1. A closer look - Muscles and their role in health and energy needs of athletes Skeletal muscles, which move bones, consist of cells fused into long fibers. These muscle fibers can increase in size when contracted, which is what happens as a result of some exercise. This animation shows the structure of vertebrate skeletal muscles. It starts at a large scale, showing the muscles of a human upper arm, and then gradually zooms in to smaller and smaller levels, labeling each structure along the way. Watch the animation and then answer the question that follows. An individual muscle fiber is one long muscle cell . An increase in skeletal muscle size, resulting in increased strength is known as