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.
During a race, the runners’ heart rate increases in a very controlled way apparently scaled to the distance of the race. So, the the runner’s body, anticipates future stresses. It would seem that athletes actively manage the level of strain on their body with an ongoing assessment of fatigue levels in relation to expectations.
Surprisingly, elite runners did not run proportionally harder than the run of the mill athletes and all the athletes had about the same heart rate responses despite a great deal of variation among them. This suggests that elite runners such as Paula Radcliffe are so good because of their basic physiological capacity, not because of the level of effort they expend. Apparently, training and innate ability both matter!
The study also looked at the phenomenon of “hitting the wall.” This is when athlete’s glycogen stores have nearly run out, so the body starts to convert (at a high rate) fat into energy. This leads to fatigue and is potentially dangerous from a medical perspective. The research suggests that this sort of catastrophic outcome occurs frequently during endurance competition because athletes either can not or simply do not slow down their heart rate.
Jonathan Esteve-Lanao, Alejandro Lucia, Jos J. deKoning, Carl Foster, Conrad P. Earnest (2008). How Do Humans Control Physiological Strain during Strenuous Endurance Exercise? PLoS ONE, 3 (8) DOI: 10.1371/journal.pone.0002943