Overuse injuries, otherwise known as cumulative trauma disorders, are described as tissue damage that results from repetitive demand over the course of time. The term refers to a vast array of diagnoses, including occupational, recreational, and habitual activities.
See Common Pediatric Sports and Recreational Injuries, a Critical Images slideshow, to help recognize some of the more common injuries and conditions associated with pediatric recreational activities.
The pathophysiology of overuse injuries is based on the idea that tissues adapt to the stresses placed on them over time. These stresses include shear, tension, compression, impingement, vibration, and contraction. Mechanical fatigue within tendons, ligaments, neural tissue, and other soft tissues results in characteristic changes depending on their individual properties. This fatigue is theorized to initially lead to adaptations of these tissues. As the tissues attempt to adapt to the demands placed on them, they can incur injury unless they have appropriate time to heal. The rate of injury simply exceeds the rate of adaptation and healing in the tissue. Evidence also suggests that chemical mediators are involved in the initiation and propagation of overuse injuries.
Nerve tissues are at particular risk for ischemic injuries. This ischemia leads to characteristic changes in the nerve itself. The timeline generally begins with subperineurial edema, followed by thickening of the perineurium, thickening of the internal and external epineurium, thinning of the peripheral myelin, and, eventually, axonal degeneration.
One hypothesis is that the development of muscular pain originates from the nearly continuous activation of low-threshold motor units that occurs in muscles performing continuous or slow, repetitive tasks, causing depletion of adenosine 5'-triphosphate (ATP) in those fibers. With insufficient ATP, sarcoplasmic reuptake of Ca++ could be reduced, resulting in high concentrations in the cytosol, allowing Ca++ –dependent activation of phospholipase, the generation of free radicals, and damage to the muscle fibers involved. This theory has a rational physiologic basis, but it remains to be proven. Multiple studies have shown that patients with more significant work-related, upper extremity disorders exhibit more muscular activity on electromyelography (EMG) findings; however, these studies are observational and not designed to exhibit causality.
Increasing data in in vitro and in vivo human and animal models shows that there are tissue-level changes associated with repetitive stress. Prostaglandin E2 has been found to be present in high quantities in overuse tissues in rat and chicken models.  This mediator has been suggested to influence cell proliferation, increase collagenase, and decrease collagen synthesis. Increasing loads on these tissues alters the amount of nitric oxide and prostaglandin E2. However, another hypothesis based on rat-model observations suggests that overuse may lead to an understimulation of tendon cells, rather than to overstimulation. 
Alterations in the regulation of genes within tendons undergoing overuse have been shown in the rat model.  These changes include upregulation of genes associated with cartilage, and down-regulation of genes associated with tendon. This suggests that overuse may cause a morphologic alteration of tendon tissue, causing it to become more cartilaginous.
Moderate (40 N) and high (60 N) cyclic loads are reported to create an acute neuromuscular disorder characterized by delayed hyperexcitability in the lower back. This delay is characteristic of an inflammatory state. Microtears within muscle tissue have been shown to be related to higher repetition loads and cyclic rate. [4, 5, 6, 7]
Psychosocial factors have been implicated in overuse injuries for decades.  A partial list includes work satisfaction, perceived physical health, perceived mental health, coping mechanisms of the patient and his/her family, perception of work-readiness, and anxiety.
A review of the English-language literature revealed specific articles focusing on ultrasonographers, equestrian athletes, ballet dancers, bicyclists, baseball players, swimmers, triathletes, golfers, bull riders, martial artists, sign language interpreters, skeletally immature patients, college students, heavy computer users, assembly line workers, tailors (seamstresses), surgeons, dentists, and nurses. This list dramatizes the point that at least the perception exists that many common and some uncommon ailments are associated with repetitive motion.
The Medscape Reference topic Nerve Entrapment Syndromes may also be of interest.
The incidence of overuse injuries as a whole is nearly impossible to estimate given the sheer volume of included diagnoses, as well as the difficulty in establishing clear diagnostic criteria.  However, several long-term, retrospective, work-related studies have estimated the annual incidence of upper extremity disorders at 4.5-12.7% per year. The frequency of injury in each diagnostic category is more appropriately left to more focused literature.
A study by Roos et al indicated that compared with high school sports in the United States, the rate of overuse injuries in college sports is 3.28 times greater. The study, which used data from the National Collegiate Athletic Association Injury Surveillance System and the High School Reporting Information Online (High School RIO) sports injury surveillance system, also found that in sex-comparable sports, the rate of overuse injuries was higher in females than in males by ratios of 1.25 (college sports) and 1.55 (high school sports). 
A study by Schroeder et al found that in US high school sports, the highest rates of overuse injuries were associated with girls’ track and field and girls’ field hockey. The study, using data from the High School RIO surveillance system, also determined that overuse injuries made up 7.7% of all injuries sustained in high school sports. 
Overuse injuries are not associated with direct mortality. Morbidity, however, is significant. The impact of these injuries varies from the occasional annoyance to loss of function as a result of frank tissue destruction. In many performing artists, musicians, craftsmen, and workers, loss of function at even a minor level can result in a significant loss of livelihood (leading to the various difficulties associated with this loss). The direct economic impact of overuse injury in the workplace is immense. The indirect impact is nearly incalculable if the number of health care dollars involved is considered. Of particular note, one interesting review of worker demographic data suggested that workers with cumulative trauma disorders were subjected to employment discrimination. Depression and quality-of-life issues have been described after a diagnosis of chronic overuse injury.
Race is not a differentiating factor for overuse injury incidence.
For a variety of hypothesized reasons, differences in sex play a role in certain overuse injuries.  Most notably, a significant female predominance in carpal tunnel syndrome has been noted. This has a variety of possible causes, including anatomical differences in the carpal tunnel, hormonal differences, and, importantly, differences in the activities performed by men and woman. Other biomechanical differences have also been implicated; elbow carrying angles, Q-angles, femoral anteversion, and lean body mass are the most commonly stated. Psychosocial and cultural phenomena also play roles.
Age would be expected to be an independent risk factor for overuse injury; however, given the dependence of overuse injury on activity and the changes in activity that typify aging, the contribution of age as a risk factor is difficult to determine. 
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