This tutorial explains how I go about doing the muscles and skintones using Roger Cruz's Victory picture. It uses a custom gradient which I have made up after much experimentation with tones and colours, and use of the airbrush and smudge tools.
Having done the flats layers it's now time to start painting over the top of them. you may well end up completely covering the skin flats in this picture's case but each picture will be different. First step is to make a circular selection. Don't worry if it goes over the hair region as you can use the eraser tool to rub away any extra areas afterwards.
Notice my custom gradients visible in the navigator palette. Use this to make up your own!
Next, use the custom gradient to make a circular fill. You might want to take the extent of the gradient beyond the selection area to make the last shades a bit lighter. Well, it's what I did anyway... Rub back any gradient that goes over the hair region.
Make another selection following the pencilled lines as a guide. This is where having a good artist who knows their anatomy comes in handy and makes your life a lot easier than having all sorts of odd stray muscles everywhere.
Keep making selections and filling with the gradient tool as shown. You may have to use the airbrush set to the colour of the skintones plus using a diffuse brush, to adjust the highlights and shadows of the gradient as required.
Now what about those hard edges? This is where the smudge tool comes in handy. Select a fairly large diffuse brush and just wiggle it along those edges, blurring them into each other (but not too much) until it looks blended enough.
And that's pretty much it! Now all you need to do is keep building this layer up, smudging the edges as you go along. You may need to make new layers to make minor local adjustments but it's just a matter of fiddling until it looks right. This example screenshot is by no means finished but it gives you the idea at least.