Abstracts of the 17th Annual Meeting of ESMAC, Oral Presentations / Gait & Posture 28S (2008) S1–S48 S39 at the elbow and two pairs at the wrist joints. The computer simulation model was evaluated by comparing simulations to a backhand topspin trial performed by a 22-year-old elite tennis player. A Vicon 624 System with twelve M2 strobe cameras operating at 250 Hz was used to record the tennis stroke with thirty-three 25mm retro-reflective markers on the subject’s body (standard marker set) and six markers on the tennis racket. The simulation model was matched to the performance for the period from 200 ms before ball-racket impact to 50 ms after impact by varying the activation profiles in order to minimize the difference between simulation and performance in terms of joint angles and racket angles. Once matched the location of the ball-racket impact was perturbed using single simulations (fixed activation profiles) and the effect on the loading at the wrist and elbow observed along with the changes in kinematics. Figure 1. The computer simulation model. Results: A good match was obtained with the average RMS difference of 2º between the joint angles in the simulation and performance. Perturbing the ball-racket impact location had a substantial affect on the elbow, wrist and racket kinematics, with for example up to 24º more flexion at the wrist for an off-centre impact. Perturbing the impact location increased the internal joint reaction force at the wrist by up to 22% and by up to 27% at the elbow. In addition, the wrist joint torques increased by up to 20% due to eccentric loading of wrist extensors. Discussion: A backhand stroke performed by an elite player was used as the basis for this study and the effects of off-centre impacts were examined. In the future, this work could be extended to consider the effect of the player technique, for example, it may be that some hitting techniques are more likely to result in off-centre impacts and might also be more susceptible to increased loading due to off-centre impacts. References [1] Giangarra et al., American Journal of Sports Medicine 21(3): 394– 399, 1993. O057 The influence of increased patellar tendon length on knee extension biomechanics N.E. Akalan 1 , Y. Temelli 2 , M. ¨ Ozkan 3 . 1 Child Neurology, Istanbul Medical School; 2 Orthopedics and Traumathology, Istanbul Medical School; 3 Biomedical Engineering, Bo˘ gazi¸ ci University, Turkey Summary: The purpose of the study was to investigate the affects of increased patellar tendon length (IPTL) on knee biomechanics during knee extension exercise. We developed an anatomical dynamic knee model which includes patello-femoral and tibio- femoral joints and compared forces on anterior and posterior bundles of anterior cruciate ligament (aACL, pACL) and posterior cruciate ligament (aPCL, pPCL), anterior, oblique and deep portions of medial colletaral ligaments (aMCL, oMCL, dMCL) and lateral collateral ligament (LCL) during knee extension with medium level quadriceps activation. Conclusions: It is found that IPTL changes the normal biomechanical harmony of the knee during knee extension exercise. These biomechanical changes may contributory factors of gonarthrosis and recurrence on crouch gait pattern after surgical interventions. The biomechanical changes of IPT during stance phase of gait are also worth to be analyzed. Introduction: IPTL and related patella alta have commonly seen in prior adolescent cerebral palsy population with crouch gait pat- tern [1]. The treatments have included stretching, bracing, casting, or lengthening of the hamstring tendons have often ineffective, especially in older children. Even when the knee capsule is surgi- cally released and passive extension is attained, active extension of the knee is impaired by quadriceps weakness and associated developmental elongation of the patellar tendon. Severely affected patients mostly end up with the distal extension osteotomie, distal transfer of tibial tubercule with PCL release interventions [2]. The purpose of the study was to investigate the biomechanical changes in the knee joint by increasing patellar tendon length. This is a pilot study which is valuable to consider analyzing the kinetic and kinematic behavior of the knee joint in stance phase of gait cycle which is seen in patients with crouch gait. Patients/Materials and Methods: A three-dimensional anatom- ical dynamic model that predicts the three-dimensional dynamic response of the natural tibiofemoral joint was used in which attach- ments of the bundles of the ligaments and the articular surfaces in medial and the lateral components [3]. The tibiofemoral model was integrated in to the dynamic patella-tibio-femoral model. The behavior of the knee model was with normal IPTL tested by simulating knee extension exercise and the results compared with the literature [4] (Figure 1). To analyze the biomechanical influences of patella alta, we lengthen the patellar tendon 10% and 40% of the original length and re-simulated the knee extension exercise. Results: During knee extension exercise, the tension on aACL, pACL (Figure 2), LCL and aMCL reduced, controversially the maximum force on aPCL, pPCL, increased by IPTL. The load changes on the rest of the ligament bundles were negligible. Discussion: Increasing patellar tendon length puts more tension on PCL. However the load on ACL, LCL and aMCL reduced by increasing patellar tendon length which may create slackness of the related ligament bundles. To test the correctness of our results,