Effect of physical training on circulation during prolonged severe exercise

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 4: evolution, thermal adaptation and unsupported theories of thermoregulation

This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 3: Heat and cold tolerance during exercise

In this third installment of our four-part historical series, we evaluate contributions that shaped our understanding of heat and cold stress during occupational and athletic pursuits. Our first topic concerns how we tolerate, and sometimes fail to tolerate, exercise-heat stress. By 1900, physical activity with clothing- and climate-induced evaporative impediments led to an extraordinarily high incidence of heat stroke within the military. Fortunately, deep-body temperatures > 40 °C were not always fatal. Thirty years later, water immersion and patient treatments mimicking sweat evaporation were found to be effective, with the adage of cool first, transport later being adopted. We gradually acquired an understanding of thermoeffector function during heat storage, and learned about challenges to other regulatory mechanisms. In our second topic, we explore cold tolerance and intolerance. By the 1930s, hypothermia was known to reduce cutaneous circulation, particularly at the extremities, conserving body heat. Cold-induced vasodilatation hindered heat conservation, but it was protective. Increased metabolic heat production followed, driven by shivering and non-shivering thermogenesis, even during exercise and work. Physical endurance and shivering could both be compromised by hypoglycaemia. Later, treatments for hypothermia and cold injuries were refined, and the thermal after-drop was explained. In our final topic, we critique the numerous indices developed in attempts to numerically rate hot and cold stresses. The criteria for an effective thermal stress index were established by the 1930s. However, few indices satisfied those requirements, either then or now, and the surviving indices, including the unvalidated Wet-Bulb Globe-Thermometer index, do not fully predict thermal strain.

Contribution of oxygen extraction fraction to maximal oxygen uptake in healthy young men

We analysed the importance of systemic and peripheral arteriovenous O₂ difference ( a-v¯O2 difference and a‐v_fO₂ difference, respectively) and O₂ extraction fraction for maximal oxygen uptake ( V˙O2max). Fick law of diffusion and the Piiper and Scheid model were applied to investigate whether diffusion versus perfusion limitations vary with V˙O2max. Articles (n = 17) publishing individual data (n = 154) on V˙O2max, maximal cardiac output ( Q˙max; indicator‐dilution or the Fick method), a-v¯O2 difference (catheters or the Fick equation) and systemic O₂ extraction fraction were identified. For the peripheral responses, group‐mean data (articles: n = 27; subjects: n = 234) on leg blood flow (LBF; thermodilution), a‐v_fO₂ difference and O₂ extraction fraction (arterial and femoral venous catheters) were obtained. Q˙max and two‐LBF increased linearly by 4.9‐6.0 L · min^–1 per 1 L · min^–1 increase in V˙O2max (R² = .73 and R² = .67, respectively; both P < .001). The a-v¯O2 difference increased from 118‐168 mL · L^–1 from a V˙O2max of 2‐4.5 L · min^–1 followed by a reduction (second‐order polynomial: R² = .27). After accounting for a hypoxemia‐induced decrease in arterial O₂ content with increasing V˙O2max (R² = .17; P < .001), systemic O₂ extraction fraction increased up to ~90% ( V˙O2max: 4.5 L · min^–1) with no further change (exponential decay model: R² = .42). Likewise, leg O₂ extraction fraction increased with V˙O2max to approach a maximal value of ~90‐95% (R² = .83). Muscle O₂ diffusing capacity and the equilibration index Y increased linearly with V˙O2max (R² = .77 and R² = .31, respectively; both P < .01), reflecting decreasing O₂ diffusional limitations and accentuating O₂ delivery limitations. In conclusion, although O₂ delivery is the main limiting factor to V˙O2max, enhanced O₂ extraction fraction (≥90%) contributes to the remarkably high V˙O2max in endurance‐trained individuals.

Fan cooling after cardiovascular drift does not reverse decrements in maximal oxygen uptake during heat stress

Cardiovascular (CV) drift, the progressive increase in heart rate (HR) and decrease in stroke volume (SV) during constant rate, moderate intensity exercise, is related to reduced maximal oxygen uptake (V̇O_2max) during heat stress. Once it has already occurred, it is unknown whether the detrimental effects of CV drift on V̇O_2max can be reversed. This study tested the hypothesis that fan cooling after CV drift has occurred attenuates decrements in V̇O_2max associated with CV drift. Eight men completed a control graded exercise test (GXT) in 22°C to measure V̇O_2max. Then on separate, counterbalanced occasions, they completed one 15-min (15MIN) and two 45-min bouts (45NF and 45FAN) of cycling in 35°C, 40% RH at 60% V̇O_2max, each immediately followed by a GXT to measure V̇O_2max. For one of the 45-min trials (45FAN), fan airflow (4.5 m/s) was directed at participants beginning ~5 min before the GXT and continuing throughout the remainder of exercise. The purpose of the separate 15- and 45-min trials was to measure V̇O_2max during the same time interval that CV drift occurred. HR increased (13.8% and 11.4%) and SV decreased (14.4% and 14.1%) for 45NF and 45FAN, respectively; trials were not different (all P > 0.05). Despite a decrease in mean skin temperature of ~1°C with fan use, V̇O_2max decreased similarly between conditions (17% vs. 15% for 45NF and 45FAN, P = 0.54). Fan cooling after CV drift was insufficient to reverse the negative consequences of CV drift on V̇O_2max after prolonged exercise in a hot environment. Abbreviations: 15MIN: 15-min trial; 45FAN: 45-min, fan trial; 45NF: 45-min, no fan trial; ANOVA: Analysis of variance; CV: Cardiovascular; GXT: Graded exercise test; HR: Heart rate; SV: Stroke volume; T̅_b: Mean body temperature; T_re: Rectal temperature; T̅_sk: Mean skin temperature; V̇O_2max: Maximal oxygen uptake.

Cardiovascular drift during heat stress: implications for exercise prescription

Cardiovascular drift, the progressive increase in heart rate and decrease in stroke volume that begins after approximately 10 min of prolonged moderate-intensity exercise, is associated with decreased maximal oxygen uptake, particularly during heat stress. Consequently, the increased heart rate reflects an increased relative metabolic intensity during prolonged exercise in the heat when cardiovascular drift occurs, which has implications for exercise prescription.

Reversed drift in heart rate but increased oxygen uptake at fixed work rate during 24 h ultra-endurance exercise

In this paper we report a reversed drift in heart rate (HR) but increased oxygen uptake (VO(2)) during ultra-endurance exercise. Nine well-trained male athletes performed 24-h exercise in a controlled laboratory setting, with alternating blocks of kayaking, running and cycling. Each block included 110 min of exercise and 10 min of rest, with an average work intensity of approximately 55% of respective VO(2peak). Blood samples were taken and HR and VO(2) measured every 6th hour during steady-state cycling at fixed work rate. As assumed HR was increased at 6 h by 15 +/- 6 beats/min compared with initial level (0 h). Thereafter the drift did not progress continuously, but instead unexpectedly returned toward initial values, although the plasma levels of catecholamines increased continuously during exercise. VO(2) was increased by 0.22 +/- 0.15 L/min (10%) at 6 h and 0.37 +/- 0.18 L/min (17%) at 12 h compared with 0 h, and thereafter remained stable. This implies an increased oxygen pulse (VO(2)/HR) by approximately 10% at the last half of the 24-h exercise compared with 0 h. Consequently, sole use of HR would give inaccurate estimates of exercise intensity and energy expenditure during endurance exercise lasting more than 6 h, and different patterns of cardiovascular drift need to be taken into account.

Effect of ambient temperature on cardiovascular drift and maximal oxygen uptake

PURPOSE

This study tested the hypothesis that the magnitude of cardiovascular (CV) drift and decrease in maximal oxygen uptake (V[spacing dot above]O2max) would be greater at 35 degrees C than at 22 degrees C.

METHODS

The increase in HR and decrease in stroke volume (SV) between 15 and 45 min of cycling at 59.2 +/- 1.9% V[spacing dot above]O2max (CV drift) was measured in hot (HEAT, 35 degrees C) and cool (COOL, 22 degrees C) ambient temperatures in 10 endurance-trained men (age = 23 +/- 3 yr, V[spacing dot above]O2max = 64.7 +/- 8.7 mL.kg.min). V[spacing dot above]O2max was measured immediately after the 45 min of cycling and again under both ambient temperature conditions on separate days after 15 min of cycling. This design permitted assessment of V[spacing dot above]O2max between the same time points that CV drift occurred. Fluid to replace sweat losses was provided during all trials.

RESULTS

CV drift and the associated decrease in V[spacing dot above]O2max was greater (P < 0.05) in HEAT versus COOL. HR increased 11% (P < 0.05), SV decreased 11% (P < 0.05), and V[spacing dot above]O2max fell 15% (P < 0.05) between 15 and 45 min in HEAT, whereas HR and SV changed less (+2% and -2% for HR and SV, respectively, P < 0.05), and there was no significant decrease in V[spacing dot above]O2max (5%, P > 0.05) between 15 and 45 min in COOL.

CONCLUSION

These data demonstrate the magnitude of CV drift during prolonged submaximal exercise, and the accompanying decrease in V[spacing dot above]O2max measured immediately thereafter is greater in a hot than in a cool environment.

Body cooling attenuates the decrease in maximal oxygen uptake associated with cardiovascular drift during heat stress

Previous research suggests cardiovascular drift (CV drift) is associated with decreased maximal oxygen uptake (VO(2)(max)) during heat stress, but more research manipulating CV drift with subsequent measurement of VO(2)(max) is needed to assess whether this relationship is causal. To assess causation, VO(2)(max) was measured during the same time interval that CV drift occurred (between 15 and 45 min of submaximal exercise under different conditions of body cooling intended to manipulate CV drift). Ten men completed a control graded exercise test (GXT) in 22 degrees C to measure VO(2)(max) then on separate occasions they cycled in 35 degrees C at 60% VO(2)(max) for 15 min (15 max), 45 min with no cooling (NC), and 45 min with fan airflow (FAN) beginning at approximately 18 min into exercise, and each bout was immediately followed by a GXT to measure VO(2)(max) In NC, VO(2)(max) decreased 18%, heart rate (HR) increased 16%, and stroke volume (SV) fell 12% (P < 0.05) from min 15 to min 45. In FAN, VO(2)(max) fell less (5.7%, P < 0.05) , HR rose less (4%, P < 0.05) and SV decreased less (3%, P < 0.05) from 15 to 45 min. The fall in VO(2)(max) associated with CV drift during exercise in a hot environment is attenuated with body cooling via fan airflow. The findings support the notion that a causal link exists between CV drift that occurs during prolonged exercise in a hot environment and a decrease in VO(2)(max).

Fluid Ingestion Attenuates the Decline in V̇O2peak Associated with Cardiovascular Drift

INTRODUCTION/PURPOSE

This study investigated whether manipulation of cardiovascular drift (CV drift) by changing exercise duration or by fluid ingestion is associated with altered peak oxygen uptake VO(2peak).

METHODS

VO(2peak) was measured in 11 trained men immediately after they cycled at 60% control VO(2max) in 30 degrees C, 40% relative humidity for 15, 60, and 120 min with no fluid (15 NF, 60 NF, 120 NF) or 120 min with fluid (120 F). Stroke volume (SV), heart rate (HR), and related measures were measured in 120 NF and 120 F at 15, 60, and 120 min.

RESULTS

Body mass decreased 0.7, 2.3, and 3.7% in 120 F, 60 NF, and 120 NF. SV at the end of submaximal exercise and VO(2peak) measured immediately thereafter were reduced significantly (P < 0.05) from 15-min values in 120 NF (13.8 and 8.7%) but not in 60 NF (4.6 and 1.2%) or 120 F (2.1 and 1.9%).

CONCLUSIONS

The progressive decline in SV during prolonged, constant-rate submaximal exercise in a warm environment, reflective of increased cardiovascular strain associated with hyperthermia, dehydration, and other changes that occur over time, reduces VO(2peak). Fluid ingestion improves performance in prolonged exercise, in part, by mitigating the decline in SV and its determinants, and preserving VO(2peak).

Cardiovascular Drift Is Related to Reduced Maximal Oxygen Uptake during Heat Stress

INTRODUCTION/PURPOSE

This study investigated whether the progressive rise in heart rate (HR) and fall in stroke volume (SV) during prolonged, constant-rate, moderate-intensity exercise (cardiovascular drift, CVdrift) in a hot environment is associated with a reduction in VO(2max).

METHODS

CVdrift was measured in nine male cyclists between 15 and 45 min of cycling at 60% VO(2max) in 35 degrees C that was immediately followed by measurement of VO(2max). VO(2max) also was measured after 15 min of cycling on a separate day, so that any change in VO(2max) between 15 and 45 min could be associated with the CVdrift that occurred during that time interval. This protocol was performed under one condition in which fluid was ingested and there was no significant body weight change (0.3 +/- 0.4%), and under another in which no fluid was ingested and dehydration occurred (2.5 +/- 1%, P < 0.05).

RESULTS

Fluid ingestion did not affect CVdrift or change in VO(2max). A 12% increase in HR (151 +/- 9 vs 169 +/- 10 bpm, P < 0.05) and 16% decrease in SV (120 +/- 12 vs 101 +/- 10 mL.beat(-1), P < 0.05) between 15 and 45 min was accompanied by a 19% decrease in VO(2max) (4.4 +/- 0.6 vs 3.6 +/- 0.4 L.min(-1), P < 0.05) despite attainment of a higher maximal HR (P < 0.05) at 45 min (194 +/- 5 bpm) vs 15 min (191 +/- 5 bpm). Submaximal VO(2) increased only slightly over time, but VO(2max) increased from 63 +/- 5% at 15 min to 78 +/- 8% at 45 min (P < 0.05).

CONCLUSION

We conclude CVdrift during 45 min of exercise in the heat is associated with decreased VO(2max) and increased relative metabolic intensity. The results support the validity of using changes in HR to reflect changes in relative metabolic intensity during prolonged exercise in a hot environment in which CVdrift occurs.

Hemodynamics and O2 uptake during maximal knee extensor exercise in untrained and trained human quadriceps muscle: effects of hyperoxia

To elucidate the potential limitations on maximal human quadriceps O2 capacity, six subjects trained (T) one quadriceps on the single-legged knee extensor ergometer (1 h/day at 70% maximum workload for 5 days/wk), while their contralateral quadriceps remained untrained (UT). Following 5 wk of training, subjects underwent incremental knee extensor tests under normoxic (inspired O2 fraction = 21%) and hyperoxic (inspired O2 fraction = 60%) conditions with the T and UT quadriceps. Training increased quadriceps muscle mass (2.9 +/- 0.2 to 3.1 +/- 0.2 kg), but did not change fiber-type composition or capillary density. The T quadriceps performed at a greater peak power output than UT, under both normoxia (101 +/- 10 vs. 80 +/- 7 W; P < 0.05) and hyperoxia (97 +/- 11 vs. 81 +/- 7 W; P < 0.05) without further increases with hyperoxia. Similarly, thigh peak O2 consumption, blood flow, vascular conductance, and O2 delivery were greater in the T vs. the UT thigh (1.4 +/- 0.2 vs. 1.1 +/- 0.1 l/min, 8.4 +/- 0.8 vs. 7.2 +/- 0.8 l/min, 42 +/- 6 vs. 35 +/- 4 ml x min(-1) x mmHg(-1), 1.71 +/- 0.18 vs. 1.51 +/- 0.15 l/min, respectively) but were not enhanced with hyperoxia. Oxygen extraction was elevated in the T vs. the UT thigh, whereas arteriovenous O2 difference tended to be higher (78 +/- 2 vs. 72 +/- 4%, P < 0.05; 160 +/- 8 vs. 154 +/- 11 ml/l, respectively; P = 0.098) but again were unaltered with hyperoxia. In conclusion, the present results demonstrate that the increase in quadriceps muscle O2 uptake with training is largely associated with increases in blood flow and O2 delivery, with smaller contribution from increases in O2 extraction. Furthermore, the elevation in peak muscle blood flow and vascular conductance with endurance training seems to be related to an enhanced vasodilatory capacity of the vasculature perfusing the quadriceps muscle that is unaltered by moderate hyperoxia.

Test-retest reliability of symptom-limited cycle ergometer tests in patients with chronic obstructive pulmonary disease

BACKGROUND

Symptom-limited exercise tests are widely used to evaluate the effects of pulmonary rehabilitation in patients with chronic obstructive pulmonary disease (COPD), but the reliability of these tests is not well established in COPD patients.

OBJECTIVES

We compared test-retest reliability of two repeated symptom-limited exercise tests between COPD patients and healthy elderly subjects and between male and female patients.

METHOD

Fifty-six COPD patients (40 men, 16 women) and 16 healthy subjects (6 men, 10 women) performed two symptom-limited exercise tests approximately 2 weeks apart. Measures of oxygen uptake (VO2), minute ventilation (VE), heart rate, and ratings of breathlessness and leg fatigue were obtained at peak exercise at each symptom-limited exercise test.

RESULTS

Repeated measures of peak exercise responses were stable for patients and healthy subjects and for male and female patients. Although mean percent error (absolute difference/mean) for peak exercise responses was low, some individuals' values exceeded 10%. There was no difference in the percent error between COPD patients and healthy subjects or between men and women with COPD. Test-retest reliability was lower for breathlessness ratings than for other peak exercise responses for all groups.

CONCLUSIONS

Repeated symptom-limited exercise tests are reliable in COPD patients and healthy subjects. However, some individuals are less reliable, and these patients may require more than one exercise test to establish reliable performance.

Uniqueness of interval and continuous training at the same maintained exercise intensity

The present study sought to evaluate the inconsistencies previously observed regarding the predominance of continuous or interval training for improving fitness. The experimental design initially equated and subsequently maintained the same relative exercise intensity by both groups throughout the program. Twelve subjects were equally divided into continuous (CT, exercise at 50% maximal work) or interval (IT, 30 s work, 30 s rest at 100% maximal work) training groups that cycled 30 min day-1, 3 days.week-1, for 8 weeks. Following training, aerobic power (VO2max), exercising work rates, and peak power output were all higher (9-16%) after IT than after CT (5-7%). Vastus lateralis muscle citrate synthase activity increased 25% after CT but not after IT. A consistent increase in adenylate kinase activity (25%) was observed only after IT. During continuous cycling testing the CT group had reduced blood lactate (lab) levels and respiratory quotient at both the same absolute and relative (70% VO2max) work rates after training, while the IT group displayed similar changes only at the same absolute work rates. By contrast, both groups responded similarly during intermittent cycling testing with lower lab concentrations seen only at absolute work rates. These results show that, of the two types of training programs currently employed, IT produces higher increases in VO2max and in maximal exercise capacity. Nevertheless, CT is more effective at increasing muscle oxidative capacity and delaying the accumulation of lab during continuous exercise.

Alterations in plasma volume, electrolytes and protein during incremental exercise at different pedal speeds

We investigated the effects of pedal speed on changes in plasma volume, electrolytes and protein during incremental exercise. Ten adult males participated in two, 30 minute incremental cycle ergometer exercise tests at room temperature (22 degrees C, rh = 56%). Exercise load was increased from 20 to 70% of peak VO2. Five minutes were spent at each of six stages which were equally spaced in exercise intensity. Subjects pedaled at 50 (50 RPM) and 90 (90 RPM) rev.min-1. Venous blood samples were drawn prior to exercise and during the last minute of each stage. Relative plasma volume changes showed a progressive hemoconcentration during the exercise. There were no significant differences due to pedal speed as plasma volume loss averaged -7.3% during exercise. [Na+], [Cl-], and [K+] increased significantly during exercise but were not influenced by pedal speed. Changes in plasma protein and albumin concentrations indicated that there was a loss of globulin from the vascular volume in both conditions and an addition of albumin to the plasma in 50 RPM. The difference in plasma albumin dynamics was possibly related to an effect of pedal speed on movement of fluid in the lymphatic vessels of the legs.

The effects of body position and exercise on plasma volume dynamics

We examined the plasma volume changes associated with a protocol of either exercise or controlled rest under identical positional and ambient conditions. Nine healthy adult males rode (E) and on another occasion sat quietly (C) on a cycle ergometer for 30 min. Ten minutes of cycle exercise immediately followed the resting C protocol. Ambient temperature was 30 degrees C (rh = 35%) and exercise load was equal to 50% of peak VO2. Venous blood samples were obtained with subjects both in the supine and seated positions prior to all experiments. Additional blood was drawn during minutes 1, 5, 10, and 30 in both experimental conditions. A final sample was taken during C after the 10 min exercise. Moving from the supine to a seated position resulted in an average loss of 162 ml of plasma across all experiments. During the E condition a further reduction in plasma volume (76 ml) occurred by one minute of exercise. Plasma volume stabilized by 5 min of exercise under the E protocol. During the C condition, subsequent fluid loss (98 ml) was not apparent until 10 min after the first seated sample and totalled 176 ml at the end of 30 min of rest. Ten minutes of cycling at the end of the C experiment resulted in a further plasma volume reduction of 137 ml. Plasma protein and albumin contents decreased by 5 min of exercise in E and by 30 min of rest in C. [Na+] and [Cl-] did not change in either condition but a rapid increase in [K+] during exercise indicated an addition of potassium to the vascular volume.(ABSTRACT TRUNCATED AT 250 WORDS)

Physical capacity in twins

A comparison of physical capacity, (measured by vital capacity, muscular strength and physical work capacity) in twin boys and controls failed to indicate significant differences. Intrapair correlations showed the MZ twins to be much more similar than the DZ twins in all the capacity measures. The correlations were lower, however, for both MZ and DZ twins for physical work capacity than for the other two capacity variables. When amount of physical exercise during leisure time is kept under control, the DZ twins tend to be more similar for physical work capacity or muscular strength, and the correlations tend to be of the same magnitude for MZ twins. Physical work capacity therefore appears, in this study, to be a more environmentally influenced variable than either vital capacity or muscular strength.