The benefits of physical activity are widely known. However, the risk of a musculoskeletal injury is an unfavourable consequence in physical training. Age, gender, injury history, body size, local anatomy and biomechanics, aerobic fitness, muscle strength, imbalance and tightness, ligamentous laxity, central motor control, psychological and psychosocial factors as well as general mental ability are factors in the predisposition to injury. Junior (15 to 16 years) and senior athletes seem to be at a higher risk of injury in many types of sport. However, the relationship between age and injuries apparently depends on both the type and intensity of activity practiced. The majority of injured athletes in many studies have been males. Men are, however, more likely to participate in vigorous exercise and sport and it is not known if men are at a generally higher risk of injury when the exposure is taken into account. Certain lesions, such as sprains, strains and dislocations, tend to recur. Previous injuries may necessarily not cause a repetition of injury if treated adequately, but certain individuals may be at a higher risk of injury due to injury-prone biological characteristics. Excessive height and weight have been shown to predispose to stress injuries in physical training. Idiopathic or acquired abnormalities in the anatomy or biomechanics in any joint may lead to a local injury. However, physical requirements vary widely between different types of activity and predisposition to injury due to anatomical or biomechanical factors seems to be characteristic for each type of exercise. Lack of fitness, muscle weakness, joint looseness and poor general flexibility have been suggested as factors in the outcome of athletic injuries but no definite conclusions can be made on the basis of the existing literature. Long simple reaction times to visual stimuli and long choice reaction times to visual stimuli have recently been related to musculoskeletal injuries. No exceptional personality dimension in injury proneness as a whole has been found and the results from specific groups cannot be extrapolated generally. Accumulation of life stress apparently predisposes to an athletic injury. Musculoskeletal injuries seem to be more common in subjects with lower scores in intelligence tests but no causation has been shown yet. Altogether, a complex network of risk factors for athletic injuries has been found. However, no prospective study including all the recognised injury risk factors has been presented in the literature.(ABSTRACT TRUNCATED AT 400 WORDS)