When muscles go in adults it's due to individual muscle cells getting bigger and not due to increase in muscle cells. It's generally believed that no new muscle cells get produced in human adults. Sometimes muscle cells die. I would expect that as humans age therefore the count of muscle cells will go down. There are frequently claims that there's a limited number of hallmarks of aging that if repaired would stop aging. Neither SENS list nor the 2013 paper seem to include muscle cell count as one mark of aging. Is there a good reason for this?