title: The effects of a systematic increase in relative humidity on thermoregulatory and circulatory responses during prolonged running exercise in the heat.
abstract: This study examined the thermoregulatory and circulatory responses, and exercise performance of trained distance runners during exercise in the heat (31°C) at varying relative humidity (RH). In a randomized order, 11 trained male distance runners performed 5 60 min steady-state runs at a speed eliciting 70% of VO2max in RH of 23, 43, 52, 61 and 71%. This was followed immediately with an incremental exercise test to volitional exhaustion. Core (Tre) and mean skin temperature (T¯sk), cardiac output (Q), heart rate (HR), and stroke volume (SV) were recorded at regular intervals. A significant (P= 0.003) main effect was detected for RH on mean body temperature (Tb), with a significantly higher Tbdetected during steady-state exercise in the 61 and 71% RH compared to that in the 23% RH. During the steady-state exercise, no differences were detected in whole body sweat loss (P= 0.183). However, a significant main effect of RH was observed for HR and SV (P= 0.001 and 0.006, respectively) but not Q (P= 0.156). The time to exhaustion of the incremental exercise test was significantly reduced at 61 and 71% RH compared with 23% RH (P= 0.045 and 0.005, respectively). Despite an increase in dry heat loss, a greater thermoregulatory and circulatory stress was evident during steady-state exercise at 61 and 71% RH. This ultimately limits the capacity to perform the subsequent incremental exercise to exhaustion. This study highlighted that in a warm environment, the range of the prescriptive zone progressively narrows as RH increases.
authors.affiliation: Lifestyle Science Cluster, Advanced Medical and Dental Institute, Universiti Sains Malaysia , Penang, Malaysia.
authors.forename: Ahmad Munir
authors.initials: AM
authors.lastname: Che Muhamed
authors.affiliation: Independent Scholar , Canberra, Australia.
authors.forename: Kerry
authors.initials: K
authors.lastname: Atkins
authors.affiliation: School of Sport and Exercise, Massey University , Palmerston North, New Zealand.
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Stannard
authors.affiliation: School of Sport and Exercise, Massey University , Palmerston North, New Zealand.
authors.forename: Toby
authors.initials: T
authors.lastname: Mündel
authors.affiliation: Discipline of Exercise and Sport Science, Faculty of Health Science, The University of Sydney , New South Wales, Australia.
authors.forename: Martin William
authors.initials: MW
authors.lastname: Thompson
keywords: circulation; prolonged exercise; relative humidity; thermoregulation