) previous surgeries of the upper extremities and/or spine, or (v) participation in competitive sports during or in the six months prior to the study measurements.
Anthropometric measurements (body mass and height) were obtained using a stadiometer (model 220; Seca, Germany). All isometric strength measurements were performed on the dominant upper limb, determined by asking the participants which hand is used to hold a pen and throw a tennis ball. In the case of discrepancies between the test results, participants were asked to indicate their preferred hand. The maximum isometric muscle strengths of the shoulder flexors, shoulder abductors, shoulder medial rotators, shoulder lateral rotators, elbow flexors, elbow extensors, wrist flexors, and wrist extensors were assessed with HHD using a calibrated hand-held dynamometer (MMT 01165, Lafayette Manual Muscle Test System, Lafayette, IN, USA). The order of the muscles tested, the standardized positions, and hand-held dynamometer placement for each muscle group were based on previous reports and are described in Table 1 [4, 6, 16, 17]. The maximum isometric grip strength of the extrinsic and intrinsic hand muscles was assessed with hand-grip dynamometry (HGD) using a calibrated hand-grip dynamometer (Jamar 5030J1, Sammons Preston Rolyan, Bolingbrook, IL, USA) according to the recommendation of the American Society of Hand Therapists .
***Table 1 near here***
The principal researcher trained the evaluator (a physical therapist) in all dynamometry procedures for twelve hours. Participants wore sport clothes, were barefoot, and all measures were taken during one testing session (one hour per participant). To assess the maximum isometric strength of the shoulder, elbow and wrist muscle groups, a make test was used. The evaluator kept the dynamometer stationary while the participant exerted maximum strength against it [1, 12, 15]. For all assessments, the participants were asked to perform the maximum isometric strength on the dynamometer (HHD or HGD, as appropriate), while the evaluator encouraged the participants with a standardized phrase, "harder, harder, harder". For all muscle groups, three attempts were completed, and the peak force (N) was recorded for each one attempt. The average of the three attempts was used for the analysis. Before each muscle group assessment, the participant performed one submaximal contraction. This was performed as a warm-up and to ensure that the task was well understood, and also to ensure that joint stabilization was adequate . Each contraction (attempt) was progressive and was held for five seconds, followed by a 30-second rest period to minimize fatigue affects. The recorded force (N) of all muscle groups, including the extrinsic and intrinsic hand muscles, was divided by the body mass (kg) of each individual. The total upper limb was calculated by adding the normalized maximum isometric strength of the shoulder flexors, shoulder abductors, shoulder medial rotators, shoulder lateral rotators, elbow flexors, elbow extensors, wrist flexors, and wrist extensors.
The software SPSS 25.0 was used to perform the statistical analysis of the data. In all tests, an alpha level < 0.05 was considered. To complete the primary objective of this research, the mean and standard deviation were calculated for each measurement of maximum isometric strength, grouped by age and sex. Data were considered to be outliers if they were > 3 standard deviations from the mean; in these cases, the data was removed. Intrarater reliability was assessed for maximum isometric strength using the standard error of the measurement (SEM) and intraclass correlation coefficient (ICC; two-way random, absolute agreement, average measure). The Shapiro-Wilk test was used to assess the normality distribution of all the data.
To complete the first secondary objectives, a two-factor analysis of variance (ANOVA) (age and sex) was performed for the variable maximum isometric strength of each muscle group and the total upper limb strength. In the case of significant interactions, a post-hoc analysis was performed with Bonferroni’s multiple comparison test. To complete the second secondary objective, the narrowest age range of strength was identified by significant differences (p < 0.05) observed between the lowest and highest age of the interval selected.
Finally, to meet the third secondary objective, the relationship between the maximum isometric strength of each muscle group and the total upper limb strength was analyzed using Pearson’s test, in which a correlation coefficient (r) from 0–0.4 was considered as weak, 0.41–0.7 as moderate, and 0.71–1.0 as strong. A stepwise multiple linear regression analysis was then performed. The dependent variable was the total upper limb strength, and the independent variables were the maximum isometric strength of each muscle group, adjusted for sex, height, and age. For this method, the independent variable that showed the strongest, simple, significant correlation with the total upper limb strength was initially selected for the analysis. The remaining variables that showed simple significant correlations (from highest to lowest correlation) were consecutively added to this model. The goodness of fit was determined by means of the R2 coefficient and its percentage of change. In addition, collinearity diagnoses were verified through values less than 0.10 tolerance and the identification of the Variance Inflation Factor, opting to eliminate the variables that showed collinearity with a Variance Inflation Factor > 10, in order to define the definitive multiple linear regression model.