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Session: Poster session I
Session starts: Thursday 24 January, 15:00

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Bart Bolsterlee (Delft University of Technology)
Frans van der Helm (Delft University of Technology)
DirkJan Veeger (Delft University of Technology)

Abstract:
Introduction One important error source of musculoskeletal models is a mismatch between the anatomy of the model (mostly derived from cadaver studies) and the subject or patient that is analysed. This hampers the applicability of these models above the level of general applications, or “what if” questions. A subject-specific model can in principle be used on more specific, patient- or subject-related questions, but requires the model’s anatomy to be adapted to fit the subject. Personalization of the muscle’s physiological cross-sectional area (PCSA) is expected to decrease differences between model predictions and experimental data. Methods Of five subjects with strongly different builds, the maximal isometric force they could exert in six different directions on a handle that was gripped by the right hand with the elbow 90° flexed, was measured. The right shoulders of the same five subjects were MRI-scanned. By manually outlining muscle boundaries and summing the volume of the selected voxels, muscle volumes were obtained. Default PCSA values (measured on a single cadaver) that are used in the Delft Shoulder and Elbow Model (DSEM, [1]) were scaled with the ratio of muscle volume between subject and default model, divided by the ratio of muscle lengths. Two subject-specific PCSA-sets were made, namely one that uses the same volume scale factor for all muscles, and one that uses different scale factors for different muscles. For all five subjects and six force directions and with both the default and two subject-specific PCSA sets, the lowest value of maximum muscle stress (σmax, maximum force a muscle can produce per area of cross-section) for which the DSEM could reproduce the recorded force, was calculated. σmax is generally considered to be constant across muscles and individuals. Results With the default DSEM, a value of σmax=94.9 N/cm2 ±32.2 was required to reproduce the measured forces with the model. A lower and more consistent value across subjects resulted after uniform scaling, σmax=62.9 N/ cm2±4.9. Muscle-specific scaling did not lead to a significant difference with respect to uniform scaling, σmax=63.4 N/cm2 ±4.3. Discussion σmax varied substantially across subjects when using the default model (standard deviation 32.2 N/cm2). After uniform scaling, σmax estimation was much more consistent (62.9 N/cm2 ±4.9), indicating that PCSA scaling improves the fit of model predictions on experimental data. Because the relative strength per direction is dependent on relative PCSA values between muscles, muscle-specific scaling was expected to lead to a more consistent σmax across force directions but this was not the case. Other factors such as inter-individual differences in muscle moment arms might be of more influence. In conclusion, a musculoskeletal model with for each muscle individualized PCSA values can predict the maximal hand force consistently across different subjects, but does not perform better than a model of which the PCSA values are scaled by a single factor. REFERENCES [1] A.A. Nikooyan et al., Development of a comprehensive musculoskeletal model of the shoulder and elbow, Med Biol Eng Comput 49, pp. 1425-35, (2011).