Jodie M. Stocks

Sustained and generalized extracellular fluid expansion following heat acclimation

We measured intra‐ and extravascular body‐fluid compartments in 12 resting males before (day 1; control), during (day 8) and after (day 22) a 3‐week, exercise–heat acclimation protocol to investigate plasma volume (PV) changes. Our specific focus was upon the selective nature of the acclimation‐induced PV expansion, and the possibility that this expansion could be sustained during prolonged acclimation. Acclimation was induced by cycling in the heat, and involved 16 treatment days (controlled hyperthermia (90 min); core temperature = 38.5°C) and three experimental exposures (40 min rest, 96.9 min (s.d. 9.5 min) cycling), each preceded by a rest day. The environmental conditions were a temperature of 39.8°C (s.d. 0.5°C) and relative humidity of 59.2% (s.d. 0.8%). On days 8 and 22, PV was expanded and maintained relative to control values (day 1: 44.0 ± 1.8; day 8: 48.8 ± 1.7; day 22: 48.8 ± 2.0 ml kg−1; P < 0.05). The extracellular fluid compartment (ECF) was equivalently expanded from control values on days 8 (279.6 ± 14.2versus 318.6 ± 14.3 ml kg−1; n= 8; P < 0.05) and 22 (287.5 ± 10.6 versus 308.4 ± 14.8 ml kg−1; n= 12; P < 0.05). Plasma electrolyte, total protein and albumin concentrations were unaltered following heat acclimation (P > 0.05), although the total plasma content of these constituents was elevated (P < 0.05). The PV and interstitial fluid (ISF) compartments exhibited similar relative expansions on days 8 (15.0 ± 2.2%versus 14.7 ± 4.1%; P > 0.05) and 22 (14.4 ± 3.6%versus 6.4 ± 2.2%; P= 0.10). It is concluded that the acclimation‐induced PV expansion can be maintained following prolonged heat acclimation. In addition, this PV expansion was not selective, but represented a ubiquitous expansion of the extracellular compartment.

Cardiovascular and thermal consequences of protective clothing: a comparison of clothed and unclothed states

We have undertaken a laboratory-based examination of the cardiovascular and thermal impact of wearing thermal (heat) protective clothing during fatiguing exercise in the heat. Seven males completed semi-recumbent, intermittent cycling (39.6°C, 45% relative humidity) wearing either protective clothing or shorts (control). Mean core and skin temperatures, cardiac frequency (f c), stroke volume (Q), cardiac output (), arterial pressure, forearm blood flow ( f), plasma volume change, and sweat rates were measured. In the clothed trials, subjects experienced significantly shorter times to fatigue (52.5 vs. 58.9 min), at lower peak work rates (204.3 vs. 277.4 W), and with higher core (37.9° vs. 37.5°C) and mean skin temperatures (37.3° vs. 36.9°C). There was a significant interaction between time and clothing on f c, such that, over time, the clothing effect became more powerful. Clothing had a significant main affect on , but not Q, indicating the higher was chronotropically driven. Despite a greater sweat loss when clothed (923.0 vs. 547.1 g.m−2·h − 1; P < 0.05), f and plasma volume change remained equivalent. Protective clothing reduced exercise tolerance, but did not affect overall cardiovascular function, at the point of volitional fatigue. It was concluded that, during moderately heavy, semi-recumbent exercise under hot, dry conditions, the strain on the unclothed body was already high, such that the additional stress imparted by the clothing ensemble represented a negligible, further impact upon cardiovascular stability.

Whole-body fluid distribution in humans during dehydration and recovery, before and after humid-heat acclimation induced using controlled hyperthermia

This experiment was designed to test the hypothesis that the plasma volume is not selectively defended during exercise‐ and heat‐induced dehydration following humid‐heat acclimation.

Cardiorespiratory dynamics: sensitivity of the on-transition to endurance-training status.

Abstract This project investigated the sensitivity of oxygen uptake (V˙O2) dynamics to training-induced physiological changes, across a broad spectrum of endurance-training histories. Forty subjects participated: sedentary (n = 10), active healthy (n = 10), regular runners (n = 10), and competitive distance runners (n = 10). Subjects completed a cycle step-function protocol, to elicit a steady state at 60% maximal work rate. Breath-by-breath data were collected for V˙O2 and cardiac frequency (ƒc), and modelled mathematically, and used to determine the average response times to attain 20%, 40%, 60%, 80% and 100% of the respective steady states. The between-group comparisons for both V˙O2 and ƒc revealed significantly faster response times to 40%, 60%, 80% and 100% of the induced response, for the better trained subjects (P < 0.05). In general, this technique permitted differentiation between the V˙O2 and ƒc response dynamics of non-elite subjects from a broad range of endurance-training histories, with differences becoming more pronounced as subjects approached the steady state.

Compartmental changes in the body-fluid contributions to the plasma volume restoration during recovery from dehydration following heat acclimation

Since water moves freely among fluid compartments, it was of interest to track whole-body fluid movements during a resting recovery from extended exercise in the heat, but without rehydration. This mechanism was investigated before, during and following an extended heat-acclimation experiment.