Sentence:  Microscopic studies of the gastrocnemius , pectoralis major , transversus abdominis , biceps brachii , and diaphragm showed atrophy as well as varying degrees of injury ranging from swelling and vacuolization to focal necrosis of the <WSD>muscle</WSD> fibers . 
Meaning of the word marked with <WSD> tags:  an elongated contractile cell that forms the muscles of the body