Notably, during the first 1-2 days of immobility a parallel activation of the ubiquitin-proteasome pathway and the IGF-1/Akt pathway seem to occur along with a deactivation of PGC-1α and PGC-1β, suggesting that cellular proteolysis plays an important role in the initiation of human disuse atrophy in both young and old muscle, whereas the concurrent regulation in protein synthesis signalling and proteolysis inhibition appears to affect young adults more pronouncedly compared to older adults.
The results reveal that irrespectively of age, mRNA expression levels of MuRF-1 and Atrogin-1 increased in the very initial phase (2-4 days) of human disuse-muscle atrophy along with a marked reduction in PGC-1α and PGC-1β (1-4 days) and a ~10% decrease in myofiber size (4 days).