BACKGROUND: Although measures of exercise performance and physical activity are known to be related to 6-minute walk test (6mWT), the role of the strength of each muscle group of the lower limb in the locomotor capacity during the development period is unknown. OBJECTIVE: To identify the role of the isometric strength of lower limb muscles and cardiovascular parameters on the locomotor capacity in children and adolescents, controlling for participants sex, age, and height. METHODS: Participants (7 to 15 years old; female, n = 113; male, n = 128) were assessed for isometric strength (seven lower limb muscles), cardiovascular parameters (diastolic pressure, oxygen saturation, resting heart rate, respiratory rate), and 6mWT. Participants were split into nine age groups and separated by 1-year intervals. RESULTS: Hip flexors-controlled for sex, age, and height-explained a meaningful percentage of the variance (R= 2 0.45; p < 0.001) for 6mWT distance, and the resting heart rate explained a change in R 2 of only 2% (p = 0.008). CONCLUSIONS: Isometric strength of hip flexors explained ∼ 50% of the 6mWT distance, suggesting the importance of strength from an early age. Other factors, such as cardiovascular parameters, while relevant, may exert a secondary role on youth’s capacity.