PURPOSE: This study determined anteroposterior knee-joint muscle activation differences among children and adult males and females landing from a self-initiated vertical jump (VJ) under normal and offset-target conditions to further understand physical maturation's influence on anterior cruciate ligament (ACL) injury risk. METHODS: Fifty-five recreationally active volunteer subjects grouped by age (children = 9.5 +/- 0.9 yr; adult = 23.9 +/- 2.8 yr) and gender (females = 28; males = 27) completed motion analysis, ground reaction force, and surface electromyography (SEMG) data collection during a two-footed landing under straight (midline-target) and offset-target (adult = 45.7 cm; child = 30.5 cm) conditions. Target height was 50% of maximum VJ height. Co-contraction ratios (CCR) (hamstrings (HAMS)/vastus medialis (VM) activity) from normalized SEMG root mean squares were analyzed in the prelanding (PRE) (100 ms before initial contact (IC)), reflexive (REF) (100 ms after IC), and voluntary (VOL) (end of REF to maximum knee flexion) muscle activity phases. Repeated-measures statistical analyses determined significant gender, physical maturation, and target differences (P < 0.05) in CCR and associated HAMS and VM activity across landing phases. RESULTS: A significant interaction (P < 0.0001) indicated similar CCR for children and adults during the REF and VOL phases, but during the PRE phase adult CCR (619.04 + 52.01) were two times greater than children's (308.32 +/- 51.04). Significantly more HAMS activity, not less VM activity, increased adult PRE-CCR. Gender and target CCR differences were absent. CONCLUSIONS: Children's decreased preparatory co-contraction about the knee does not seem to be linked to increased ACL injury risk. Thus, adults may strive for preparatory co-contraction levels about the knee that permit adaptability to varied landing tasks.