There are 'muscle suits' you can purchase, some are molded latex, some are a matte, flesh colored spandex with soft padded muscles sewn in. Both are hot to wear, both look great under costumes.
quilting technique I so often sing the praises of is frequently used to create the softer version of these suits. You get two leotards/ pairs of leggings, put on one pair, draw in with fabric marker where & what you want emphasized, sew the two layers together along those lines (so for muscles, mostly big oblong shapes), snip a hole in the inside layer, use polyfil to stuff to the desired size, sew up the hole.
For a full body muscle suit I'd use a traditional leotard (snap crotch versions are mighty convenient) and leggings, put the leggings on first to hid the lump a waistband invariably shows.