Age changes of myofibrils of human minor pectoral muscle

Micro- and ultramicromeasuring studies have been made on the age changes of surgically resected minor pectoral muscles in 200 Japanese females of various ages (26-80 years). With advancement of age, muscle fibers of type I decreased in number and increased in size, and the age-related increase in size is not considered to be due to an increase in number of myofibrils but seems to be due to an increase in their volume resulting from an increase in the number of myofilaments.