Can Saggital Motion Help Estimate Frontal Displacement in Treadmill Walking?

[btd]

Hello everybody,
I hope I am posting this under right topic.
I am running a research project and we are recording videos of subjects from both frontal (back) and saggital (side) plane while they are walking on the treadmill.
I put 10cm long calibration marker on the back of the subject. This way, when I analyze the video recording I can calculate how much center of mass displaced during walking. But the problem is subjects do not walk on the same spot on the treadmill all the time. They go front and back and this makes my 10 cm, 8cm or 12cm (estimation) on the recording
I am just wondering if I measure the displacement from saggital recording, can this help to calculate how much it makes change on 10 cm in frontal recording.

or is there a another way?
Thanks
Tanner

 

[AJ Bentley]

I think I understand - you are looking at how much a marker between two body parts shrinks and grows as they move?.
Can you place another marker by the side of the first but make it fixed so that it only changes size because of the saggital motion?
Then you simply divide one measurement by the other.