Thermoregulatory responses in open water and pool swimming: Presentation of hypothermia and hyperthermia within and outside of World Aquatics water temperature thresholds

BACKGROUNDS: Water submersion challenges human thermoregulatory (Treg) homeostasis [typically a core temperature (T_C) of ~37 °C], accelerating rates of heat loss (cold water; hypothermia) and gain (warm/hot water; hyperthermia) when swimming, and increasing susceptibility to heat/cold illness. Particularly, during open water swimming (OWS) competition where race dropout and fatalities occur within a broad mandated water temperature (Tw) range (16-31 °C). Conversely, pool-based swimming (PBS) competition (performed within a narrow-mandated Tw range 25-28 °C) schedules encompass circadian T_C changes alongside multiple cycles of warmups, transition phases and races. Understanding Treg responses within OWS and PBS would: (i) enhance knowledge regarding swimmer's Treg demands; (ii) inform competition health and safety policy from a heat/cold illness perspective; and (iii) underpin Treg orientated performance enhancement. OBJECTIVES: Systematically review available Treg response data in OWS and PBS. METHODS: An open science framework registered (https://osf.io/v4ags/) systematic literature search was performed using Web of Science, MEDLINE, and SPORTDiscus to identify studies up to September 2024 using a string of key words relating to body temperature and swimming. Data with body temperature measurement during OWS or PBS training and/or competition, in participants of all sporting levels and abilities, were reviewed. RESULTS: Sixty-two studies were included (43 PBS, 19 OWS, 1 both) with a total of 605 participants (72 % male, 27 % female, 5 % Para-athletes). Twenty-one percent of PBS Treg responses and 63 % of OWS responses were collected below the respective World Aquatics (WA) Tw thresholds (PBS: 25 °C; OWS: 16 °C). Forty-two percent of PBS and no OWS studies were above the WA Tw thresholds (PBS: 28 °C; OWS: 31 °C). Mild hypothermia (35-36 °C; 19 % PBS; 63 % OWS studies) and hypothermia (<35 °C; 9 % PBS; 42 % OWS studies) were evident within studies; 39 of 191 OWS participants suffered from mild and/or hypothermia, and no individual data were reported for PBS studies. Mild hyperthermia was evident in 19 % of PBS and 16 % of OWS studies, individual data was reported for 5 of 414 PBS participants. CONCLUSIONS: Some reviewed evidence (21 % PBS and 63 % OWS: <WA Tw thresholds; 42 % PBS and 0 % OWS: >WA Tw thresholds) lacked external validity to competition mandated Tws. Prevalence of mild hypothermia/hypothermia and mild hyperthermia was evident within competition mandated Tws. Characterising Treg responses (using continuous, ingestible methods) in competition-relevant Tws (with increased female and Para-athlete representation) is necessary towards mitigating in-race Treg illness/risk and dropouts (OWS) and informing Treg performance optimisation strategies (PBS and OWS).