How to animate (highlight) muscle parts (e.g. Leg muscles) on a moving person

Here is a tutorial where I manipulated my arm using mocha (tracker, which is bundled with Ae) and my extension MochaImport+. MochaImport+ is not free, but a bit cheaper than Lockdown. The approach is very flexible. Inside the stabilized precomp you can really do any kind of modifications.

https://mamoworld.com/tutorials/cyborg-arm-effect-your-next-action-film

The biggest difference between Lockdown and mocha / MochaImport+ is that the latter always works with just one track, whereas Lockdown can combine multiple tracks to also deal with deformations (and not just movements) of the surface. This adds some complexity, but helps if you have complex, warping surfaces.

An even simpler approach based on MochaImport+ is shown in this tutorial

If you have a budget I suggest Lockdown from AE Scripts. 

It's well worth the money if you have a fairly powerful system. The tracking and analysis take a long time. Lockdown is the best system I have found for distorting the overlay to match the distortions on the surface. 

If you don't have much of a budget then it's probably Mocha AE and a bunch of corner pin tracks of body parts. There is a pretty good tutorial on how to use Mocha AE to track a face and insert a really scary graphic over at Video Copilot. You would use basically the same technique to track different muscles. 

Neither technique is automatic. I recommend setting up comps that include only one shot and are only a couple of frames longer than the final effect is going to be. Trying to do this for 6 or 7 different muscle groups in a 30-second shot is going to turn into a nightmare but if you set up a comp that shows only five or six seconds of the hamstring being stretched, which is probably all you need to say something like "the hamstrings are activated when you do this." then you edit the final in Premiere Pro you'll be a lot better off.