Introduction:​ One of the most common injuries involving the knee joint is the Anterior Cruciate Ligament (ACL) tear. Female basketball and soccer players have about 3 times higher risk of ACL injury versus male athletes (Prodromos et al., 2007). Female athletes alter their motion pattern during the landing phase differently than male athletes. That is one of the potential reasons why females have a higher risk of ACL injury during sports (Chappell et al., 2002). Possible mechanisms of ACL injury are the deceleration of the knee in an extended position (Boden et al., 2000), shear forces applied to the leg during landing (Pflum et al., 2004) and large knee varus and internal rotation moments (McLean et al., 2007). The purpose of this study was to determine if mid-air postural adjustments affect the potential for ACL injury during the landing.

Methods:​ Eleven healthy college female students (age 21.8 ± 1.6 yrs, height 1.7 ± 0.5 m, mass 64.1 ± 11.7 kg) participated in the study. Three tennis balls were suspended from the ceiling. After maximum jump height was established, subjects were instructed to jump from and land on two force platforms (AMTI). Approximately 100 ms after leaving the force platform an LED positioned near one of the balls was illuminated. Subjects were asked to tap this ball using both hands. They were told to jump as high as possible for each of 21 randomly selected trials. A total of 23 retroreflective markers on the right and left lower extremities were used to determine the 3D orientation of the segments (Peak Motus). Inverse dynamics were used to calculate joint moments and reaction forces at the knee joint. Kinematics were imported to a scaled SIMM (MusculoGraphics, Inc.) model to obtain the maximal muscles forces, muscle moment arms and muscle orientations for 88 lower extremity muscles. Static optimization using a cost function that minimizes the sum of the muscle stress squared was used to estimate the individual muscle forces. The knee joint contact forces were then calculated as the sum of the muscle forces and the joint reaction forces. The peak anterior shear force, peak varus moment, and peak internal rotation moment on the knee joint were used to assess the potential for ACL injury. Two 3 x 2 (reaching direction by right vs left leg and reaching direction by ipsilateral vs contralateral leg) repeated measures ANOVAs were used to determine statistical significance.

Results:​ The peak anterior shear force was significantly greater on the right knee compared to the left and the peak external rotation moment was significantly greater on the left knee compared to the right. However, data were also analyzed without reference to which leg had the highest peak values. The average peak difference between the middle ball condition and the greater value of the ipsilateral and contralateral legs from the side reaching conditions is 0.12BW, 0.03BWm, and 0.04BWm for peak anterior shear force, peak varus moment and peak external rotation moment respectively. This shows that peak anterior shear forces, peak varus moment and peak external rotation moments all increased in one of the legs when reaching to the side.

Discussion:​ The results suggest that reaching to the side balls had a higher risk of ACL injury than reaching to the middle ball. This result was not apparent when looking at right/left leg effects or ipsilateral/contralateral leg effects because subjects adopted different strategies to deal with the mid-air adjustments that are necessary when reaching to a side ball. Some subjects always landed on their dominant leg while others always landed on the ipsilateral leg.