One Week of Bed Rest Leads to Substantial Muscle Atrophy and Induces Whole-Body Insulin Resistance.

One of the many studies looking at the atrophy that occurs from disuse and bed rest. A 2008 study by Jirikca also estimated a 12% loss in as little as a week (50% loss in 3 to 5 weeks). This study concludes:

1 week of bed rest substantially reduces skeletal muscle mass and lowers whole-body insulin sensitivity, without affecting mechanisms implicated in high-fat diet-induced insulin resistance.