What Is Muscle Weakness?
Muscle weakness is the impaired status of muscle function characterized by decreased or low muscle strength and inability to perform normal work such as lifting a pot off the stove.
Q: How do muscles work?
A: Muscles do their work by contracting or shortening. For example, to move the foot up and down at the ankle, muscles attached to the foot by tendons must contract to shorten or relax to return to their resting length. Calf muscles contract to point the foot down (flexion) while the shin muscles relax (extension). For the foot to point up, calf muscles relax while the opposing shin muscles contract.
Each muscle is made up of individual muscle fibers. A muscle fiber is a long cylindrical cell that contains many nuclei, mitochondria, and sarcomeres. Each muscle fiber is surrounded by a thin layer of connective tissue called the endomysium.
Approximately 20–80 of these muscle fibers are grouped together in a parallel arrangement called a muscle fascicle or fiber bundle that is encapsulated by a perimysium. A distinct muscle is formed by enveloping a large number of muscle fascicles in a thick collagenous external sheath extending from the tendons called the epimysium.1 
Muscles fall into three types:
- Voluntary muscles. These muscles, also called skeletal, we can control by will. Voluntary muscles function by contracting their fibers to draw one part of the body toward another in flexion while opposing muscles that extend or pull a body part away from another. They move our bones to perform activities such as walking to get somewhere, chewing to eat food, lifting to do work, and moving the eyeball to look at something.
- Involuntary muscles. These muscles work independently of our conscious control. They are needed for internal organs, sphincters, and other parts to do their work, such as peristalsis in the gut that must function at all times to digest and move food, the squirting of bile juice into the duodenum by the Sphincter of Odi in the presence of fat eaten, and action of the pupil to see.
- Cardiac muscles. These muscles are specialized to keep the heart functioning at all times.
Muscle weakness can involve all types of muscles.
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