1. Sensitivity of Musculoskeletal Models to Scaled-Generic Knee Kinematic Errors
M. Williams1, S. Martelli1, N. Sancisi2, M. Conconi2, V. Parenti-Castelli2, K. Reynolds1
1Medical Device Research Institute, School of Computer Science, Engineering and Mathematics, Flinders University, South Australia
2Department of Industrial Engineering, University of Bologna, Bologna, Italy

2. Musculoskeletal forces during activity
[1]
Increasingly used to study human motion in healthy and pathological conditions [1].
Increasing field
Modelling the mechanical system
Non-invasive
[1] Hicks et al., J Biomech Eng Feb 1;137(2):

3. Creating confidence on model calculations
Verification, validation and sensitivity studies in computational biomechanics [1,2].
No study has assessed the sensitivity of musculoskeletal model calculation to common scaled-generic knee kinematic assumptions.
Increasing field
Modelling the mechanical system
Non-invasive
[1] Anderson & Weiss, Comp meth biomech and biomed eng, 2007, 10 (3)
[2] Viceconti et al., Clin Biomech, 2005, 20 (5).

4. Aim
To assess the effect of scaled-generic knee kinematic errors on calculation of muscle and joint forces and knee ligaments elongation

5. Participants and data collection
Participants: 13 healthy women of year of age
Motion capture (10-cameras VICON system, Oxford Metrics Group, Oxford, U.K., 120Hz): walking, jumping, stair ascent, stair descent, chair up and down, fast walking
GRF (3 AMTI,Watertown, MA, USA, 1000 Hz)
CT (Aquilon CT, Toshiba, 0.5 x 0.5 x 0.5 mm, 120kV, 200mA) including pelvis and proximal tibia
Scaled-generic models validated [1] against published joint angles [2,3], hip contact forces [4] and muscle firing patterns [5]
[1] Martelli et al., J Biomech., 2015, In Press
[2] Kadaba et al., Journal of Orthop. Res., 1989, 7, 849–60.
[3] Stacoff et al., Gait Posture 21,2005, 24–38.
[4] Bergmann et al., J Biomech., 2001, 34, 859–871
[5] Inman et al., 1981, Human Walking. MD: Williams & Wilkins, Baltimore

6. Knee kinematic models 3D model with physiological constraints [4,5]
RMSE < 2.3º & 1.6 mm
The common type of model… scaled
Scaling of random subject from 90’s – produces kinematic errors
What is the effect of these errors?
Recent protocol identifies subject-specific knee motion, can now find out these effects by creating model C
We developed a method…
Planar-scaled model [1,2,3]
[1] Delp et al., IEEE Trans Biom Eng, 1990, 37 (8)
[2] Yamaguchi & Zajac, J . Biomech., 1989 , 21:1-10
[3] Martelli et al., J Biomech., 2015, In Press
[4] Ottoboni et al., P I Mech Eng H, 2010
[5] Conconi et al., IDETC/CIE 2013, Portland, USA, 2013

7. Study design Scaled-generic models from [1]
Joint Angles
Joint Moments
Muscle Forces
Joint Contact Forces
Ligament length
Scaled-generic models from [1]
Subject-specific model [2,3]
13 healthy women. Age: 60-78;
2 stages of input, motion data: Activities?
Scaled-generic vs subject-specific
(6 of 13 analysed)
[2] Ottoboni et al., P I Mech Eng H, 2010
[3] Conconi et al., IDETC/CIE 2013, Portland, USA, 2013
[1] Martelli et al., J Biomech, 2015, In Press . 

8. The calculated 3D knee kinematics
[1] Sheehan et al., J Biomech, 2007 

9. Comparison of joint angles
N = 3600 (6 subjects x 6 tasks x 100 frames)
Explain data analysis
Further work is accumulating data for more subjects
Comparing ligament extension across models

10. Comparison of joint moments
Explain data analysis
Further work is accumulating data for more subjects
Comparing ligament extension across models

11. Comparison of muscle Force
Explain data analysis
Further work is accumulating data for more subjects
Comparing ligament extension across models

12. Comparison of joint forces
Explain data analysis
Further work is accumulating data for more subjects
Comparing ligament extension across models

13. Scaled-generic ligament length
Explain data analysis
Further work is accumulating data for more subjects
Comparing ligament extension across models

14. Conclusions
Scaled-generic models are a valid solution (R2 > 0.97) for calculating patterns of joint angles, muscle and joint forces, although localized high force errors (<2105 N) have been shown.
Scaled-generic knee kinematics induce unrealistic ligament elongation (Dl/l < 60%).
Explain data analysis
Further work is accumulating data for more subjects
Comparing ligament extension across models

15. Thank you Explain data analysis
Further work is accumulating data for more subjects
Comparing ligament extension across models

17. Knee Modelling Results