One of the hallmarks of the flu is weakness, often to the point where you can barely lift your head off the pillow. That's because the virus causes inflammation of the muscle fibers. This interferes with proper contraction, according to Irwin Siegel, M.D., associate professor of orthopedic surgery at the Rush Presbyterian-St. Luke's Medical Center in Chicago. The longer you stay bedridden, however, the weaker you'll be.
Movement builds up the proteins needed for muscle contraction.
If you're barely stirring, the proteins deteriorate. When you lose fluid, you drain away salt and also trace amounts of potassium , magnesium and calcium . These signals from the brain tell the muscles when to flex. Losing fluid upsets the electrolyte balance, and the signals may get confused before contraction can occur. But muscles can't live on fluid alone.