MEDICINE & SCIENCE IN SPORTS AND EXERCISE Volume 46:5 Supplement, May 2014 Official Journal of the American College of Sports Medicine HEART RATE VARIABILITY AS BIOMARKER OF TRAINING LOAD IN PROFESSIONAL SOCCER PLAYERS Javier J. Vilamitjana 1-2 , Nestor A. Lentini 2 , Mario F. Pérez Jr. 3 , Pablo E. Verde 4 1 BYP, Bs.As. Argentina, 2 CENARD, National Sport High performance Center, Physiology Laboratory, Bs As, Argentina, 3 Al Wasl FC, Dubai, UAE, 4 University of Düsseldorf, Germany. PURPOSE The goal of the present study was to assess the potential use of heart rate variability analysis as a biomarker in the control of the impact of weekly training loads before the games in competitive season (9 months follow-up, 2011-2012 season). METHODS 17 members from a United Arab Emirates Pro-League soccer team (age: 26.1 ± 3.9 yr, weight: 70.5 ± 7.5 kg, height: 174 ± 0.1 cm, percentage of body fat: 12.6 ± 1.5; mean ± SD) were studied 2-4 times a month. Each player was monitored in two conditions: normal training during the previous days of games (NOR), or training with muscle fatigue (muscle soreness or contracture) during the days before competition (FTG). Short-term recordings (5 minutes) were made with a Polar RS800CX heart rate monitor in two positions: supine (S) and 90° head-up tilt (T), immediately after awakening (match day morning). HRV was analyzed in the time and frequency domain using RR intervals according to the specific software (HRV Analysis Software v. 1.1, Finland). Spectral components as high frequency (HF) and low frequency (LF) were obtained using nonparametric method (Fast Fourier Transform algorithm) and expressed in absolute units (milliseconds squared). LF/HF ratio in S and T positions were calculated and the quotient between LF/HF ratios in position T and S was obtained as final LF/HF ratio (F LF/HF r). A paired-sample t-test was implemented to determine significant difference between paired scores. Statistical significance was established at 0.05 level (p<0.05). RESULTS LF/HF ratios were significantly altered between two conditions, shifting towards a sympathetic predominance in S (LF/HF: 1.1 ± 0.52 in NOR vs. 1.63 ± 0.84 in FTG, t: - 4.23, p<0.01) and parasympathetic prevalence in T (3.57 ± 1.58 vs. 1.86 ± 1.13 respectively, t: 7.93, p<0.01). Hence, F LF/HF r was significantly higher in NOR than in FTG (3.48 ± 0.99 vs. 1.30 ± 0.70 respectively, t: 9.58, p<0.01). Most of players monitored in FTG (94.1 %), finished the match injured (85 % from total were muscle injuries). As a counterpart, a hundred percent of players in NOR condition completed the games without injuries. CONCLUSIONS The results showed that the autonomic activity in soccer players might vary according to muscle fatigue conditions. Significant difference of F LF/HF ratio reflects a sympathovagal imbalance, suggesting the potential use of HRV in follow-up and control of training. It could be relevant to have into account especially within a congested match schedule, when a complete recovery is essential to prevent injuries. Further investigations should concentrate on autonomic responses in different fatigue conditions along the competitive season.