How Do Humans Control Physiological Strain during Strenuous Endurance Exercise?

Background: Distance running performance is a viable model of human locomotion. Methodology/Principal Findings: To evaluate the physiologic strain during competitions ranging from 5-100 km, we evaluated heart rate (HR) records of competitive runners (n=211). We found evidence that: 1) physiologic strain (% of maximum HR (% HRmax)) increased in proportional manner relative to distance completed, and was regulated by variations in running pace; 2) the % HRmax achieved decreased with relative distance; 3) slower runners had similar % HRmax response within a racing distance compared to faster runners, and despite differences in pace, the profile of % HRmax during a race was very similar in runners of differing ability; and 4) in cases where there was a discontinuity in the running performance, there was evidence that physiologic effort was maintained for some time even after the pace had decreased. Conclusions/Significance: The overall results suggest that athletes are actively regulating their relative physiologic strain during competition, although there is evidence of poor regulation in the case of competitive failures.