Also known as involuntary muscle, smooth muscles are found in the body organs such as the walls of the stomach or blood vessels. The nervous system controls the smooth muscles in both humans and canines, so these muscles function automatically without active thought or other form of human (or canine) control. Digestion, breathing, blood flow and other involuntary functions are also associated with smooth muscles.
Skeletal muscles are found throughout both human and canine bodies and are attached to the skeletal system or bones, producing the shape and appearance of the whole body. These muscles are striated, as opposed to smooth muscles, and considered voluntary in the sense that they are controlled through active thought and habitual movement. Skeletal muscles are responsible for the body's ability to move around, whether human or canine, and they produce heat during exercise to regulate the body's temperature.
Cardiac muscles are similar in texture to skeletal muscles (striated) but different from other muscles in the body. The cardiac muscles are also similar to smooth muscles in that the heart is involuntarily controlled. In both humans and dogs, the cardiac muscle is unique from all other muscles in the body because the heart never reaches fatigue and will continue working even when exercising or active. This is because the structure of the heart, with its steady blood supply, can accommodate sustained effort in exercise or other activity, provided there are no underlying conditions that compromise natural healthy heart function.
Dogs can have tense and sore muscles from overuse, just as humans can who suffer from a hard workout or repetitive activity. Dogs can also suffer atrophy, or the loss of muscle from not using it. Canines can suffer heart disease from poor diet and lack of sufficient exercise, as well as obesity, just as humans can. Canine diets, however, generally include larger amounts of protein than typical human diets, and for most dog breeds increasing protein and exercise will not result in serious "bulking up" like they will in humans who increase both to build muscle. Some dog breeds are selectively bred to produce large, strong muscles, such as American Pitbull Terriers, while a Chihuahua may grow strong and healthy from activity, though it will not look like it spends hours at the gym.