Skeletal muscle is one of three types of muscle in the human body. It is voluntarily controlled and allows for movements of the skeletal system and maintenance of posture. Physical activity, specifically weight and resistance training, allows for increases in skeletal muscle mass through hypertrophy of muscle fibers.
Microscopic Anatomy
Skeletal muscles are made up of muscle fibers, which are long cylindrical cells with multiple nuclei. These muscle fibers are in bundles and are surrounded by dense connective tissues. Each bundle of muscle fibers is called a fascicle. The functional unit of skeletal muscle is called a sarcomere. The sarcomere is a repetitive unit that serves as the muscle's contractile apparatus. The primarily contractile proteins in skeletal muscle are actin and myosin. Another important anatomical component of skeletal muscle is the sarcoplasmic reticulum and tranverse tubules, a network of membranes that allows calcium to move to its site of action within the cell.
Gross Anatomy
As the name implies, skeletal muscles are attached to the bony skeleton via tendons. There are five skeletal muscle shapes in the human body -- circular, convergent, parallel, pennate and fusiform. Circular muscle usually serve as sphincters or openings to body orifices. An example of a circular muscle is the orbicularis oris, which is the muscle around the mouth. Convergent muscles have a wide origin and a narrow bony insertion. An example of convergent muscle is the pectoralis major, the muscle of the chest. Parallel muscles are also called strap muscles. An example of a parallel muscle is the sartorius, a long muscle in the anterior thigh. Pennate muscles are arranged diagonally against the tendon and can be either unipennate, bipennate, or multipennate. An example of a bipennate muscle is the rectus femoris, one of the quadriceps group of muscles. Lastly, the fusiform muscles tend to have a wide muscle belly. An example of this muscle is the biceps.
Types of Muscle Fibers
There are two distinct types of muscle fibers -- fast twitch and slow twitch fibers. Type I or slow twitch fibers are more efficient at slow utilization of oxygen to make energy. Thus, these fibers are more useful and predominant during endurance exercises. Type II or fast twitch fibers can quickly make energy using anaerobic metabolism. Thus, these fibers are more useful in power activities such as sprinting or lifting weights.
Other types of muscle
Other types of muscle in the human body are cardiac muscle and smooth muscle. Cardiac muscle is a special type of muscle only found in the heart. It is unique in that electrical signals propagate very quickly through the muscle so that it can contract in a coordinated fashion. Smooth muscle is found primarily in the gastrointestinal system and in blood vessels. Unlike skeletal muscle, smooth muscle cannot be voluntarily controlled.
References
- "Junqueira's Basic Histology: Text & Atlas, Ch. 10"; Anthony L. Mescher, Ph.D.; 2010
- Teach PE: Muscle Shapes
- Fast Twitch and Slow Twitch Muscle Fibers
