You can create the illusion of highlighting muscles by using the percentage dial for opacity in a program like Photoshop. 1) Create an opaque highlight overlay of the muscle in question. 2) Save the texture map as a series of highlight gradients from 0% to 75% (in increments of 5% to 10% should be sufficient). 3) Assign each incremental texture map to the figure as needed throughout the flexing motion. That should do it for you. Keep it simple and you'll be happy.