Although a number of muscles may be involved in an action, the principal muscle involved is called the prime mover, or . A muscle with the opposite action of the prime mover is called an .

Bone marrow is the tissue comprising the center of large bones. It is the place where new blood cells are produced. Bone marrow contains two types of stem cells: hemopoietic (which can produce blood cells) and stromal (which can produce fat, cartilage, and bone).