No abstract

Heat stress is common and arises endogenously and exogenously. It can be acutely hazardous while also increasingly advocated to drive health and performance-related adaptations. Yet, the nature of strain (deviation in regulated variables) imposed by different heating modes is not well established, despite the potential for important differences. We, therefore, compared three modes of heat stress for thermal, cardiovascular and perceptual strain profiles during exposure and recovery when experienced as a novel stimulus and an accustomed stimulus. In a crossover design, 13 physically active participants (five females) underwent 5 days of 60-min exposures to hot water immersion (40 • C), sauna (55 • C, 54% relative humidity) and exercise in the heat (40 • C, 52% relative humidity), and a thermoneutral water immersion control (36.5 • C), each separated by ≥4 weeks. Physiological (thermal, cardiovascular, haemodynamic) and psychophysical strain responses were assessed on days 1 and 5.Sauna evoked the warmest skin (40 • C; P < 0.001) but exercise in the heat caused the largest increase in core temperature, sweat rate, heart rate (post hoc comparisons all P < 0.001) and systolic blood pressure (P ≤ 0.002), and possibly decrease in diastolic blood pressures (P ≤ 0.130), regardless of day. Thermal sensation and feeling state were more favourable on day 5 than on day 1 (P ≤ 0.021), with all modes of heat being equivalently uncomfortable (P ≥ 0.215). Plasma volume expanded the largest extent during immersions (P < 0.001). The current data highlight that exercising in the heat generates a more complex strain profile, while passive heat stress in humid heat has lower tolerance and more cardiovascular strain than hot water immersion.

Current evidence indicates that to achieve maximum health benefits, regular resistance exercise should be a key component of structured physical activity. Several studies have revealed that regular resistance exercise may be associated with impaired vascular function, although this finding is inconsistent. Proposed explanations for impairment include substantial increases in blood pressure and increased retrograde blood flow in active limbs promoted by resistance exercise. However, few studies have examined the acute haemodynamics of resistance exercise in active-and even fewer in inactive-limbs. The purpose of this study was to characterise the haemodynamic responses in peripheral arteries in active and inactive limbs in response to resistance exercise using upper and lower limbs. Ten participants (five male, five female) familiar with resistance training performed three sets of 10 isotonic repetitions of right-sided bicep curls or knee extensions on separate days. Blood flow, shear rate and muscle oxygenation in the active and inactive limb, and blood pressure were measured before and for 3 min after each set. Blood flow increased in response to resistance exercise in the active limb (∼8-fold and ∼6-fold for the upper and lower limb respectively), with concurrent significant increases in mean and antegrade shear rate. In the inactive limb, blood flow more than doubled for both upper and lower limb exercise, transiently, with no significant change in retrograde shear rate. These acute blood flow profiles following resistance exercise are not indicative of long-term vessel impairment based on current understanding of blood flow and shear stress patterns.

New Findings What is the central question to the study?Are primary indices of heat adaptation (e.g., expansion of plasma volume and reduction in resting core temperature) differentially affected by the three major modes of short‐term heat acclimation, that is, exercise in the heat, hot water immersion and sauna? What it the main finding and its importance?The three modes elicited typical adaptations expected with short‐term heat acclimation, but these were not significantly different between modes. This comparison has not previously been made and highlights that individuals can expect similar adaptation to heat regardless of the mode used. Abstract Heat acclimation (HA) can improve heat tolerance and cardiovascular health. The mode of HA potentially impacts the magnitude and time course of adaptations, but almost no comparative data exist. We therefore investigated adaptive responses to three common modes of HA, particularly with respect to plasma volume. Within a crossover repeated‐measures design, 13 physically active participants (five female) undertook four, 5‐day HA regimes (60 min/day) in randomised order, separated by ≥4 weeks. Rectal temperature (Tre) was clamped at neutrality via 36.6°C (thermoneutral) water immersion (TWI; i.e., control condition), or raised by 1.5°C via heat stress in 40°C water, sauna (55°C, 52% relative humidity), or exercise in humid heat (40°C, 52% relative humidity; ExH). Adaptation magnitude was assessed as the pooled response across days 4–6, while kinetics was assessed via the 6‐day time series. Plasma volume expansion was similar in all heated conditions but only higher than TWI in exercise in the heat (ExH) (by 4%, P = 0.036). Approximately two‐thirds of the expansion was attained within the initial 24 h and was moderately related to that present on day 6, regardless of HA mode (r = 0.560–0.887). Expansion was mediated by conservation of both sodium and albumin content, with little evidence for these having differential roles between modes (P = 0.706 and 0.320, respectively). Resting Tre decreased by 0.1–0.3°C in all heated conditions, and systolic blood pressure decreased by 4 mmHg, but not differentially between conditions (P ≥ 0.137). In conclusion, HA mode did not substantially affect the magnitude or rate of adaptation in key resting markers of short‐term HA.