Final answer:
Muscle atrophy with age, or sarcopenia, primarily involves the reduction in the sizes of muscle fibers (option C) , contributing to the loss of muscle mass and strength. Although exercise can partially counteract this atrophy, the decline due to sarcopenia is largely irreversible.
Step-by-step explanation:
Muscle atrophy that progresses with aging is predominantly caused by a reduction in the sizes of muscle fibers. This process, known as sarcopenia, is characterized by the loss of skeletal muscle mass and strength. The main cause of sarcopenia is not fully understood, but it may be linked to a decrease in the responsiveness to growth factors necessary for maintaining muscle mass.
Sarcopenia is further influenced by the death of muscle fibers, which are then replaced by adipose and connective tissues, leading to a loss of muscle strength and function. Factors such as reduction in motor units and a shift to more SO (slow oxidative) fibers from FG (fast glycolytic) fibers, which generate less powerful movements, also contribute to the decline in muscle mass and strength.
While exercise can offset some effects of muscle atrophy by adding structural proteins and causing cellular changes that increase mitochondrial numbers, capillary density, and connective tissue strength, the age-related sarcopenia is largely irreversible. Consequently, less muscle tension is produced resulting in impairments that negatively affect quality of life in the elderly.