In Praise of Project Planning, Pilots, Points and Processing

Knowing where to place your Markers

Marker placement is a huge subject and can be tricky to grasp if you are a newcomer to Motion Capture. Even highly qualified and experienced biomechanists can struggle when presented with a new project and a blank sheet. If your mathematical background doesn’t include a detailed knowledge of Vector Algebra where do you start?

If you can find similar studies in the published literature you may be able to use a previously developed marker-set and modeling protocol. Many lower limb studies will be based on the so-called Helen Hayes method (though it is also known by many other names). This is often regarded as the gold standard for Gait Analysis, despite being from the 1980’s. It was perfect for the camera systems of the day, using the absolute minimum number of markers to save all the post processing time. It solves problems that 21st century systems no longer have. Though take care as almost all models now are ‘modified-Helen-Hayes’ and they vary in subtle ways.

If you can’t ‘stand on the shoulders of giants’ where do you begin? 

Let’s consider Markers, Virtual Markers and Points (ie coordinates in 3d space). Each segment that you want to track will need at least three, non-linear, points to give all six degrees of freedom, XYZ and 3 Rotations. An individual marker tells you a position; singularly it can’t give you an orientation.

These can be real markers or virtual ones, which are points calculated from marker data but offset in a rigorously defined way. For example if you want to know where a Knee Joint Centre (KJC) is, you can’t place a marker inside the body but if you can place two, carefully on the surface where the knee joint axis line passes through the skin it is possible to readily define the KJC, as a Virtual Marker, halfway between them. Motion Analysis’s highly regarded Cortex software has an easy to use graphical interface that presents this and even more complex Vector Algebra with beautiful simplicity.

Three markers for every segment can soon add up, but often a single, carefully defined point (real or virtual marker) can be shared. That KJC point, or the outside knee marker can be used for the thigh and shank segments.

After you have decided on a marker set that covers the segments that you need to know the angles between, plus all the other spatial–temporal parameters which Cortex will also calculate for you, you’ll be ready to get on with the fun part: the motion capture of all your subjects. Or can you?

You may need to revise your ideal marker set due to the reality of you systems camera count and the specific move you want to analyze. For example, if your laboratory has cameras permanently fixed high on the walls and you are studying push-ups, any markers on the front of the subject will be impossible to see. For the chest you could just place three or more markers on the subject’s back and remove the front markers. Or better still leave the front markers, add additional back markers and use predefined tools to recreate the ‘lost’ markers with virtual ones calculated from the relative positions of these extras. There are gap filling interpolation tools but they are making up data, whereas these ‘virtual joins’ are using genuine information.

It’s worth consider a Pilot Study

If this is all seeming a bit difficult to grasp, then we highly recommend a Pilot Study. Often the best way to find out what snags are going to catch you out is to get in the lab and try it. Set aside time to try a couple of subjects with different marker sets. Work through this data and find out where markers need adding, or which ones are not needed. It will also help you decide which parts of the myriad results are essential for your study.

Time and effort spent on a Pilot Study early on always saves you later down the line! It can help prevent hours of clean up and post processing, and most often it highlights a tiny tweak that could have prevented you answering the actual question you set out to study.

If you don’t have a lab yet but are thinking of investing in one then please contact Motion Analysis, we’ll be happy to discuss your experiment. We’ve helped set up thousands of labs around the world and will be happy to assist.

Important considerations when purchasing a motion capture system

If you’re still in the process of deciding on the motion capture system you need, we have put together a helpful checklist to keep in mind during your decision making process.