Effects of autonomic tone on short distance performance in swimmers

High volume and frequency aerobic training usually shifts the autonomic control of heart rate (HR) towards parasympathetic predominance. However, the enhanced vagal tone may affect performance during swimming on 50 and 100 m distances.
Resting autonomic tone and swimming performance on 50 and 100 m crawl were evaluated in 12 national-level highly-trained swimmers (males, crawl specialists; age 25±3 years). HR was recorded at rest in supine position for 15 minutes in the morning. Heart rate variability indexes were calculated from time (RMSSD, pNN50, SD1) and frequency (LF, HF; LF/HF) domains. The anaerobic threshold was evaluated by an incremental swimming test with lactate measurements.
HR ranged from 40 to 68 bpm and 50% of the athletes showed a HR>60 bpm. RMSSD was 69±23 ms, pNN50 was 43±15% and SD1 was 48.9±16.1 ms. LF was 62.1±12.8 and HF 37.9±12.8, (normalized units) with LF/HF ratio of 2.2±2.0. Significant correlations were found between 50 m time and the indexes RMSSD (r=0.75), pNN50 (r=0.79), LFnu (r=-0.63) and absolute HF power (r=0.75)(p<0.05 for all comparisons). Anaerobic threshold correlated with 100 m (r=0.75; p=0.005) but not with 50 m performance.
Despite high training volume, only 50% of the athletes developed a hypervagotonia. However, high vagal tone appeared somehow detrimental on 50 m swimming performance, but not on 100 m. HRV indexes predicted 50 m events, whereas performance in 100 m events are better predicted by the anaerobic threshold.