i would flatten a ball of putty over the general shape, then use a clay shaper to indent the central groove. then use it to form the horizontal lines and then tidy up the corners to make the individual muscles more rounded
If I were sculpting this in any kind of two-part putty freakinacage\'s method is pretty much how I\'d do it too.
There\'d be more fiddling around to get the right degree of softness - maybe a bit of brushing over with a soft brush and solvent for example - but basically I\'d just put on a slab of putty and indent the texture, much as I\'d do for any muscle definition.