Perform Inverse Dynamics (ID)

Kinetics

Task 7: Perform Inverse Dynamics (ID). Use the IK solutions from Task 5 and 6 together with the ground

reaction forces provided to perform inverse dynamics on both of the walking trials. Analyze ONLY the FIRST right

foot strike for each trial. Note that your lecturer has been kind, and already exported the reaction forces from

the C3D file (jw_ngait_og1_grf.mot and jw_wpgait_ln4_grf.mot). However, before running ID, double check that

the forces are in the same coordinate system as your marker data by ‘associating’ the Force Plate.mot file to the

IK solution. You should see the green force vector going through the right foot. You will need to apply the forces

to the calcaneus (Review the External Loads tab of the Inverse Dynamics tool). Save the results from your ID

analysis.

Task 8: Calculate medial-lateral contact forces. During this motion capture session, we also measured knee

contact forces directly using four load cells embedded in the tibial tray of the knee implant

(jw_ngait_og1_knee_forces.csv and jw_wpgait_ln4_knee_forces.csv). The raw data files collected from

instrumented knee need to be converted to a force (N) using a set of calibration regression equations. These

calibrations were performed using an Instron machine prior to surgery. Use these regression equations below to

convert the raw data into medial and lateral contact forces.

3Prepare a report that answers the following questions. For each question write a short paragraph that

introduces the problem and then refers to the figure or table, where appropriate. Include tables or figures

with appropriate axes and labelling to highlight your findings.

1. Create a table showing the left and right hip joint centre locations (i.e. xyz coordinates in pelvis CS) using

the functional method (Task 1), the regression equation (Task 2), and the scaled OpenSim model (Task

3). Add a caption to the table that describes the data and highlights the main differences that you

observe. (5 marks)

2. What was the cut off frequency you chose to filter the marker data prior to calculating the functional hip

joint centres and justify why you selected this frequency. Discuss the limitation(s) of the functional

method (Task 1). (3 marks)

3. Create a figure comparing the hip, knee and ankle joint flexion-extension angles for the right limb

between normal walking gait and the walking pole gait (ie. Plot the results from Task 5 AND Task 6). Plot

data for the stance phase only (i.e. time-normalise your data to 100 points to enable a direct

comparison across trials as a % stance – use the InterpolateData.ipynb Jupyter notebook in the

SplineTimeNorm Python folder in Tutorial 1). Add a caption to explain the figure, highlighting the main

findings. (5 marks)

4. Plot the vertical ground reaction forces for the normal walking gait trial and the walking pole gait trial,

time-normalised to the stance phase. Comment on the difference in the peak vertical ground reaction

force and why you think this difference exists. (3 marks)

5. Plot the knee adduction moment for the two walking trials (from Task 7), time-normalised over stance

phase, and briefly comment on the difference between the normal walking trial and the walking pole

trial in the figure caption. (4 marks)

6. Comment on whether you should constrain (or clamp) the varus-valgus (abduction-adduction) or

internal-external rotation degrees of freedom at the knee when you perform inverse kinematics. Use

your knowledge of normal knee motion to justify this answer. (2 marks)

7. Plot the medial-lateral joint contact forces measured from the implant across the stance phase for the

two walking trials (from Task 8). Comment on the difference between the normal walking trial and the

walking pole trial in a figure caption. (3 marks)

8. Is the knee adduction moment calculated from inverse dynamics a reasonable estimate of the knee

contact forces for these two trials? Justify your answer given your knowledge of inverse dynamics and

the measured contact forces. (2 marks)

9. Using the data from your analysis make a recommendation to a patient who has medial tibiofemoral

osteoarthritis whether or not they should use walking poles to potentially reduce the progression of the

disease. Justify your response. (3 marks)