$74 Journal of Biomechanics 2006, Vol. 39 (Suppl 1) 4223 Tu, 16:00-16:15 (P24) Role of tibial constraint on knee joint biomechanics under muscle loads W. Mesfar, A. Shirazi-Adl. Department ef Mechanical Engineering, Ecole Polytechnique, Montr6al, Qu6bec, Canada To counterbalance the moments of quadriceps/hamstrings forces while pre- serving the joint flexion, in vitro investigations restrain tibial A-P translation at a distance away from the joint. Such constraints, however apart from flexor/extensor moments, introduce artefact A-P shear forces that in turn in- fluence joint kinematics and forces in cruciate ligaments. Using a 3D nonlinear finite element model of the entire joint, this study aims to investigate the effect of the tibial constraint (by a pure moment or by restraining forces at 20 cm or 30cm distal to joint) on joint mechanics and cruciate ligament forces under 205.5 N hamstrings and/or 411 N quadriceps forces. The model consists of tibia, femur, and patella plus their articular cartilage layers, menisci, six major ligaments, patellar tendon, quadriceps and hamstrings muscle force vectors (each by 3 distinct components). The tibial constraint has an important effect on A-P translations and, hence, ACL/PCL forces. This is due to artefact shear forces generated at the tibial restraint location as compared with cases constrained by a pure moment. In isolated quadriceps loading, artefact forces resulted in posterior tibial trans- lations which significantly diminished ACL forces but increased PCL forces. In contrast, under hamstrings activation, restraining forces reversed direction causing tibial anterior translations that diminished PCL forces while increasing ACL forces. The relative extent of foregoing changes depends on the magni- tude of restraining forces that in turn alter as a function of muscle activity and lever arm. Rehabilitation exercises following ligament reconstructions should account for the effect of the lever arm on ligament forces in order to reduce the risk on grafts following surgery. Near joint placement of the resistance is recommended as it decreases forces in ACL grafts in extension exercises at small flexion and in PCL grafts in flexion exercises at larger flexion angles. Hamstrings co-activation plays an important role to protect the knee with ACL injury or graft by reducing the tibial anterior translation of the knee. On the other hand, the risk to the PCL or its graft increases under hamstrings activity at larger flexion angles. 5225 Tu, 16:15-16:30 (P24) Moment potential balance at the knee is effected by total knee arthroplasty W.L. Buford Jr., Z.S. Stinson, F.M. Ivey. University of Texas Medical Branch, Galveston, TX, USA Total Knee Arthroplasty (TKA) may affect the muscles operating at the flex- ion/extension (rE) or internal /external rotation (IE) axes. This study tested the hypotheses that TKA will change the mechanical balance of the knee joint by altering the moment arms of muscles acting about two separate axes of rotation, and removal of the Posterior Cruciate Ligament (PCL) during TKA will alter the moment arm balance to a different extent than a TKA that retains the PCL. Extending previous work [1], this study measured knee muscle moment arms for both the FE and IE axes. 14 fresh-frozen hemi-pelvis cadaver specimens were used to determine the effective moment arms of 12 muscles. Mea- surements for excursion and angular rotation were recorded as the leg was manually rotated through the full ROM about the FE axis and then the full ROM about the IE axis at 6 different positions of knee flexion. Moment arms were determined for the normal knee, the knee after a PCL-sparing TKA and the knee after a PCL-sacrificing TKA. Moment arms for the individual muscle ten- dons were multiplied by the muscle's tension fraction (fractional physiological cross-sectional area [PCSA]) [2] to estimate its maximum potential for moment production relative to the other muscles at the knee, and this value was labeled as the muscle's moment potential. The resultant normal knee moment potential balance tended toward internal rotation and extension. The resultant vector for both arthroplasty conditions was significantly different from the normal knee resultant (p<0.05). TKA shifted the balance towards external rotation and flexion. Results for the two arthroplasties did not differ significantly. This report provides knowledge regarding mechanical function of the knee following TKA, which can be used in rehabilitation efforts. PCL-sparing and sacrificing arthroplasty result in similar changes in muscle balance relative to normal balance at the knee implying that rehabilitation should focus on improving internal rotation and extension exercises. Acknowledgement: Supported by a research grant from Smith Nephew, Inc., Memphis, TN. References [1] Buford WL, Ivey FM, Nakamura T, et al. Knee 2001; (8)293-303. [2] Winters JM, Woo SL-Y, eds. 1990; Springer. Oral Presentations 7048 Tu, 16:30-16:45 (P24) Contact locations of the knee joint in deep knee flexion M.S. Hefzy 1, K.L. Aeschliman 1, M.J. Dennis 2. 1Biomechanics and Assistive Technology Laboratory, DepL of Mechanical, Industrial and Manufacturing Eng., The University of Toledo, Toledo, Ohio, USA, 2Department of Radiology, Medical University of Ohio, Toledo, Ohio, USA The objective of the present work is to determine the contact locations of the human knee joint when it is maximally bent at 160 degrees of flexion. Computer Tomography (CT) scans were obtained from subjects sitting in a kneeling position with the knee maximally flexed and the torso upright. Using the Scion Software and the I-DEAS software, a three dimensional representation was created for the knee at the sitting position, including the tibio-femoral and patello-femoral joints. Results show that when sitting in a kneeling position in deep flexion, the contact is highly asymmetric. The tibio-femoral contact occurs only on the medial side of the joint. On the tibia, the contact occurs on the posterior edge of the medial tibial plateau. On the femur, the contact occurs proximal to the femoral condyles, on the distal shaft of the femur. No contact occurs on the lateral side. It appears that the tibia is forced to lift-off causing impingement of the posterior tibial edge against the back of the femur. The patella was found to be very flexed with respect to the tibia, and was completely clear of the femoral grove and in contact with the femoral condyles only. We have previously used X-rays data to qualitatively describe the kinematics of the joint in this position [1]. This study provides for the first time a quantitative assessment of the contact locations of the knee joint when sitting with the knee maximally flexed. These kinematic data should be taken into considerations in the design of total knee replacements that allow individuals to sit in a kneeling position. References [1] Hefzy M.S., Kelly B.R, Cooke D.V. Kinematics of the knee joint in deep flexion: a radiographic assessment. Med. Eng. Phys. 1998; (20)302-7. 7460 Tu, 17:00-17:15 (P24) Ambulatory inertial system for 3D knee joint angles measurement during gait J. Favre 1, R. Aissaoui 1,3, B. Jolles 2, R Luthi 2, J. de Guise 3, K. Aminian 1. 1Ecole Polytechnique F6d~rale de Lausanne (EPFL), Laboratory of Movement Analysis and measurement (LMAM), Lausanne, Switzerland, 2University Hospital of Lausanne (CHUV), H6pital Orthop6dique de la Suisse Romande (HOSR), Lausanne, Switzerland, 3Ecole de technologie Sup~rieure, Laboratoire de rechemhe en imagerie et orthop~die (LIO), Montreal, Canada This study investigates a new inertial method to estimate 3D knee joint angles. A new functional and postural calibration method was defined based on bone- embedded anatomical frames (BAr). The proposed method was used with an ambulatory inertial system including a Physilog ® data-logger and two sensor- units, each consisting of a tri-axial gyroscope and a tri-axial accelerometer. A new functional calibration method using passive rotation of shank was introduced to define knee BAR 3D knee joint angles during gait were measured for four healthy subjects simultaneously using the proposed ambulatory system and an electromagnetic motion capture system as reference. The measure- ment errors due to the shank's soft tissues were also evaluated by the use of an exoskeleton. The mean ranges of motion measured with the combination of the ambulatory inertial system with the proposed calibration method were of 51.5 °, 10.0 o and 14.0 o for flexion/extension, adduction/abduction, and inter- nal/external rotation. The corresponding ranges obtained through the reference system were of 52.5 o, 10.5 o and 13.0 o. The means (and standard deviations) errors of the 3D continuous time angles averaged for four subjects were 6.3(1.7) °, 4.2(2.0) o and 1.8(1.7) °. The small standard deviation errors reflected close definition of the BAr between both methods, and the systematic errors were due to the different ways of defining the zero values (i.e. the neutral posture). The errors obtained with different attachment methods did not show significant differences. The proposed method constitutes a very promising tool for clinical evaluations. It is easy-to-use and could be applied for ambulatory and long time monitoring systems. Although in this study, the inertial system was applied to knee joint, it can be used to investigate the 3D rotations of other complex joints such as ankle, hip, elbow, and shoulder. 7441 Tu, 17:15-17:30 (P24) Influence of muscular fatigue on agonist and antagonist muscle groups moments during loaded squats G. Rao 1, D. Amarantini 2, L. Vigouroux 1, E. Berton 1. 1Mouvement et Pemeption, Universit6 de la M6diterran~e, Marseille, France, 2LAPMA, Universit6 Paul Sabatier, Toulouse, France Understanding how the neuromuscular system deals with muscular redun- dancy to achieve coordinated motions is a recurrent issue in biomechanics. Convenient optimization methods using kinematics, ground reaction and EMG data as inputs provide reliable estimates of agonist and antagonist muscle