High sensitivity of the knee joint response in stair ascent to changes in knee kinematics, kinetics and joint center positioning.

Abstract

Compared to level walking, stair ascent imposes larger mechanical demands on the knee joint involving elevated muscle, ligament, and contact forces. This study investigates knee biomechanics in stair ascent using a lower-extremity coupled musculoskeletal finite element (MS-FE) model. Focus is on the sensitivity of biomechanical response both to reported scatter in knee joint kinematics/kinetics and to the alteration in the knee joint reference point (RP) location. Three highly loaded instances of stance, 25%, 50%, and 75%, were analyzed. Six knee joint kinematics-kinetics variables of flexion-extension, adduction-abduction, and internal-external rotations/moments, were varied at five levels each using a L25 Taguchi design. Regression analyses quantified the contribution of each variable to predicted muscle, ACL, patellar tendon, tibiofemoral (TF), and patellofemoral (PF) contact forces. The effect of shifting the knee RP in anterior-posterior and medial-lateral directions was also examined in our MS-FE model and compared to its scaled OpenSim model. Flexion moment was the dominant contributor to quadriceps, patellar tendon, and PF contact forces, whereas adduction-abduction angle and not moment governed medial-lateral TF load partitioning. ACL force was sensitive to knee rotations and flexion moment suggesting a multiplanar response in stair ascent. Shifting RP location altered passive reaction moments in the MS-FE model with no effects on muscle and internal forces. In contrast, OpenSim results markedly altered with anterior-posterior shifts in RP. These results emphasize the importance of incorporating the reported scatter in measurements as well as the substantial sensitivity of all MS results to the RP positioning in stair ascent biomechanics.