Intensity and physiological strain of competitive ultra-endurance exercise in humans

The aim of this study was to determine the magnitude and pattern of intensity, and physiological strain, of competitive exercise performed across several days, as in adventure racing. Data were obtained from three teams of four athletes (7 males, 5 females; mean age 36 years, s=11; cycling VO2 peak 53.9 ml.kg(-1).min(-1), s=6.3) in an international race (2003 Southern Traverse; 96-116h). Heart rates (HR) averaged 64% (95% confidence interval: +/- 4%) of heart rate range [%HRR=(HR - HRmin)/(HRmax - HRmin) x 100] during the first 12h of racing, fell to 41% (+/- 4%) by 24h, and remained so thereafter. The level and pattern of heart rate were similar across teams, despite one leading and one trailing all other teams. Core temperature remained between 36.0 and 39.2 degrees C despite widely varying thermal stress. Venous samples, obtained before, during, and after the race, revealed increased neutrophil, monocyte and lymphocyte concentrations (P<0.01), and increased plasma volume (25 +/- 10%; P<0.01) with a stable sodium concentration. Standardized exercise tests, performed pre and post race, showed little change in the heart rate-work rate relationship (P=0.53), but a higher perception of effort post race (P<0.01). These results provide the first comprehensive report of physiological strain associated with adventure racing.