Author Identifiers

Jason R Boynton
ORCID: 0000-0003-2848-0694

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


School of Medical & Health Sciences

First Advisor

Associate Professor Chris Abbiss

Second Advisor

Dr Paolo Menaspà

Third Advisor

Associate Professor Jeremiah Peiffer


Endurance athletes typically spend the large majority of training (> 70%) at low intensities (i.e. below lactate threshold) coupled with short and intermittent bouts of high-intensity exercise or interval training (HIIT). Despite HIIT being a relatively small part of training in terms of duration, it has a substantial effect on the adaptations to endurance training. While it is well-established endurance exercise performance is affected in both hot and cold environmental conditions, the effect ambient temperature (TA; frequently referred to as environmental temperature) has on HIIT as performed by an endurance athlete population is not well understood. Therefore, the overall purpose of this thesis was to investigate the effects TA has on HIIT in an endurance trained population. Specifically, this thesis aimed to increase the understanding of how TA acutely affects performance and physiological responses during high-intensity intervals (Study 1); how repeated exposure to TA manipulates physiological responses during high-intensity intervals (Study 2), and how TA affects performance outcomes of a HIIT intervention (Study 3).

In Study 1, eleven well-trained cyclists completed 4 interval sessions at 5°C, 13°C, 22°C, and 35°C (55 ± 13% RH) in a randomised order. Each session involved 5 x 4-minute intervals interspersed with 5 minutes of recovery. During the intervals, power output, core temperature (TC), oxygen consumption (VO2), and heart rate (HR) were recorded. It was hypothesized that the 13°C condition would have the highest mean power output compared to the other TA conditions. However, mean session power output for 13°C (366 ± 32 W) was not significantly different than 5°C (363 ± 32 W), 22°C (364 ± 36 W), or 35°C (352 ± 31 W). Power output was lower in the 5th interval of the 35°C condition, compared with all other TA. TC was higher in 22°C compared with both 5°C and 13°C (P= .001). VO2 was not different across TA. HR was higher in the 4th and 5th intervals of 35°C compared with 5°C and 13°C. It was concluded well-trained cyclists performing maximal high-intensity aerobic intervals can achieve near optimal power output over a broader range of TA than previous literature may indicate.

Study 1 indicated TA had acute effects on performance and physiological responses during high-intensity aerobic intervals, especially in terms of cardiovascular stress. However, whether acute cardiorespiratory and thermoregulatory responses during high-intensity intervals change as a result of repeated TA exposures (i.e. during HIIT) was unknown. In Study 2, 20 trained cyclists and triathletes completed a 4-week (8 session) HIIT intervention in either cool (13°C) or hot (35°C) conditions. The HIIT intervention utilized the interval protocol from Study 1 and recorded cardiopulmonary and thermoregulatory measures during the first (INT8) and last (INT8) sessions. It was observed that time spent at or near maximal oxygen consumption (VO2max) during HIIT was greater in 13°C (877 ± 297 seconds) than 35°C (421 ± 395 seconds), but did not change for either TA condition between INT1 and INT8. HR was not significantly different between 13°C (164 ± 9 bpm) and 35°C HIIT (164 ± 12 bpm). TC significantly decreased in 35°C HIIT between INT1 and INT8. These results potentially indicate the relationship between time spent at or near VO2max and cardiovascular strain during HIIT is influenced by TA. Additionally, HIIT performed intermittently (~2x per week) at 35°C resulted in demonstrated evidence for heat acclimation in endurance athletes.

Study 1 and Study 2 provided findings for performance, cardiorespiratory, and thermoregulatory responses during acute high-intensity interval sessions and after repeated exposure to TA. In particular, differences in time spent at or near VO2max between 13°C and 35°C HIIT, and changes in thermoregulatory responses over the course of a HIIT intervention both have the potential to affect endurance performance outcomes and coinciding physiological responses. In order to investigate this, Study 3 evaluated submaximal warm-ups and 20 km time-trials in temperate conditions (22°C) before (TT1) and after (TT2) the HIIT interventions from Study 2. Gross mechanical efficiency (GME) was measured during the warm-up (at 50% peak power output), whilst power output and HR were measured during the 20 km TT. Rate of perceived exertion (RPE) and body temperature (TB) were measured through the warm-up and time-trial. It was demonstrated that time-trial power output was increased after HIIT interventions in both the 13°C (3%; HIIT13) and 35°C (7%; HIIT35), yet no differences between groups for power output, HR, or RPE were noted. Within subject increases for HR and RPE during the 20 km time-trial were noted in HIIT13, but not in HIIT35. GME approached a significant decrease (P= .051) in HIIT13. A significant interaction in TB was observed between groups and TT1 and TT2 during both the 20 km time-trial and submaximal warm-up. These findings indicate that HIIT performed in hot and cool conditions result in similar temperate time-trial performance outcomes. However, changes in cardiorespiratory, thermoregulatory, and subjective responses during aerobic exercise after a HIIT intervention appear to be dependent on the TA HIIT is performed in.

The results of this thesis demonstrate TA acutely affects performance, and cardiorespiratory and thermoregulatory responses during high-intensity intervals; repeated exposures to TA during HIIT can stimulate changes in thermoregulatory responses; and TA exposure during HIIT has limited effect on temperate endurance performance, yet affects coinciding cardiorespiratory, thermoregulatory, and subjective responses. These findings will assist coaches and athletes to make better informed decisions relating to HIIT prescription and acclimating endurance athletes to TA.

Available for download on Wednesday, September 08, 2021