According to a study from 2000, named Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr (J Appl Physiol (1985). 2000 Jul;89(1):81-8), generally in women, 31% of their body weight is muscle mass. So, for a human female weighing 90 pounds, her muscle mass would be of approximately 27.9 pounds (90*0.31=27.9).