O’Regan SM, Guglielmo CG, Graeme M. TaylorMeasurement of arthropod body composition using quantitative magnetic resonance. Invertebrate Biology 2012;131(3):216-223. http://sachaoregan.ca/publications/O%27Regan%20et%20al.%202012.pdf
If I were you, I would probably try western blotting for skeletal myosin. Myosin makes up ~60% of the total protein in skeletal muscle. You wouldn't be measuring muscle content directly but you would be measuring a major component of it.