December 1, 2003 - Nerve compression the cause behind most workplace RSIs; Prevention
Many of the aches and pains associated with repetitive stress injuries are caused by the compression of nerves.
Nerve tissue has many distinguishing properties that allow it to loop around and pass through contracting muscles without getting perpetually crushed and damaged. Nerves lie rather loosely attached to the muscles they enervate, and their slippery, fatty coating allows them to slide away from contracting muscle tissue while conducting electrical energy.
Crushed nerves can recover over time. The fatty myelin sheath can grow back after trauma, and severed nerves that have been repaired can regenerate at the rate of 1 mm per day.
Nerves and muscles function perfectly together during most dynamic movements such as walking, running, playing a sport or lifting items.
Prolonged and awkward postures are perhaps the most insidious cause of nerve compression injuries. Peripheral nerves--those located outside of the spinal cord--branch off of the main spinal cord as nerve roots and pass through holes in the vertebrae. They then branch out into the surrounding muscles, where they serve as "live wires" that electrically stimulate the muscles into action.
Workers who remain seated for long hours may experience a shooting pain or a burning ache. As the semisoft discs between the vertebrae compress, the bones settle closer together, pinching the nerve roots. Eventually the nerve transmits its discomfort down into the limb. This explains why constant pressure on the sciatic nerve can result in severe knee and foot pain, and compression at the neck can affect the hand.
Because nerve tissue is accustomed to a certain amount of pressure during normal muscle activity, nerve compression injuries may take a while to become apparent. Other symptoms of nerve compression can include mild tingling, itching, burning, muscle tics, electric "shocks" and numbness.
If a nerve is continually exposed to ergonomic risk factors such as constant compression or vibration, its conductive sheath can be worn off, causing complete numbness and lost dexterity. This loss of the myelin sheath is sometimes seen in the carpal tunnel during surgery.
Prolonged muscle contraction can cause the normally mobile nerve tissue to "hang up" in one or more locations along the limb. For example, a computer worker hunched over a keyboard with his head handing forward is tightening most of the supporting muscles around his neck, shoulders and upper chest. The nerves that exit the neck to serve the arms and hands can get compressed somewhere between the neck and the armpit.
In this posture, the nerve is not gliding smoothly within the muscle tissue like it should and every arm motion can yank the nerve. With traction injuries like this, the injured nerve may not respond right away. The resulting pain occurs later. This makes it difficult for the worker to determine what caused the injury or where the source of the pain actually originates.
Types of nerve compressions
Nerve compressions that occur between the neck and chest muscles are usually called thoracic outlet syndrome. TOS often affects the small and ring fingers of the hands first, and symptoms can include tingling, numbness and coldness in the hands as the artery often gets compressed as well. Poor static postures and overreaching are risk factors for TOS.
Double crush syndrome occurs when nerves that may be compressed higher up in the neck and thoracic outlet begin to affect nerves further down in the hand, such as the median nerve at the wrist. The three nerve branches that enervate the arm can refer pain down the arm, and impair use of the hand if they are compressed.
Carpal tunnel syndrome is nerve compression specific to the median nerve in the palm side of the wrist. The nerve exists in a high traffic area, passing through a dime-sized tunnel along with nine flexor tendons. If the palm is exposed to vibration or impact or if the wrist is held in a tight position while keying or mousing, this nerve can get compressed and damaged.
Forceful, repetitive gripping thrusts the median nerve against a hard ligament in the palm, and this can further compress the nerve or wear off its sheath.
Cubital Tunnel syndrome involves the ulnar nerve at the "funny bone" inside of the elbow. The ulnar nerve gets trapped as it passes around the inside knob of the elbow. This area can suffer traumatic compressions if a worker falls on his elbow.
Irritation at the elbow occurs as an RSI associated with slouching and overextending the arm, such as reaching for the mouse with the arm in full extension. Leaning on the elbow or keeping the arm flexed past 90 degrees can also contribute to ulnar nerve irritation. If serious compressions of this nerve are ignored, the small muscles inside the hand (the intrinsics) can waste away, leading to lost dexterity and strength. Radial Nerve Compressions are less common as RSIs. The radial nerve is responsible for extending and lifting the wrist and fingers, and provides a sensitive sensory branch on the thumb side of the wrist. If the hand is resting on a hard surface while mousing, this small nerve branch can be crushed.
The main branch of the radial nerve can be compressed in the top of the forearm associated with repetitive forceful gripping and lifting activities.
The forearm muscles can be come so overdeveloped by heavy labor that they get too big to fit in their compartment and begin to crush their own nerve supply. When an impact injury occurs to the forearm causing swelling, it is called "compartment syndrome" and without surgery, the nerves can die from compression.
The radial tunnel is a soft tissue tunnel deep in the top of the forearm under a small muscle called the supinator. Radial tunnel syndrome can occur with excessive use of the long finger--for instance while mouse clicking or operating phone switchboards or with repetitive work that requires forceful grasping and turning, such as using a screwdriver.
Sciatica is a common compression that occurs in the lower back where the large sciatic nerve passes between the lumbar vertebrae. A deep buttock muscle called the pyraformis is often involved.
This injury usually occurs with prolonged sitting and slouching. Symptoms include pain from the buttock to the foot and symptoms can be severe. The best prevention for nerve compression injuries is a well-supported, neutral posture and the discipline to get up and take several "micro breaks" an hour. A regular exercise program several times a week that includes flexibility training such as yoga, Tai Chi and gentle stretching will help to keep nerve and muscle tissue healthy.
RELATED ARTICLE: Risk factors for nerve compression injuries
Nerve compression injuries occur when one or more of the ergonomic risk factors are present such as:
1. Prolonged and/or awkward postures.
2. Prolonged forceful gripping.
3. Excessive exposure to vibration.
4. Repetitive motions isolated to a single body part.
5. Exposure to a cold environment.
6. Contact against hard or sharp surfaces.
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