Well, at least some of the blame anyway.
Dr. LB Verdijk and coworkers at the Masstricht University in The Netherlands have published several articles contributing insights into at least some of what goes on with the age related loss of muscle size and strength.See here and here.
Their work deals with satellite cells (SC), so called because in their resting form they hang out on the periphery of skeletal muscle cells, wedged between the basement membrane and the sarcolemma.They are the muscle's stem cells. They are poised to multiply and to differentiate into myofibers when signaled by damage such as occurs with strenuous exercise.
Their 2007 publication gives a good summary in the article's title, "Satellite cell content is specifically reduced in type II skeletal muscle fibers in the elderly" The authors performed muscle biopsies in the lateral thigh in 80 elderly subjects (age 76 +/- 1 yr) and 80 twenty year olds. The proportion and mean cross-sectional area of type II fibers was reduced in the elderly as were the number of satellite cells per fiber .A similar pattern was not noted in the slow twitch (type I) fibers.The type II fibers were smaller and fewer in number and contained fewer satellite cells per fiber .
Now for the sorta good news.
A more recent article from the same research group in the Netherlands reported that a 12 week program of resistance exercise training significantly increased both muscle fibers size and satellite cell count in type II fibers in elderly subjects. This was part of a more comprehensive study which examined muscle fiber type and satellite cell content in 165 subjects in various age ranges. 49 of which were 70-86 years of age.Also a subset of elderly subjects took part in a 12 week resistance exercise program .Muscle biopsies after the training program demonstrated increased type II fiber size and satellite cell content.
It seems plausible that the satellite cell awakening induced by resistance exercise was instrumental in muscle cell growth.There are , of course, many other factors in the muscle loss of aging including loss of sex hormones,increase in inflammatory cytokines,inactivity,poor nutrition and loss of anterior horn cells,among others. Resistance exercise in the setting of adequate protein intake is not the fountain of muscle youth but so far it seems the best we've got.
A final word.writing as someone who has run more ( much more ) than can be justified based on reasonable concerns of improving health .Running will not prevent sarcopenia. While running on level ground the quadriceps does little more than stabilizes the patella. I suppose running up and down hills may activate some type II fibers but basic jogging will not prevent the age related loss of muscle size and strength.That requires resistance exercise and adequate protein intake.Apparently you need resistance exercise to mitigate the age related loss of fast twitch muscle fibers.