Effects of Incremental Training on the Physiology and Mucosal Immunity of Adolescent Swimmers

Optimizing training intensity and volume is crucial for adolescent swimmers, as excessive high-intensity exercise may cause fatigue, overtraining, and hinder recovery. Research has shown that training load affects immune function and sleep. This study examined how progressively increasing training intensity impacts salivary immunity, sleep quality, perceived fatigue, and performance in adolescent swimmers. Eighteen male swimmers participated in a three-week protocol with daily 100-min sessions at 70%, 80%, and 90% of maximum heart rate, respectively. Saliva and questionnaire data were collected twice weekly to assess cortisol, testosterone, α-amylase, immunoglobulin A, lactoferrin, and perceived exertion. Sleep was measured using actigraphy. Performance was evaluated weekly through 200-m freestyle and 25-m lap counts within 16 min. Results showed a significant weekly increase in maximum heart rate (p < 0.001). Muscle soreness and perceived exertion were significantly higher after training at 80% and 90% intensities (p < 0.05) compared to rest days. Sleep quality and most salivary immune markers did not significantly change over time. However, the testosterone/cortisol ratio decreased after 5 days at 80% intensity (p < 0.01). Performance significantly improved at 80% and 90% intensities compared to 70%. In conclusion, a progressively incremental training program enhanced athletic performance without negatively affecting sleep or immune markers. These findings support controlled intensity increases for improving performance while maintaining physiological balance in adolescent swimmers.