There are three main types of muscle fibers defined by their speed of contraction and ATP production pathways: slow oxidative fibers that contract slowly and use aerobic pathways, fast oxidative fibers that contract quickly with moderate fatigue resistance, and fast glycolytic fibers that contract quickly but fatigue easily. Aerobic exercise increases muscle capillaries, mitochondria, and myoglobin while resistance exercise leads to muscle hypertrophy and increased mitochondria, myofilaments, and glycogen stores. Muscular dystrophy is a group of inherited muscle-destroying diseases where muscles enlarge due to fat and connective tissue deposits but muscle fibers atrophy.