Chapter 6: The Muscles of the Human Body
General Overview
Muscles are essential organs of the human body, composed of muscular tissue and connected to bones through tendons. Together, they form the systems that enable movement of the skeleton and contribute to locomotion, stability, and posture.
There are three main types of muscle in the human body:
- Cardiac muscle – forms the myocardium of the heart. It contracts rhythmically and involuntarily to sustain life.
- Smooth muscle – found in the walls of internal organs (viscera) and blood vessels, as well as in the skin and eyes. Microscopically, smooth muscle fibers lack striations. Their activity is involuntary and controlled by the autonomic nervous system.
- Skeletal (striated) muscle – the focus of kinesiology. These muscles form independent organs attached to bones, and their activity is voluntary, meaning they contract under conscious control.
Structure of Skeletal Muscle
Skeletal muscles are composed of multinucleated fibers (muscle cells). When observed under the microscope, they display transverse striations, hence the term striated fibers.
Each muscle fiber is wrapped in a thin membrane called the endomysium. Groups of fibers are bundled and covered by the perimysium, while the entire muscle is enclosed by a thicker membrane, the epimysium. These connective tissue layers provide protection, maintain muscle shape, and enable the transmission of force.
Muscle Fiber Types
Striated fibers are classified based on color, composition, and functional properties:
- Red fibers (slow-twitch, Type I)
- High content of myoglobin and mitochondria.
- Capable of sustained, prolonged contraction.
- Specialized for endurance activities.
- White fibers (fast-twitch, Type II)
- Rich in sarcoplasm and contract rapidly.
- Generate powerful movements but fatigue quickly.
- Specialized for speed and strength.
Both fiber types coexist within the same muscle, but their proportions differ depending on function and genetics.
The human body contains about 400 skeletal muscles, made up of approximately 250 million muscle fibers. The number of fibers per muscle varies according to its size and function.
- Example: the biceps brachii contains ~580,000 fibers.
- The gluteus maximus contains ~10 million fibers.
- The tensor tympani (a tiny muscle in the ear) contains ~1,100 fibers.
Muscle Attachments
Skeletal muscles attach to bones (and occasionally to skin or fascia) via tendons. Each muscle has three main parts:
- Origin (proximal attachment) – the end of the muscle attached to the more fixed or less mobile part of the skeleton, usually closer to the midline.
- Insertion (distal attachment) – the end attached to the more mobile bone, typically further from the midline.
- Belly (gaster) – the thick, fleshy central portion of the muscle between origin and insertion.
All attachments are mediated by tendons, which are made of dense connective tissue.
- In long, bulky muscles, tendons are usually cylindrical.
- In thin, flat muscles, tendons may form broad sheets known as aponeuroses.