Aggressive quadriceps loading, with the knee in slight flexion, produces significant anterior tibial translation and ACL injury. This suggests that the quadriceps is the intrinsic force in noncontact ACL injuries, producing a model for further investigation.

Greater dorsiflexion ROM was associated with greater knee-flexion displacement and smaller ground reaction forces during landing, thus inducing a landing posture consistent with reduced ACL injury risk and limiting the forces the lower extremity must absorb. These findings suggest that clinical techniques to increase plantar-flexor extensibility and dorsiflexion ROM may be important additions to ACL injury-prevention programs.

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Context: Researchers have suggested that large landing forces, excessive quadriceps activity, and an erect posture during landing are risk factors for anterior cruciate ligament (ACL) injury. The influence of knee kinematics on these risk factors has been investigated extensively, but trunk positioning has received little attention.Objective: To determine the effect of trunk flexion on landing forces and quadriceps activation during landing.Design: Two (sex) 3 2 (task) repeated-measures design. Intervention(s): Participants performed 2 drop-landing tasks. The first task represented the natural, or preferred, landing strategy. The second task was identical to the first except that participants flexed the trunk during landing.Main Outcome Measure(s): We measured peak vertical and posterior ground reaction forces and mean quadriceps electromyographic amplitude during the loading phase of landing (ie, the interval from initial ground contact to peak knee flexion).Results: Trunk flexion decreased the vertical ground reaction force (P , .001) and quadriceps electromyographic amplitude (P , .001). The effect of trunk flexion did not differ across sex for landing forces or quadriceps electromyographic activity.Conclusions: We found that trunk flexion during landing reduced landing forces and quadriceps activity, thus potentially reducing the force imparted to the ACL. Research has indicated that trunk flexion during landing also increases knee and hip flexion, resulting in a less erect landing posture. In combination, these findings support emphasis on trunk flexion during landing as part of ACL injury-prevention programs.Key Words: anterior cruciate ligament, ground reaction forces, injury prevention, risk factors Key Points N Trunk flexion during landing reduced landing forces and quadriceps activity. N The influence of trunk flexion on landing forces and quadriceps activity did not differ across sex. N Trunk flexion potentially reduces the quadriceps force requirement and subsequent load placed on the anterior cruciate ligament immediately after ground contact during landing.N Because of its influences on kinetic, kinematic, and neuromuscular risk factors for anterior cruciate ligament injury, active trunk flexion during landing might be an important component of injury-prevention programs.

Although limb differences in clinical indicators of posterior tightness exist in healthy overhead athletes, these measures appear to be influenced by humeral torsion rather than soft tissue tightness. Once torsion is accounted for, the limb differences observed clinically were minimal in healthy overhead athletes. When possible, accounting for humeral torsion when interpreting clinical measures of posterior shoulder tightness may aid in treatment decisions.