There are three different types of muscles:
1) cardiac muscle,
2) smooth muscle and
3) skeletal muscle.
Cardiac muscle is an involuntary muscle type found only in the heart. It has striated fibers with its sarcomeres packed into extremely organized groups. This type of muscle relaxes and contracts in intense short bursts. You are probably well aware of this fact, especially after completing exercises such as running where you can feel the pounding of your heart through your chest.
Smooth muscle is also an involuntary type of muscle found inside the walls of the organs of your body. Your intestines, blood vessels, stomach are a few of the structures in your body lined with smooth muscle fibers. Like cardiac muscle, smooth muscle is also under unconscious control.
Skeletal muscle is the only voluntary muscle of the three. These muscles attach to the bones of your body through a highly elastic tissue know as tendons. They are located throughout your arm, legs, and torso and provide for movements of your body such as walking and running, as well as less visible movements such as maintaining an erect posture.
Skeletal muscle is the type that all athletes seek to directly improve through various types of exercise. This is naturally due to the fact that these muscles are under your conscious control.