Track 2. Musculoskeletal Mechanics-Joint ISB Track

5727 We-Th, no. 67 (P57) The estimation of theoretical values of tendon slack length for individual muscle in humans M. Vilimek. Dept. of Mechanics, Fac. of Mech. Engineering, Czech Technical University in Prague, Prague, Czech Republic

This paper presents a numerical approach for estimation a theoretical value of musculotendon (MT) parameter tendon slack length. The tendon slack length is the length on elongation at which tendon just begins to develop force [1]. This parameter is necessary for describing a tendon dynamics and load characteristics for musculoskeletal modeling. In this approach an advantage of dependence between several MT parameters is taken, MT length, pennation angle, muscle fiber length, theoretical interval of muscle fiber length when mus- cles can produce active force and tendon slack length. Optimum muscle fiber length and pennation angle values are known (these parameters are relatively easy to measure and are also more precisely visually defined than tendon slack length) and minimum and maximum length of MT can be individually examined from joint angle mobility and positions of MT attachments. In this calculation, the cost function minimizes the difference between optimized and theoretical range of muscle fiber length (0.5L M <~LM<~ 1.5L M) when muscle can produce active force. After optimizing the interval of effective operating range of muscle length can be calculated values for tendon slack length. This method newly estimates MT parameters for individual muscle and it is not necessary to optimize values for all muscles from functionally anatomical unit together [2]. In this paper is presented application of this method to the lower limb MT actuators. The values from this finding are close to experimentally measured and published data. This method can be used for first approach and can help investigators, who can not directly measure this value. This study was supported by grant GACR 106/06/P304.

References [1] Zajac EE. Muscle and tendon: properties, models, scaling, and application to

biomechanics and motor control. Critical Reviews in Biomedical engineering 1989; 17:359-411.

[2] Garner B.A., Pandy, M.G. Estimation of musculotendon properties in the human upper limb, Annals of Biomedical Engineering 2003; 31: 207-220.

6817 We-Th, no. 68 (P57) Inverse finite element characterization of soft tissues using genetic algorithm B. Karthikeyan, A. Chawla, S. Mukherjee. Department ef Mechanical Engineering, Indian Institute of Technology, New Delhi, India

Human body finite element (FE) models for use in impact simulations require soft tissue characterization at high strain rates. The objective of the current work is to extract viscoelastic properties of passive muscle tissues at high strain rates and study their rate dependency. A procedure to identity the dynamic properties of passive muscle tissue under impact has been proposed using isolated-tissue experiments, FE simulations and Genetic Algorithm (GA) based optimization. Data from nineteen impact tests on unconfined isolated human muscles for strain rate ranging from 132/s to 262/s were used [1]. Tissues were compressed up to approximately 50% strain and the force-time response was recorded. FE simulations of these impact tests have been performed in the present study by modeling the muscle as linear viscoelastic. RMS of the deviation between the experimental and FE force data, sampled at 10 kHz, was then minimized to predict the material parameters, bulk modulus, short-term shear modulus and long-term shear modulus. This parameter identification process was automated using PAM-CRASH TM, open source GA code [2] and C++ programming. In the present study a predefined generation size is used for optimization. Optimal bulk modulus, short term shear modulus and long term shear modulus for three average strain rates were found to be 73200, 13100, 347Pa for 136/s; 278000, 26200, 1510Pa for 183/s; 317000, 34900, 5210Pa for 262/s respectively. The variation obtained in these properties with strain rates suggests that the linear viscoelastic model being used for muscle tissues is not a perfect choice for characterizing the dynamic compressive behaviour of these tissues. The proposed methodology offers a method for determining soft tissue properties at different strain rates. The usage of GA based optimization eliminates the determination of derivatives as used in conventional optimization techniques [3]. With minimal effort, this method can also be extended to other material models considered for muscle characterization.

References [1] Karthikeyan B., Mukherjee S., Chawla A. Inverse finite element characterization

using impact experiments and Taguchi methods. To appear in SAE world congress 2006.

[2] http://www.iitk.ac.in/kangal/codes/sga/sga.tar. Simple GA Code Accessed as on September 2005.

[3] Kauer M., Vuskovic V., Dual J., Szekely G., Bajka M., Inverse finite element characterization of soft tissues, Medical Image Analysis, 2002; 6: 275-287.

2.7 Musculoskeletal Modelling Meets Muscle Physiology $491

7498 We-Th, no. 69 (P57) A passive computational model of rhythmic arm cycling used for the determination of task dynamics M. Klimstra, E .P. Zehr. Rehabilitation Neuroscience Laboratory, University of Victoria, Victoria, British Columbia, Canada

The estimation of length change, force and torque production capabilities of muscle is essential to understand neural control. Computational mechanical models based on anatomical and biomechanical parameters can be driven using experimentally-derived kinematics and used to predict kinetic values through inverse dynamics. Comparing the predicted kinematics to electromyo- graphic (EMG) values obtained experimentally can help determine the relation- ship between EMG and the anatomical arrangement of arm muscles during a cycling task. EMG can also be used as a model input taking into account the relationship between EMG, static force and muscle moment arms. A passive model of a human arm was developed using anthropometric values and driven by experimentally derived crank arm kinematics from a rhythmic arm cycling task. The computer model produced joint angular displacement, velocity and acceleration about the shoulder, elbow, hand ergometer interface and crank arm. EMG recording from shoulder and elbow flexor and extensor muscles were compared to model kinematics and muscle length change. A functional model of arm cycling was developed by relating EMG, predicted muscle length change and task kinematics to the crank arm cycle. This demonstrates that muscle length change can be used to predict phases of movement and the EMG, when related to the functional movement cycle, can be used to substantiate the role of a specific muscle in the completion of a rhythmic arm cycling task. This approach has potential applications to predict task kinematics and muscle length change and it is hypothesized that these variables could be used to develop a functional movement cycle. In future this will be implemented to quantity mechanical parameters related to the neural control of rhythmic arm cycling. Supported by NSERC.

6603 We-Th, no. 70 (P57) The medio-lateral force distribution in the sheep knee during walking

W.R. Taylor 1 , C. KSnig 1 , A.D. Speirs 1 , R.M. Ehrig 2, G.N. Duda 1 , M.O. Heller 1 . 1CMSC, Charit6 - Universit~tsmedizin Berlin, Germany, 2Zuse Institute Berlin, Germany

Although sheep have become standard models for understanding healing and regeneration processes after surgical treatments, the actual mechanical forces under which healing occurs in the sheep tibio-femoral joint during osteochondral defect repair and ligament reconstruction is hardly understood. To better understand the internal loading mechanisms, we aimed to determine the medio-lateral distribution of the forces that occur in the knee during normal gait in sheep. In this study, bone pins and reflective markers were used to measure the 3D kinematics of three sheep hind limbs. Simultaneous measurement of ground reaction forces during repetitions of gait trials allowed average peak tibio- femoral contact forces of up to 2.1 times body weight to be calculated at the joint centre using inverse dynamics and optimisation techniques. The points of contact were determined by calculating the minimum distances between the opposing condylar surfaces. Using the previously determined muscle force distribution, the magnitudes of the medial and lateral joint contact forces were then computed by solving the equilibrium conditions in the frontal plane. In general, the force on the medial condyle ranged between 65 and 74% of the total contact force over the loaded stance phase of gait. At the average peak contact force (at an average flexion angle of 57°), the contact force was distributed such that 72% of the load was on the medial condyle, corresponding to 1.51 BW on the medial and 0.70BW on the lateral condyle. From the force distribution determined in this study, we have provided a detailed understanding of the mechanical loading environment that occurs in sheep knees. This data will therefore serve as an important basis for interpreting the biological healing outcome of surgical treatment performed in sheep knees and allow better design of future animal experiments. Future comparison with these conditions in humans has important implications for the interpretation of quadruped experiments and their relevance to the clinical situation.

6015 We-Th, no. 71 (P57) Assessment of patellofemoral joint contact pressure in an uninjuried knee E Araujo, C. Bernardes, G. Portella, L.F. Silveira, J. Loss. Exercise Research Laboratory/Federal University of the Rio Grande do Sul, Porto Alegre, Brazil

Understand the complex interaction between contact area, contact forces and resultant pressure in the patellofemoral joint is essential to define the mecha- nisms responsible for injuries and estabilish more effective treatment strategies (Besier et al., 2005). However, the scientific literature lacks pressure data on patellofemoral joint, during a dinamic activity, and utilizing data obtained by