_Inverse Kinematics Best Practices

Data Collection and Other Preparation:

1. When collecting experimental data, place three non-collinear markers per body segment that you want to track. You need at least three markers to track the 6-degree-of-freedom motion (position and orientation) of a body segment.
2. Place markers on anatomical locations with minimum skin/muscle motion.
3. These are just a few tips. Also review our full guide to Collecting Experimental Data.

Inverse Kinematics Settings:

1. Weight "motion" segment markers (from a triad placed on the thigh segment, for example) more heavily than anatomical markers affixed to landmarks like the greater trochanter and the acromion, which can be helpful for scaling, but are influenced by muscle and other soft tissue movements during motion.
2. Relative marker weightings are more important than their absolute values. Therefore, a weighting of 10 vs. 1 is 10 times more important, whereas 20 vs. 10 is only twice as important. Markers are not necessarily tracked better because they both have higher weightings.
3. See How Inverse Kinematics Works and How to Use the IK Tool for more information about IK settings.

Evaluating your Results:

1. Total RMS and maximum marker errors are reported in the "Messages" window. Use these values to guide changes in weightings or, if necessary, to redo marker placement and possibly scaling. Maximum marker error should generally be less than 2-4 cm, and RMS under 2 cm is achievable. These guidelines will vary depending on the nature of the model and the motion being examined.
2. If using coordinates from a motion capture system, make sure that the joint/coordinate definitions match—otherwise, you may cause more harm than good.
3. Compare your results to similar data reported in the literature. Your results from an unimpaired average adult should generally be within one standard deviation.
4. If you are unsatisfied with the results, recheck the results of scaling.