Context: Sex differences in dynamic measures have been established in physically mature populations. Gaining information on maturation's effect on dynamic performance measures implicated in injury risk may enable us to better design injury prevention programs.Objective: To examine sex differences in dynamic valgus alignment and triple-hop distance measures across maturational stages in males and females. A secondary purpose was to determine if a field test of strength and power predicts dynamic valgus alignment.Design: Cross-sectional study. Setting: Laboratory.Patients or Other Participants: 157 young athletes (78 females, 79 males) aged 9 to 18 years.Intervention(s): Subjects performed drop-jump landings and single-leg triple-hop tests as part of a broader injury screening.Main Outcome Measure(s): Maturational status was ascertained from self-report questionnaires and grouped according to Tanner stages 1 and 2 (MatGrp1), 3 and 4 (MatGrp2), and 5 (MatGrp3). Frontal-plane knee valgus displacement, which served as a measure of dynamic valgus alignment, and single-leg triple-hop distance were assessed.Results: Males demonstrated less dynamic valgus alignment during drop jumps in the latter maturational stages (MatGrp1 5 13.16 6 8.76, MatGrp2 5 9.06 6 6.26, MatGrp3 5 9.26 6 9.46), whereas females increased dynamic valgus alignment throughout maturation (MatGrp1 5 11.56 6 6.96, MatGrp2 5 12.86 6 8.86, MatGrp3 5 15.56 6 8.76). Thus, in the more mature groups, males had less dynamic valgus alignment than females. Both males (MatGrp1 5 N Although both males and females improved their performance in the triple-hop test as they matured, males showed greater improvement.393.5 6 63.7 cm, MatGrp2 5 491.8 6 95.1 cm, MatGrp3 5 559.3 6 76.3 cm) and females (MatGrp1 5 360.3 6 37.1 cm, MatGrp2 5 380.1N The triple-hop test did not predict dynamic valgus alignment by sex or level of maturation.M usculoskeletal injury risk is reported to increase with age 1-4 and physical maturation status. 5 In a recent review across sports, adolescents (those older than 13 years) were at a consistently increased risk of injury compared with younger children. 6 This risk appears to coincide with physical maturation, because Michaud et al 5 reported that Tanner stages 4 and 5 were associated with an increased occurrence of sport injuries. With authors 6 of a comprehensive literature review reporting that adolescents are consistently at an increased risk of injury with respect to a younger population, we need to focus on the maturing youth population to better understand potential relevant risk factors for injury. Specifically, functional performance changes that occur in males and females as part of the maturation process should be considered further, because young females have a greater risk of straining or spraining the knee and ankle than young males. 7