Measurements of Tibiofemoral Kinematics During Soft and Stiff Drop Landings Using Biplane Fluoroscopy

Background: Previous laboratory studies of landing have defined landing techniques in terms of soft or stiff landings according to the degree of maximal knee flexion angle attained during the landing phase and the relative magnitude of the ground-reaction force. Current anterior cruciate ligament injury prevention programs are instructing athletes to land softly to avoid excessive strain on the anterior cruciate ligament. Purpose: This study was undertaken to measure, describe, and compare tibiofemoral rotations and translations of soft and stiff landings in healthy individuals using biplane fluoroscopy. Study Design: Controlled laboratory study. Methods: The in vivo, lower extremity, 3-dimensional knee kinematics of 16 healthy adults (6 male and 10 female) instructed to land softly and stiffly in different trials were collected in biplane fluoroscopy as they performed the landing from a height of 40 cm. Results: Average and maximum relative anterior tibial translation (average, 2.8 +/- 1.2 mm vs 3.0 +/- 1.4 mm; maximum, 4.7 +/- 1.6 mm vs 4.4 +/- 0.8 mm), internal/external rotation (average, 3.7 +/- 5.1 degrees vs 2.7 degrees +/- 4.3 degrees; maximum, 5.6 degrees +/- 5.5 degrees vs 4.9 degrees +/- 4.7 degrees), and varus/valgus (average, 0.2 degrees +/- 1.2 degrees vs 0.2 degrees +/- 1.0 degrees; maximum, 1.7 degrees +/- 1.2 degrees vs 1.6 degrees +/- 0.9 degrees) were all similar between soft and stiff landings, respectively. The peak vertical ground-reaction force was significantly larger for stiff landings than for soft landings (2.60 +/- 1.32 body weight vs 1.63 +/- 0.73; P < . 001). The knee flexion angle total range of motion from the minimum angle at contact to the maximum angle at peak knee flexion was significantly greater for soft landings than for stiff (55.4 degrees +/- 8.8 degrees vs 36.8 degrees +/- 11.1 degrees; P < .01). Conclusion: Stiff landings, as defined by significantly lower knee flexion angles and significantly greater peak ground-reaction forces, do not result in larger amounts of anterior tibial translation or knee rotation in either varus/valgus or internal/external rotation in healthy individuals. Clinical Relevance: In healthy knees, the musculature and soft tissues of the knee are able to maintain translations and rotations within a small, safe range during controlled landing tasks of differing demand. The knee kinematics of this healthy population will serve as a comparison for injured knees in future studies. It should be stressed that because the authors did not compare how the loads were distributed over the soft tissues of the knee between the 2 landing styles, the larger ground-reaction forces and more extended knee position observed during stiff landings should still be considered dangerous to the anterior cruciate ligament and other structures of the lower extremities, particularly in competitive settings where movements are often unanticipated.