How to have a line move with a body part during an exercise demo video

I would use Mocha Pro to track the Translation only of the body parts where you want the circles to be. One track for each body part. One at the knee, one at the foot, one at the hip. If the shot has enough detail, you should be able to get a good track for the joints of each moving body part. 

You can then apply the tracks to a null for each joint. 

You then draw a shape layer path with as many vertices as you need for each limb. That would be three for the foot, knee, and hip. Then you open the Create Nulls from Paths script from the window menu, select the path, then apply Points follow Nulls. Then, all you have to do is use a simple pick whip expression to tie the position of each Path null to the appropriate Track Null, and you have your animated line. 

Do the same thing for each circle layer. Just tie the position of the layers to the position of each Path Null with simple Shift + Parenting or an expression.

The only way this will work is if you have enough detail in the body parts to get a decent translation (transform) track in Mocha AE. AE's feature tracker requires significant detail in a small area, which is totally unsuited for this kind of tracking.

If you can post a screenshot of a frame of your video, I can tell you if there is any chance for a good track. Just drag it to the reply field instead of using the "Drag & Drop Here..." area so we can easily see it without downloading.

If tracking in Mocha AE does not work for any or all of the points, you will have to animate the position of some nulls by hand, then tie the corners and the vector points to your animated nulls.

Here are the basics of using Mocha AE: Boris FX Mocha AE Training

Hi @Greg5FB0,

I'll move your post to the After Effects forum. I hope that's OK.

Thanks,

Kevin