Muscle is a soft tissue that has the ability to contract. Contractions are achieved using protein filaments (actin and myosin) that slide past one another, decreasing the length and the shape of the cell. There are three types of muscles within the human body: smooth, skeletal, and cardiac.
Smooth or visceral muscles are found within the digestive tract, blood vessels, and other organs. They contract to move food within the digestive tract, open the pupils in the eye, propel the baby from the uterus during childbirth, and raise the hair on the surface of the body. These muscles are not voluntarily controlled.
Skeletal muscles are attached to the skeletal system and used to produce forces needed for body movement. These muscles can be voluntarily controlled.
Biomechanically, they serve as actuators for the skeletal system. Muscles act as the effort in the third-class levers of the body, such as the arm, where the elbow is the pivot and the forearm is the lever lifting itself and any load in the hand. The mechanical advantage is less than 1, but there is a speed multiplier proportional to the distance ratio of the bicep to the elbow.
Each skeletal muscle is comprised of several fascicles, bundles of muscle fibers. These cells are multinucleated cells or myocytes that contain multiple mitochondria for energy production as well as myofibrils, which are the contractile elements within the cell.
Each myofibril consists of a series of small units called sarcomeres that are connected end to end. Inside each sarcomere are myofilaments consisting of the proteins actin and myosin. These small units utilize the chemical energy stored in ATP (adenosine triphosphate) to contract the sarcomere. Contraction of the myofibrils causes contraction of the muscle cell, muscle fascicle, and the muscle overall.
The individual fibers are enclosed in a layer of connective tissue called the endomysium, and surrounded by blood vessels. The bundles are wrapped in the connective tissue called perimysium, and the entire muscle is surrounded in turn by a tough layer of connective tissue called the epimysium. The epimysium protects the muscle, allows it to slide against other surfaces in the body, and attaches to tendons at the end of the muscle.
Cardiac muscle is specialized connective tissue found only in the heart. Resistant to the squeezing forces required to pump blood, this muscle begins beating during fetal development and continues beating an average of 70 times per minute until the end of life. Cardiac muscle is not voluntarily controlled.
