How Do Humans Control Physiological Strain during Strenuous

[jliddil]

Free Full Text:
http://www.plosone.org/article/info%3Ad ... ne.0002943

-Primary outcome: HR response and exercise intensity according to race distance
As demonstrated in the top panel of figure 1, the HR response expressed relative to the %HRmax during each event was fairly consistent.

-Secondary outcome: HR response according to running ability
There was little evidence that the %HRmax sustained varied with running ability, as the mean %HRmax essentially did not decrease in runners who required more time to complete their events, i.e., we found low Pearson's correlation coefficients for the relationship between mean %HRmax during each event and time to complete each event, especially up to 21 km (Figure 5).

Methodological limitations

We aware that the main methodological limitation of using HR recordings to monitor physiological strain is due to the phenomenon known as “cardiac drift” [42]. Cardiac drift is characterized by a gradual increase in HR values that tends to occur during prolonged exercise involving large muscle mass (e.g., running) despite maintaining “external load” (e.g., running velocity) and which does not solely reflect an increase in actual physiological intensity (“internal load”). This phenomenon is most marked in hot environments. (In this regard, environmental conditions were relatively benign in our study, with temperature consistently <25°C, as detailed in the Methods section). Further, the magnitude of the HR drift phenomenon is expected to increase with exercise duration. As such, it would be higher in the slowest compared to the fastest runners within a given long distance race (i.e., half marathon and above), and thus would artificially inflate exercise intensity in the former.