Rodent motor units, muscle fibers, and motor proteins undergo significant aging-related changes. Such changes include spatial organization and physiological properties of fast- and slow-twitch single motor units, regulation of contractile speed and force generation capacity at the muscle fiber level, and altered functional properties of the motor protein myosin. In addition to specific changes, there also appears to be a "disorganization" of the coordinated expression of contractile, sarcoplasmic reticular, and mitochondrial protein isoforms in aging skeletal muscle. This is suggested to have a strong impact on aging-related impairments in muscle function in addition to the changes in specific muscle proteins.