Scientists at Harvard University have determined significant variations among skeletal, cardiac and smooth muscles, though all the muscular movements use actin against myosin to create contraction. Skeletal muscles are anchored by tendons to bone, and control all movements of the human body including the postural control. An average adult male is made up of 40% to 50% of skeletal muscle and an adult female of 30 to 40%. It is pertinent to note that smooth muscle is not always under conscious control. Cardiac muscle is a specialized kind of muscle found only within heart. The latter two types are packed into highly regular arrangements of bundles. Heart muscles contract and relax in short, intense bursts, whereas smooth muscles sustain longer contractions. Skeletal muscles are arranged in discrete manner, connected by tendons to processes of the skeleton. The fibers of cardiac muscle are typically branched like a tree and are connected to other cardiac muscle fibers through inter-locked discs. Contrary to popular belief, the number of muscle fibers can't be increased through exercise. Instead the muscle cells get bigger. In skeletal muscle, contraction is always regulated by internal pacemaker cells.