Acute and adaptive responses to endurance swimming in warm water

Abstract

Human thermoregulation and exercise performance have been investigated extensively in warm terrestrial environments but not warm aquatic environments. Open-water swimming (OWS) is an increasingly popular sport, with more events in latitudes where warm water and air create a seemingly heat-stressful environment. Water is unique, especially in relation to autonomic and behavioural thermoregulation. Immersion nullifies evaporative heat loss but provides high convective and conductive transfer, and it exerts hydrostatic pressure with a prone posture (if swimming). Swimming effectively clamps skin temperature (Tsk) homogeneously and below usual levels, making it potentially hazardous for behavioural thermoregulation yet thereby also providing a potential research model to examine thermosensitivities. Uncertainty also surrounds warm water as a medium for heat acclimating to improve performance or safety in OWS. Therefore, the purpose of this thesis was to examine acute and adaptive effects of competitive-intensity endurance swimming, particularly in warm water, on thermoregulation-related physiological and psychophysical responses and performance. Acute responses (Chapter Three) were examined in swimmers undertaking up to ten swims; a cross-over design of three durations (20, 60 and 120 min) and four water temperatures (Tw; 20, 27, 30 and 32 °C). Chapter Four presents small validation studies of (i) site of Tc measurement (rectal (Tre) vs. oesophageal (Toes)), (ii) laboratory- vs. field-based data, (iii) effects of training overload (as a model of systemic inflammation), and (iv) effects of solar heat load on physiological and psychophysical responses and performance. Adaptive responses (Chapter Five) were examined using a RCT, with physiological and performance testing in temperate and warm environments before and after swimming 60 min·d-1 for 6-7 d in Tw28 or Tw33. Competitive-intensity swims in warm water (≤Tw33) elicited modest increases in Tre. Conversely, in Tw20, only 50% of swimmers completed 120 min. Toes increased substantially more than Tre during Tw32 swims and decreased substantially more in Tw20 in some individuals. Adiposity-related changes in Tre were evident and more adipose swimmers maximised 120-min swim performance in Tw20, whereas others performed best in Tw27; all performed worst in ≥Tw30. Swimmers accurately perceived body temperature changes in Tw32, whereas accuracy became poorer with decreasing Tw. Tc provided a relatively stronger input than Tsk to thermal perceptions in this setting of large Tc perturbations. Field race data (Tw28) revealed Tc peaked at 40 °C in one swimmer but increased modestly in others. Swim performance and physiological responses were unaffected by 5 d of training overload or simulated solar loading. Repeated swimming in warm water did not elicit the adaptive responses commonly seen with terrestrial acclimation. In summary, competitive-intensity endurance swimming over common durations and in relevant Tw resulted in modest Tre increases in Tw32 that were appropriately transduced to drive behavioural thermoregulation. However, diverse Tre responses in Tw20, with reduced accuracy of thermal perceptions, indicates that cool water poses more risk to athlete safety, especially those with lower adiposity. Validity concerns remain regarding site of Tc measurement and laboratory-based testing. Finally, swimmers seem unlikely to gain heat adaptations or ergogenic benefit from daily swimming even in markedly warm water (Tw33).