Other aspects of muscle use and misuse include the motor control patterns for various work tasks, which depend on force level, rate of force development, type of contraction, duration and the precision of the muscle task (Sjøgaard et al. 1995). Individual muscle fibres are "recruited" for these tasks, and some recruitment patterns may induce a high load on individual motor units even when the load on the muscle as a whole is small. Extensive recruitment of a particular motor unit will inevitably induce fatigue; and occupational muscle pain and injury may follow and could easily be related to the fatigue caused by insufficient muscle blood flow and intramuscular biochemical changes due to this high demand (Edwards 1988). High muscle tissue pressures may also impede muscle blood flow, which can reduce the ability of essential chemicals to reach the muscles, as well as the ability of the blood to remove waste products; this can cause energy crises in the muscles. Exercise can induce calcium to accumulate, and free radical formation may also promote degenerative processes such as the breakdown of muscle membrane and the impairment of normal metabolism (mitochondrial energy turnover) (). These processes may ultimately lead to degenerative changes in the muscle tissue itself. Fibres with marked degenerative characteristics have been found more frequently in muscle biopsies from patients with work-related chronic muscle pain (myalgia) than in normal subjects. Interestingly, the degenerated muscle fibres thus identified are "slow twitch fibres", which connect with low-threshold motor nerves. These are the nerves normally recruited at low sustained forces, not high force related tasks. The perception of fatigue and pain may play an important role in preventing muscle injury. Protective mechanisms induce the muscles to relax and recover in order to regain strength (Sjøgaard 1990). If such biofeedback from the peripheral tissues is ignored, the fatigue and pain may eventually result in chronic pain.
The prevalence estimates of low-back pain vary depending on the definitions used in different surveys. The prevalence rates of low-back pain syndromes in the Finnish general population over 30 years of age are given in . Three in four people have experienced low-back pain (and one in three, sciatic pain) during their lifetime. Every month one in five people suffers from low-back or sciatic pain, and at any point in time, one in six people has a clinically verifiable low-back pain syndrome. Sciatica or herniated intervertebral disc is less prevalent and afflicts 4% of the population. About half of those with a low-back pain syndrome have functional impairment, and the impairment is severe in 5%. Sciatica is more common among men than among women, but other low-back disorders are equally common. Low-back pain is relatively uncommon before the age of 20, but then there is a steady increase in the prevalence until the age of 65, after which there is a decline.
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The prevalence of degenerative changes in the lumbar spine increases with increasing age. About half of 35- to 44-year-old men and nine out of ten men 65 years or older have radiographic signs of disc degeneration of the lumbar spine. Signs of severe disc degeneration are noted in 5 and 38%, respectively. Degenerative changes are slightly more common in men than in women. People who have degenerative changes in the lumbar spine have low-back pain more frequently than those without, but degenerative changes are also common among asymptomatic people. In magnetic resonance imaging (MRI), disc degeneration has been found in 6% of asymptomatic women 20 years or younger and in 79% of those 60 years or older.
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The low-back pain in most people has mechanical causes, which include lumbosacral sprain/strain, degenerative disc disease, spondylolisthesis, spinal stenosis and fracture. Here only mechanical low-back pain is considered. Mechanical low-back pain is also called regional low-back pain, which may be local pain or pain radiating to one or both legs (sciatica). It is characteristic for mechanical low-back pain to occur episodically, and in most cases the natural course is favourable. In about half of acute cases low-back pain subsides in two weeks, and in about 90% within two months. About every tenth case is estimated to become chronic, and it is this group of low-back pain patients that accounts for the major proportion of the costs due to low-back disorders.
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Low-back pain is a common ailment in populations of working age. About 80% of people experience low-back pain during their lifetime, and it is one of the most important causes for short- and long-term disability in all occupational groups. Based on the aetiology, low-back pain can be classified into six groups: mechanical, infectious (e.g., tuberculosis), inflammatory (e.g., ankylosing spondylitis), metabolic (e.g., osteoporosis), neoplastic (e.g., cancer) and visceral (pain caused by diseases of the inner organs).
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Low-back pain is also associated with frequent or prolonged twisting, bending or other non-neutral trunk postures. Motion is necessary for the nutrition of the intervertebral disc and static postures may impair the nutrition. In other soft tissues, fatigue can develop. Also prolonged sitting in one position (for instance, machine seamstresses or motor vehicle drivers) increases the risk of low-back pain.