Science is quietly revising one of its oldest assumptions about aging: that muscle loss is a problem of movement, not of survival. New research reveals that sarcopenia — the gradual wasting of muscle tissue in older adults — directly weakens the immune system, because muscle itself serves as a biological reservoir of the proteins and amino acids the body needs to fight infection. What has long been treated as an inevitable diminishment of strength is now understood as a slow erosion of the body's defenses, one that may be neither inevitable nor beyond our reach to slow.