Abstract Purpose: To determine the isometric strength profile of the upper limb muscles of children and adolescents between 7–15 years of age. Furthermore, to (i) identify the age at which differences in isometric strength are observed between sexes; to (ii) determine the age range at which significant progression of isometric strength could be observed in girls and boys; and (iii) identify the role of the isometric strength of each muscle group on the total upper limb strength. Methods: Cross-sectional study that evaluated the maximum isometric strength of 9 muscle groups of the upper limb of 243 Chilean children and adolescences (female n = 114; male n = 129), split into 9 age groups, separated by 1-year intervals. For this, hand-held dynamometry (make test) and hand-grip dynamometry were used. The mean and standard deviation were calculated for each measurement of maximum isometric strength, grouped by age and sex. A two-factor analysis of variance (age and sex) was performed for the maximum isometric strength of each muscle group and the total upper limb strength. 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, sex, height, and age. Results: From 11 years of age, wrist flexors were the first muscle group that revealed a significant difference in isometric strength in favor of boys ( p = 0.0143). In boys, the narrowest and earliest age range in the progression of isometric strength was 10 to 12 years for wrist flexors ( p = 0.0392). Shoulder flexors was the main factor that explained the performance of the total upper limb strength (R 2 = 0.742; p < 0.001). Conclusions: The most progressive isometric strength development occurred from age 10 years in the ventral and distal muscles of the upper limb; and from this age the boys begin to present a greater isometric strength than girls. In addition, the isometric strength of shoulder flexors explained the higher total upper limb strength performance.