What Are Voluntary Muscles (Skeletal Muscles)?

Voluntary muscles are the skeletal muscles of the body that attach to bones and control movement of the limbs, head, neck, and body under conscious control. Skeletal muscles are controlled by neuromuscular signals from the brain that communicate with individual muscle fibers and cause them to contract.

What Is the Difference Between Voluntary and Involuntary Muscles?

Voluntary muscles are skeletal muscles that contract and relax under conscious control. These muscles attach to bones and regulate movement of the body.

Involuntary muscles, on the other hand, are not under conscious control. They contract and relax automatically and receive signals from the autonomic nervous system, which regulates your internal bodily functions.

Voluntary Muscles

Voluntary muscles are skeletal muscles that make up 40% of your body weight and consist of 50% to 75% of your total body’s proteins. Skeletal muscles can convert chemical energy into mechanical energy to cause voluntary muscle contraction and movement.

Skeletal muscle is composed of fascicles, bundled units of multiple muscle fibers or muscle cells. Each muscle fiber consists of a cross-banded structure that is further divided into myofibrils that contain thick (myosin) and thin (actin) myofilaments, which give muscle its stripe appearance. This structure gives skeletal muscle a characteristic striated structure.

Muscle contraction occurs when these myofilaments move closer together when stimulated by the release of the neurotransmitter acetylcholine from a nerve cell that communicates with the muscle fiber.

Common examples of skeletal muscles include major muscle groups that control movement of the arms, legs, and body, such as the biceps, triceps, glutes, quadriceps, hamstrings, and abdominals.

Involuntary Muscles

Involuntary muscles are muscles that are not under conscious control. Their contraction and relaxation are regulated by the autonomic nervous system, which controls the activity of organs and blood vessels needed for essential daily functions, such as heartbeat regulation, circulation, breathing, digestion, and urination.

Most involuntary muscles are made up of smooth muscle. Smooth muscle lacks the striated structure of skeletal muscle and instead consists of sheets or layers of smooth muscle cells. When stimulated by the autonomic nervous system to contract from the release of hormones or other chemical signals, smooth muscle cells shorten via the movement of actin and myosin myofilaments.

Involuntary smooth muscles include the diaphragm, intestines, bladder, and walls of blood vessels.

The one exception of an involuntary muscle is the myocardium, or heart muscle. The myocardium is composed of a specialized type of muscle cell called cardiac muscle that is only found in the heart.

Cardiac muscle is striated like skeletal muscle, but is controlled by both the autonomic nervous system and its own pacemaker cells, causing it to contract automatically and rhythmically. 

Weak Voluntary Muscles: Skeletal Muscle Diseases, Neuromuscular Disorders, and Other Causes

Neuromuscular disorders, also called skeletal muscle disorders, are conditions that affect the nerves that send electrical signals to voluntary skeletal muscles to control movement.

When the nerves are damaged, communication between the nerves and muscles becomes disrupted. This results in significant muscle weakness, atrophy, and loss of function. The majority of neuromuscular disorders are genetic or caused by problems with the immune system.

Nerves communicate with muscles through the release of neurotransmitters at the neuromuscular junction, the space between a nerve cell and muscle fiber. Neuromuscular disorders can damage the nerve itself or the neuromuscular junction, where the signal is transmitted from a nerve to a muscle.

Symptoms of neuromuscular disorders include:

• Muscle weakness
• Muscle atrophy (wasting)
• Muscle twitches, cramps, or spasms
• Muscle pain
• Numbness and tingling
• Decreased coordination
• Droopy eyelids and double vision due to eye muscle weakness
• Difficulty swallowing due to weakness of the pharynx
• Difficulty breathing due to weakness of the diaphragm
• Poor balance

Common neuromuscular disorders include:

• Amyotrophic lateral sclerosis (ALS), also called Lou Gehrig’s disease, is a genetic disorder that results from hardening of the spinal cord. It causes damage to the nerves that control muscles and voluntary movement. 
• Charcot-Marie-Tooth disease is a class of peripheral nerve disorders that cause muscle weakness and atrophy, as well as loss of sensation, most commonly in the legs and feet. It is a genetic disorder caused by a gene mutation that damages myelin, an insulating sheath that surrounds all nerves and aids in the conduction of electrical signals.
• Multiple sclerosis (MS) causes degeneration of the myelin sheath surrounding nerves, which reduces the impulses sent along these nerves to muscles. It can result in muscle weakness, which is often more severe on your dominant side of the body. There are many forms of MS, but the condition is often progressive and gets worse over time if left untreated. 
• Muscular dystrophies are a group of genetic diseases characterized by gradual loss of motor function, muscle weakness and atrophy, gait problems, progressive respiratory failure, and cardiomyopathy. There are nine different types of muscular dystrophy, all caused by genetic mutations.
• Myasthenia gravis is an autoimmune disease that causes inflammation throughout the body. An autoimmune disease occurs when your immune system attacks healthy cells by mistake. With myasthenia gravis, the body produces antibodies that attack the receptors for acetylcholine, reducing the body’s ability to contract muscles. This leads to muscle weakness, atrophy, and fatigue.
• Myopathies, meaning diseases of muscles, cause muscle weakness and atrophy. Depending on the type, they may progress and get worse over time.

Muscle weakness can also result from electrolyte imbalances, such as altered levels of sodium, potassium, calcium, or magnesium. 

Summary

Skeletal muscles that attach to bones such as the muscles of your arms, legs, neck, back, and trunk are voluntary muscles that you can consciously control. Weakness or inability to control voluntary skeletal muscles can signal a health issue like a neuromuscular disorder or electrolyte imbalance. Involuntary muscles include those involved in automatic internal processes needed for survival that control your blood vessels and organs like your heart, lungs, and digestive system.

A Word From Verywell

Skeletal muscles are muscles under voluntary control that cause parts of your body like your arms, legs, and body to move when contracted. If you experience skeletal muscle weakness, make sure to discuss your type and duration of symptoms with your doctor, as this might be a sign of a medical condition such as a neuromuscular disorder. Always seek immediate medical attention for any sudden, unexplained weakness in your muscles.