Background The knee malalignment is a risk factor of injuries such as anterior cruciate ligament tear. Most studies that analyses the knee valgus in jump tasks, investigates only this pattern at landing. It would be interesting to study the knee valgus at takeoff, because at this specific moment the active components of the movement are the primary injury contributors.

Objective to compare the frontal plane knee angle (FPKA) during takeoff and landing at countermovement jump in athletes.

Design Observational cross sectional.

Setting Volleyball and basketball players were evaluated at Minas Tennis Club and at Sports Prevention and Rehabilitation Laboratory (CENESP-UFMG).

Participants 101 athletes, mean age 15.88 (0.35) years were included in this study. All participants were healthy, with no current complaints of lower limb injury or pain and no history of lower limbs previous surgery.

Assessment of risk factors High-FPKA. Reflexive markers were located in ASIS, femoral condyles and in a middle point of the medial and lateral maleolus.

Main outcome measurements The FPKA was determined in SIMI motion 2D software.

Results The mean angle during takeoff was 3.020 and 3.13 0 for dominant and non-dominant leg. For landing, it was 2840 and 2210 respectively. There was no significant statistical differences between the two means on both legs (p=0.73 dominant leg and p=0.13 non-dominant leg). Meanwhile, the variance was greater in the impulsion analysis (37.2 right leg and 48.3 left leg) compared to landing (24.4 and 29.8 respectively).

Conclusion The analysis of FPKA during takeoff provides data about the active contribution of certain variables in knee injuries and must be better understood. Differences between the FPKA during takeoff and landing were not found in this study, although in the first movement there was a major variance. This inconsistency may demand a more thorough investigation.