abdominals and chest

The ‘six pack’ is a lay term referring to the divisions of the rectus abdominus which are two vertical long muscles, joined by the vertical connective tissue called 'linea alba' and transversely divided by three fibrous bands (tendinous intersections), which are what give it the '6 pack look'.

I feel it should be called the 8 pack or the 7 pack, because the three fibrous bands make four divisions, the lower one is vertically divided but not usually obvious, hence why I say 7!! Anyhow '6-pack'  is a lay term, not to be confused with the actual muscles.

LET'S LOOK AT DRAWING MALE ABS and CHEST FIRST (for no particular reason!!) Follow the images and notes here, then watch the

As you can see in these models, there are more muscles coming into play around the rectus abdominus which look somewhat different in males to females, and also of course in toned and untoned bodies.

See if you can identify the serratus anterior, the external oblique and the ingruinal ligament.(by using your anatomy reference)

There are more muscle layers underneath, as you see in the anatomy reference but these ones are more likely to make an external mark which is what you need to familiarise yourself with them most of all for drawing.

Below I have added our typical rough stickman proportions over the top of this dude and a rough skeleton. 

FEMALE ABS

This photo shows a non-body-builder female who is lean. You can faintly see the demarcation of her rectus abdominus. In general, softer female abdomens and breasts are best drawn and practised from photos and life. They all have a belly button, they all have nipples, but each one you draw will have its own variations on the general shapes. If you are drawing leaner or more muscular feminine abs or chest, you might see similarities with the male shapes that make the light and shade areas.

This is a helpful thing to remember when drawing the figure, that the Human body is not really made up of solid lines. Think of it instead, as a series of shapes and tonal values.