What is it called when skeletal muscle fibers break down?

Apoptosis, due to: either ischemia or mechanical stress beyond what the phospholipid+cholesterol bilipid layer membranes of the muscle cell can endure without inducing cell death. When large quantities of muscle fibers die, commonly used terms include: rhabdomyolysis, myocardial infarction, ischemic infarction of bowel, etc., etc.

Rhabdomyolisis: Rhabdomyolisis is the breakdown of skeletal muscle fibers. More about rhabdomyolisis http://www.medicinenet.com/rhabdomyolysis/article.htm .