Upper respiratory tract infections (URTI) are one of the most common illnesses affecting the general population and particularly athletes, often reducing the individual capacity of physical performance. Epidemiological data suggest that intensive exercise, training and competition may increase susceptibility to respiratory infections. On the other hand, some studies defend that less active subjects may reduce their risk of URTI by engaging in moderate exercise training. Nevertheless, reports are not unanimous and frequently contained various limitations. The higher incidence of infections in elite athletes has been widely attributed to immune suppression induced by exhaustive exercise, however this has not been clearly demonstrated. The present series of studies aimed to investigate the relationship between physical activity and the incidence of URTI in young healthy females with a range of physical activity levels, from sedentary to elite athletes (tennis players). Additionally, it was intended to explore the temporal association between specific characteristics of tennis training and competition, the incidence of URTI, and salivary lgA levels (μg.mr1 ; μg.min-1 ) and changes, in elite female tennis players over a 12-week period. To accomplish the objectives described, a practical method for quantifying tennis play was validated and applied. Major findings of this research include the greater incidence of URTI symptomatology in girls with low or extremely high levels of physical activity as compared to those with moderate levels. Moreover, the incidence of URTI in elite tennis players was directly correlated with the training load and competition level, on a weekly basis. In the subsequent study, one hour of intense tennis play produced a significant drop in salivary lgA secretion rate (S-lgA), and the magnitude of the immune suppression was directly associated with the amount of training undertaken during the previous day and week (P<0.05). Nevertheless, tennis training did not seem to suppress chronically salivary lgA as positive correlations were found between resting salivary lgA levels of concentration and secretion rate, and the amount of training undertaken previously. Finally, it was noted a sharper post-exercise drop in S-lgA in occasions preceding an URTI episode compared to occasions when the infection did not develop subsequently (within 7 days). However, this parameter was not a specific predictor of URTI, in this cohort of athletes
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