Core temperature regulation of heart rate during exercise in humans

The relationship between two abnormalities of exercise physiology in chronic heart failure patients was investigated: chronotropic incompetence and decrease in core temperature. While at rest, 13 heart failure patients had an average sinus heart rate that was significantly higher than seven normals (92 +/- 13 vs. 82 +/- 10 min-1, P less than 0.05). However, during exercise, the trend of increase in sinus heart rate as a function of work load and O2 uptake was significantly greater in normals compared with heart failure (P less than 0.05), and the absolute increase in heart rate at 50 W of cycle ergometry was larger in normals compared with heart failure (38 +/- 17 vs. 22 +/- 13 min-1, P less than 0.05). Differences in core temperature regulation were also observed. In the normals, core temperature increased from 37.13 +/- 0.33 degrees C at rest to 37.37 +/- 0.31 degrees C at 50 W of exercise (P less than 0.01). In the heart failure patients, core temperature decreased from 36.99 +/- 0.33 degrees C at rest to 36.66 +/- 0.39 degrees C at 50 W of exercise (P less than 0.01). As expected, significant differences in hemodynamic and gas exchange variables were observed between the normals and the heart failure patients both at rest and during exercise. A multiple linear regression analysis was performed of heart rate changes as the dependent variable and thermoregulatory and hemodynamic changes as the independent variables to test for their influence on heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of cold, exercise, and caffeine on catecholamines and metabolism in men

Recently we found that caffeine ingestion did not enhance either thermal or fat metabolic responses to resting in cold air, despite an increase in plasma epinephrine and free fatty acids. Theophylline, another methylxanthine, has been shown to be effective during exercise but not at rest during cold stress. Therefore we hypothesized that caffeine ingestion before exercise in cold air would have a thermal-metabolic impact by increasing fat metabolism and increasing oxygen consumption. Young adult men (n = 6) who did not normally have caffeine in their diet performed four double-blind trials. Thirty minutes after ingesting placebo (dextrose, 5 mg/kg) or caffeine (5 mg/kg) they either exercised (60 W) or rested for 2 h in 5 degrees C air. Cold increased (P less than 0.05) plasma norepinephrine while both caffeine and exercise increased (P less than 0.05) epinephrine. Serum free fatty acids and glycerol were increased, but there were no differences between rest and exercise or placebo and caffeine. Caffeine had no influence on either respiratory exchange ratio or oxygen consumption either at rest or during exercise. The exercise trials did not significantly warm the body, and they resulted in higher plasma norepinephrine concentrations and lower mean skin temperatures for the first 30 min. The data suggest that skin temperature stimulates plasma norepinephrine while caffeine has little effect. In contrast, caffeine and exercise stimulate plasma epinephrine while cold has minimal effect. Within the limits of this study caffeine gave no thermal or metabolic advantage during a cold stress.     

Effect of cold air inhalation on core temperature in exercising subjects under heat stress

Whether increasing respiratory heat loss (RHL) during exercise under heat stress can contain elevation of rectal temperature (Tre) was examined. Eight men cycled twice at 45-50% their maximum work rate until exhaustion at ambient temperature and relative humidity of 38 degrees C and 90-95%, respectively. They inspired either cold (3.6 degrees C) or ambient air in random sequence. When subjects breathed cold air during 23 min of exercise, a ninefold increase in RHL was observed vs. similar work during hot air inhalation (32.81 vs. 3.46 W). Respiratory frequency (f) and rate of rise in Tre decreased significantly (P less than or equal to 0.004 and P less than or equal to 0.002, respectively). The rise in skin temperature in each inhalant gas condition was accompanied by a parallel almost equal increase in core temperature above basal (delta Tre) for equivalent gains in skin temperature. The increase in tidal volume and decreased f in the cold condition allowed more effective physical conditioning of cold inspirate gas in the upper airways and aided RHL. Cold air inhalation also produced a significant (P less than or equal to 0.05) decrease in heart rate vs. hot air inhalation in the final stages of exercise. Insignificant changes in O2 consumption and total body fluid loss were found. These data show that cold air inhalation during exercise diminishes elevation of Tre and suggest that both the intensity and duration of work can thus be extended. The importance of the physical exchange of heat energy and any physiological mechanisms induced by the cold inspirate in producing the changes is undetermined.     

Reduced stroke volume during exercise in postural tachycardia syndrome.

Postural tachycardia syndrome (POTS) is characterized by excessive tachycardia without hypotension during orthostasis. Most POTS patients also report exercise intolerance. To assess cardiovascular regulation during exercise in POTS, patients (n = 13) and healthy controls (n = 10) performed graded cycle exercise at 25, 50, and 75 W in both supine and upright positions while arterial pressure (arterial catheter), heart rate (HR; measured by ECG), and cardiac output (open-circuit acetylene breathing) were measured. In both positions, mean arterial pressure, cardiac output, and total peripheral resistance at rest and during exercise were similar in patients and controls (P > 0.05). However, supine stroke volume (SV) tended to be lower in the patients than controls at rest (99 +/- 5 vs. 110 +/- 9 ml) and during 75-W exercise (97 +/- 5 vs. 111 +/- 7 ml) (P = 0.07), and HR was higher in the patients than controls at rest (76 +/- 3 vs. 62 +/- 4 beats/min) and during 75-W exercise (127 +/- 3 vs. 114 +/- 5 beats/min) (both P < 0.01). Upright SV was significantly lower in the patients than controls at rest (57 +/- 3 vs. 81 +/- 6 ml) and during 75-W exercise (70 +/- 4 vs. 94 +/- 6 ml) (both P < 0.01), and HR was much higher in the patients than controls at rest (103 +/- 3 vs. 81 +/- 4 beats/min) and during 75-W exercise (164 +/- 3 vs. 131 +/- 7 beats/min) (both P < 0.001). The change (upright - supine) in SV was inversely correlated with the change in HR for all participants at rest (R(2) = 0.32), at 25 W (R(2) = 0.49), 50 W (R(2) = 0.60), and 75 W (R(2) = 0.32) (P < 0.01). These results suggest that greater elevation in HR in POTS patients during exercise, especially while upright, was secondary to reduced SV and associated with exercise intolerance.     

Comparison in men of physiological responses to exercise of increasing intensity at low and moderate ambient temperatures

In six male subjects the sweating thresholds, heart rate (fc), as well as the metabolic responses to exercise of different intensities [40%, 60% and 80% maximal oxygen uptake (VO2max)], were compared at ambient temperatures (Ta) of 5 degrees C (LT) and 24 degrees C (MT). Each period of exercise was preceded by a rest period at the same temperature. In LT experiments, the subjects rested until shivering occurred and in MT experiments the rest period was made to be of exactly equivalent length. Oxygen uptake (VO2) at the end of each rest period was higher in LT than MT (P less than 0.05). During 20-min exercise at 40% VO2max performed in the cold no sweating was recorded, while at higher exercise intensities sweating occurred at similar rectal temperatures (Tre) but at lower mean skin (Tsk) and mean body temperatures (Tb) in LT than MT experiments (P less than 0.001). The exercise induced VO2 increase was greater only at the end of the light (40% VO2max) exercise in the cold in comparison with MT (P less than 0.001). Both fc and blood lactate concentration [1a]b were lower at the end of LT than MT for moderate (60% VO2max) and heavy (80% VO2max) exercises. It was concluded that the sweating threshold during exercise in the cold environment had shifted towards lower Tb and Tsk. It was also found that subjects exposed to cold possessed a potentially greater ability to exercise at moderate and high intensities than those at 24 degrees C since the increases in Tre, fc and [1a]b were lower at the lower Ta.     

Pulmonary gas exchange in humans during exercise at sea level

Previous studies have shown both worsening ventilation-perfusion (VA/Q) relationships and the development of diffusion limitation during exercise at simulated altitude and suggested that similar changes could occur even at sea level. We used the multiple-inert gas-elimination technique to further study gas exchange during exercise in healthy subjects at sea level. Mixed expired and arterial respiratory and inert gas tensions, cardiac output, heart rate, minute ventilation, respiratory rate, and blood temperature were recorded at rest and during steady-state exercise in the following order: rest, minimal exercise (75 W), heavy exercise (300 W), heavy exercise breathing 100% O2, repeat rest, moderate exercise (225 W), and light exercise (150 W). Alveolar-to-arterial O2 tension difference increased linearly with O2 uptake (VO2) (6.1 Torr X min-1 X 1(-1) VO2). This could be fully explained by measured VA/Q inequality at mean VO2 less than 2.5 l X min-1. At higher VO2, the increase in alveolar-to-arterial O2 tension difference could not be explained by VA/Q inequality alone, suggesting the development of diffusion limitation. VA/Q inequality increased significantly during exercise (mean log SD of perfusion increased from 0.28 +/- 0.13 at rest to 0.58 +/- 0.30 at VO2 = 4.0 l X min-1, P less than 0.01). This increase was not reversed by 100% O2 breathing and appeared to persist at least transiently following exercise. These results confirm and extend the earlier suggestions (8, 21) of increasing VA/Q inequality and O2 diffusion limitation during heavy exercise at sea level in normal subjects and demonstrate that these changes are independent of the order of performance of exercise.     

Maximal vascular leg conductance in trained and untrained men

Lower leg blood flow and vascular conductance were studied and related to maximal oxygen uptake in 15 sedentary men (28.5 +/- 1.2 yr, mean +/- SE) and 11 endurance-trained men (30.5 +/- 2.0 yr). Blood flows were obtained at rest and during reactive hyperemia produced by ischemic exercise to fatigue. Vascular conductance was computed from blood flow measured by venous occlusion plethysmography, and mean arterial blood pressure was determined by auscultation of the brachial artery. Resting blood flow and mean arterial pressure were similar in both groups (combined mean, 3.0 ml X min-1 X 100 ml-1 and 88.2 mmHg). After ischemic exercise, blood flows were 29- and 19-fold higher (P less than 0.001) than rest in trained (83.3 +/- 3.8 ml X min-1 X 100 ml-1) and sedentary subjects (61.5 +/- 2.3 ml X min-1 X 100 ml-1), respectively. Blood pressure and heart rate were only slightly elevated in both groups. Maximal vascular conductance was significantly higher (P less than 0.001) in the trained compared with the sedentary subjects. The correlation coefficients for maximal oxygen uptake vs. vascular conductance were 0.81 (trained) and 0.45 (sedentary). These data suggest that physical training increases the capacity for vasodilation in active limbs and also enables the trained individual to utilize a larger fraction of maximal vascular conductance than the sedentary subject.     

Endurance training slows down the kinetics of heart rate increase in the transition from moderate to heavier submaximal exercise intensities

To find out whether endurance training influences the kinetics of the increases in heart rate (fc) during exercise driven by the sympathetic nervous system, the changes in the rate of fc adjustment to step increments in exercise intensities from 100 to 150 W were followed in seven healthy, previously sedentary men, subjected to 10-week training. The training programme consisted of 30-min cycle exercise at 50%-70% of maximal oxygen uptake (VO2max) three times a week. Every week during the first 5 weeks of training, and then after the 10th week the subjects underwent the submaximal three-stage exercise test (50, 100 and 150 W) with continuous fc recording. At the completion of the training programme, the subjects' VO2max had increased significantly (39.2 ml.min-1.kg-1, SD 4.7 vs 46 ml.min-1.kg-1, SD 5.6) and the steady-state fc at rest and at all submaximal intensities were significantly reduced. The greatest decrease in steady-state fc was found at 150 W (146 beats.min-1, SD 10 vs 169 beats.min-1, SD 9) but the difference between the steady-state fc at 150 W and that at 100 W (delta fc) did not decrease significantly (26 beats.min-1, SD 7 vs 32 beats.min-1, SD 6). The time constant (tau) of the fc increase from the steady-state at 100 W to steady-state at 150 W increased during training from 99.4 s, SD 6.6 to 123.7 s, SD 22.7 (P less than 0.01) and the acceleration index (A = 0.63.delta fc.tau-1) decreased from 0.20 beats.min-1.s-1, SD 0.05 to 0.14 beats.min-1.s-1, SD 0.04 (P less than 0.02). The major part of the changes in tau and A occurred during the first 4 weeks of training. It was concluded that heart acceleration following incremental exercise intensities slowed down in the early phase of endurance training, most probably due to diminished sympathetic activation.     

O2 Uptake in hyperthyroidism during constant work rate and incremental exercise

To investigate the effect of hyperthyroidism on the pattern and time course of O2 uptake (VO2) following the transition from rest to exercise, six patients and six healthy subjects performed cycle exercise at an average work rate (WR) of 18 and 20 W respectively. Cardiorespiratory variables were measured breath-by-breath. The patients also performed a progressively increasing WR test (1-min increments) to the limit of tolerance. Two patients repeated the studies when euthyroid. Resting and exercise steady-state (SS) VO2 (ml.kg-1.min-1) were higher in the patients than control (5.8, SD 0.9 vs 4.0, SD 0.3 and 12.1, SD 1.5 vs 10.2, SD 1.0 respectively). The increase in VO2 during the first 20 s exercise (phase I) was lower in the patients (mean 89 ml.min-1, SD 30) compared to the control (265 ml.min-1, SD 90), while the difference in half time of the subsequent (phase II) increase to the SS VO2 (patient 26 s, SD 8; controls 17 s, SD 8) were not significant (P = 0.06). The O2 cost per WR increment (delta VO2/delta WR) in ml.min-1.w-1, measured during the incremental period (mean 10.9; range 8.3-12.2), was always within two standard deviations of the normal value (10.3, SD 1). In the two patients who repeated the tests, both the increment of VO2 from rest to SS during constant WR exercise and the delta VO2/delta WRs during the progressive exercise were higher in the hyperthyroid state than during the euthyroid state.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiovascular response to static handgrip in trained and untrained men

The influence of aerobic capacity on the cardiovascular response to handgrip exercise, in relation to the muscle mass involved in the effort, was tested in 8 trained men (T) and 17 untrained men (U). The subjects performed handgrip exercises with the right-hand (RH), left-hand (LH) and both hands simultaneously (RLH) at an intensity of 25% of maximal voluntary contraction force. Maximal aerobic capacity was 4.3 l.min-1 in T and 3.21 l.min-1 in U (P less than 0.01). The endurance time for handgrip was longer in T than in U by 29% (P less than 0.05) for RH, 38% (P less than 0.001) for LH and 24% (P less than 0.001) for RLH. Heart rate (fc) was significantly lower in T than in U before handgrip exercise, and showed smaller increases (P less than 0.01) at the point of exhaustion: 89 vs 106 beats.min-1 for RH, 93 vs 100 beats.min-1 for LH and 92 vs 108 beats.min-1 for RLH. Stroke volume (SV) at rest was greater in T than in U and decreased significantly (P less than 0.05) during handgrip exercise in both groups of subjects. At the point of exhaustion SV was still greater in T than in U: 75 vs 57 ml for RH, 76 vs 54 ml for LH and 76 vs 56 ml for RLH. During the last seconds of handgrip exercise, the left ventricular ejection time was longer in T than in U. Increases in cardiac output (Qc) and systolic blood pressure did not differ substantially between T and U, nor between the handgrip exercise tests.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in structure and function of the human left ventricle after acclimatization to high altitude

To analyse the role of changes in structure and function of the left ventricle in determining cardiac function at rest and during exercise, several two-dimensional and Doppler echocardiographic measurements were performed on 11 healthy subjects immediately before an Himalayan expedition (Nun, 7135 m), during acclimatization (3 weeks) and 14 days after the return. At rest decreases were found in cardiac index (CI) (3.23 l.min-1.m-2, SD 0.4 vs 3.82 l.min-1.m-2, SD 0.58, P less than 0.01), left ventricular mass (55.3 g.m-2, SD 9.4 vs 65.2 g.m-2, SD 13.5, P less than 0.005) and left ventricular end-diastolic volume (LVEDV) (53.9 ml.m-2, SD 6.9 vs 64.8 ml.m-2, SD 9.1, P less than 0.001) after acclimatization; by contrast the coefficient of peak arterial pressure to left ventricular end-systolic volume (PAP/ESV) (7.8, SD 1.6 vs 6.0, SD 1.8, P less than 0.005) and mean wall stress [286 kdyn.cm-2, SD 31 vs 250 kdyn.cm-2, SD 21 (2.86 N.cm-2, SD 0.31 vs 2.50 N.cm-2, SD 0.21), P less than 0.005] increased. After return to sea level, low values of CI and mass persisted despite a return to normal of LVEDV and preload. A reduction of PAP/ESV was also observed. At peak exercise, PAP/ESV (8.7, SD 2.4 vs 12.8, SD 2.0, P less than 0.0025), CI (9.8 l.min-1.m-2, SD 2.5 vs 11.6 l.min-1.m-2, SD 1.6, P less than 0.05) and the ejection fraction (69%, SD 6 vs 76%, SD 4, P less than 0.05) were lower after return to sea level than before departure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermoregulatory responses to exercise at low ambient temperature performed after precooling or preheating procedures

Seven male skiers exercised for 30 min on a cycle ergometer at 50% of maximal oxygen uptake and an ambient temperature of 5 degrees C. The exercise was preceded either by cold exposure (PREC) or active warming-up (PREH). The data were compared with control exercise (CONT) performed immediately after entering the thermal chamber from a thermoneutral environment. Cold exposure resulted in negative heat storage (96.1 kJ.m-2, SE 5.9) leading to significantly lower rectal, mean body and mean skin temperatures at the onset of exercise in PREC, as compared to PREH and CONT. The PREC-PREH temperature differences were still significant at the end of the exercise period. During exercise in the PREC test, oxygen uptake was higher than in PREH test (32.8 ml.kg-1.min-1, SE 1.5 vs 30.5 ml.kg-1.min-1, SE 1.3, respectively). Heart rate showed only a tendency to be higher in PREC than in PREH and CONT tests. In the PREH test skin and body temperatures as well as sweat rate were already elevated at the beginning of exercise. Exercise-induced changes in these variables were minimal. Heat storage decreased with the duration of the exercise. Exercise at low ambient temperature preceded by a 30-min rest in a cold environment requires more energy than the same exercise performed after PREH.     

Saliva flow and composition in humans exposed to acute altitude hypoxia

The effects of acute hypoxia (2 days at 4350 m) on whole saliva flow and composition were studied on 12 sea-level natives, at rest and following a maximal exercise. Exercise, performed in normoxia and hypoxia, did not induce variations in saliva flow rate, saliva potassium or alpha-amylase concentrations. In contrast, acute hypoxia did lead to an increase in mean saliva flow rate both at rest (0.63 ml.min-1 to 0.93 ml.min-1, P less than 0.01) and after exercise (0.56 ml.min-1 to 1.06 ml.min-1, P less than 0.05) and a decrease in mean saliva potassium concentration at rest (20.8 mmol.l-1 to 14.7 mmol.l-1, P less than 0.01) as well as after exercise (21.7 mmol.l-1 to 16.5 mmol.l-1, P less than 0.05). This effect might be the consequence of a hypoxia-induced stimulation of the parasympathetic nervous system.     

Comparison of thermal responses between rest and leg exercise in water

This study examined both the thermal and metabolic responses of individuals in cool (30 degrees C, n = 9) and cold (18 degrees C, n = 7; 20 degrees C, n = 2) water. Male volunteers were immersed up to the neck for 1 h during both seated rest (R) and leg exercise (LE). In 30 degrees C water, metabolic rate (M) remained unchanged over time during both R (115 W, 60 min) and LE (528 W, 60 min). Mean skin temperature (Tsk) declined (P less than 0.05) over 1 h during R, while Tsk was unchanged during LE. Rectal (Tre) and esophageal (Tes) temperatures decreased (P less than 0.05) during R (delta Tre, -0.5 degrees C; delta Tes, -0.3 degrees C) and increased (P less than 0.05) during LE (delta Tre, 0.4 degrees C; Tsk, 0.4 degrees C). M, Tsk, Tre, and Tes were higher (P less than 0.05) during LE compared with R. In cool water, all regional heat flows (leg, chest, and arm) were generally greater (P less than 0.05) during LE than R. In cold water, M increased (P less than 0.05) over 1 h during R but remained unchanged during LE. Tre decreased (P less than 0.05) during R (delta Tre, -0.8 degrees C) but was unchanged during LE. Tes declined (P less than 0.05) during R (delta Tes, -0.4 degrees C) but increased (P less than 0.05) during LE (delta Tes, 0.2 degrees C). M, Tre, and Tes were higher (P less than 0.05), whereas Tsk was not different during LE compared with R at 60 min.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanism of enhanced cold tolerance by an ephedrine-caffeine mixture in humans

The influence of a thermogenic mixture of ephedrine- (1 mg/kg) caffeine (2.5 mg/kg) on cold tolerance was investigated in nine healthy young male subjects during two seminude exposures to cold air (3 h at 10 degrees C). The drug ingestion reduced the total drop in core, mean skin, and mean body temperatures (P less than 0.01), thus producing significantly warmer final core, mean skin, and mean body temperatures compared with the placebo ingestion. The drug ingestion increased the total 3-h energy expenditure by 18.6% compared with that of the placebo ingestion in the cold (P less than 0.01). By means of the nonprotein respiratory exchange ratio to calculate the rates of substrate oxidation, it was found that the drug ingestion increased carbohydrate oxidation by as much as 41.7% above that of the placebo (P less than 0.05). In contrast, the drug mixture had no significant influence on lipid or protein metabolism. The results demonstrate that the ingestion of an ephedrine-caffeine mixture improves cold tolerance in humans by significantly increasing body temperatures in the cold. These improvements were not caused by an increased conservation of heat but by a greater energy expenditure, which appears to be dependent on an enhanced carbohydrate utilization.     

Diving response and arterial oxygen saturation during apnea and exercise in breath-hold divers.

This study addressed the effects of apnea in air and apnea with face immersion in cold water (10 degrees C) on the diving response and arterial oxygen saturation during dynamic exercise. Eight trained breath-hold divers performed steady-state exercise on a cycle ergometer at 100 W. During exercise, each subject performed 30-s apneas in air and 30-s apneas with face immersion. The heart rate and arterial oxygen saturation decreased and blood pressure increased during the apneas. Compared with apneas in air, apneas with face immersion augmented the heart rate reduction from 21 to 33% (P < 0.001) and the blood pressure increase from 34 to 42% (P < 0.05). The reduction in arterial oxygen saturation from eupneic control was 6.8% during apneas in air and 5.2% during apneas with face immersion (P < 0.05). The results indicate that augmentation of the diving response slows down the depletion of the lung oxygen store, possibly associated with a larger reduction in peripheral venous oxygen stores and increased anaerobiosis. This mechanism delays the fall in alveolar and arterial PO(2) and, thereby, the development of hypoxia in vital organs. Accordingly, we conclude that the human diving response has an oxygen-conserving effect during exercise.     

Thermal responses of men and women during cold-water immersion: influence of exercise intensity

The influence of exercise intensity on thermoregulation was studied in 8 men and 8 women volunteers during three levels of arm-leg exercise (level I: 700 ml oxygen (O2).min-1; level II: 1250 ml O2.min-1; level III: 1700 ml O2.min-1) for 1 h in water at 20 and 28 degrees C (Tw). For the men in Tw 28 degrees C the rectal temperature (Tre) fell 0.79 degree C (P less than 0.05) during immersion in both rest and level-I exercise. With level-II exercise a drop in Tre of 0.54 degree C (P less than 0.05) was noted, while at level-III exercise Tre did not change from the pre-immersion value. At Tw of 20 degrees C, Tre fell throughout immersion with no significant difference in final Tre observed between rest and any exercise level. For the women at rest at Tw 28 degrees C, Tre fell 0.80 degree C (P less than 0.05) below the pre-immersion value. With the two more intense levels of exercise Tre did not decrease during immersion. In Tw 20 degrees C, the women maintained higher Tre (P less than 0.05) during level-II and level-III exercise compared to rest and exercise at level I. The Tre responses were related to changes in tissue insulation (I(t)) between rest and exercise with the largest reductions in I(t) noted between rest and level-I exercise across Tw and gender. For mean and women of similar percentage body fat, decreases in Tre were greater for the women at rest and level-I exercise in Tw 20 degrees C (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Metabolic and cardiovascular adjustment to work in air and water at 18, 25, and 33 degrees C.

By use of successive increments of discontinuous work with an arm-leg cycle ergometer the VO2, Q, SV, and HR were studied in six male subjects at rest and during exercise in air and in water at 18, 25, and 33 degrees C. The Q values obtained by CO2 rebreathing were reproducible. VO2 was linearly related to work with the plots for air and 33 degrees C water being similar. However, during work in 25 and 18 degrees C water, the VO2 averaged 9.0% (150 ml) and 25.3% (400 ml) higher, respectively, than values observed in 33 degrees C water, with the largest differences observed in leaner subjects. The plot of HR-VO2 was linear and almost identical during work in air and 33 degrees C water, but shifted significantly to the right in cooler water. VO2 averaged 250-700 ml higher in cold water compared to air and 33 degrees C water at a given mean heart rate. The Q vs. VO2 line was similar during work in air and in water with no effect of water or temperature. At similar levels of VO2, SV was significantly larger (P less than 0.05) in 25 and 18 degrees C water than in air or 33 degrees C water. Consequently, the reduction in heart rate during work in cold water was entirely compensated for by a proportionate increase in the SV of the heart. Q was therefore maintained at similar levels of energy expenditure in air and in 18, 25, and 30 degrees C water.     

Effect of anaerobic and aerobic exercise of equal duration and work expenditure on plasma growth hormone levels

Growth hormone (GH) and lactic acid levels were measured in five normal males before, during and after two different types of exercise of nearly equal total duration and work expenditure. Exercise I (aerobic) consisted of continuous cycling at 100 W for 20 min. Exercise II (anaerobic) was intermittent cycling for one minute at 285 W followed by two minutes of rest, this cycle being repeated seven times. Significant differences (P less than 0.01) were observed in lactic acid levels at the end of exercise protocols (20 min) between the aerobic (I) and anaerobic (II) exercises (1.96 +/- 0.33 mM X 1(-1) vs 9.22 +/- 0.41 mM X 1(-1), respectively). GH levels were higher in anaerobic exercise (II) than in aerobic (I) at the end of the exercise (20 min) (2.65 +/- 0.95 micrograms X 1(-1) vs 0.8 +/- 0.4 micrograms X 1(-1); P less than 0.10) and into the recovery period (30 min) (7.25 +/- 6.20 micrograms X 1(-1) vs 2.5 +/- 2.9 micrograms X 1(-1); P less than 0.05, respectively).     

Energy substrate partitioning and efficiency in individuals with atherogenic lipoprotein phenotype

Individuals with an atherogenic lipoprotein phenotype (ALP) characterized by increased levels of small dense low-density lipoprotein (LDL) particles tend to have greater adiposity compared to unaffected subjects. We sought to determine whether this may be related to alterations in energy substrate partitioning or efficiency. These were assessed by indirect calorimetry in men with ALP (ALP(+), n = 7) and unaffected controls (ALP(-), n = 8) during rest (30 min) and exercise (10 min). Gross, net and delta efficiencies were calculated during graded leg-cycle ergometry at workloads of 10 and 50 W. Respiratory exchange ratios (RER) were significantly (P < 0.05) higher in ALP(+) vs. ALP(-) during rest (0.86 ± 0.01 vs. 0.83 ± 0.02) and exercise at 10 W (0.88 ± 0.02 vs. 0.84 ± 0.02) and 50 W (0.92 ± 0.01 vs. 0.87 ± 0.01, respectively) (P < 0.05). Lipid oxidation (kcal/min) was lower in ALP(+) vs. ALP(-) during rest (0.56 ± 0.02 vs. 0.71 ± 0.07) and exercise at 10 W (1.52 ± 0.25 vs. 2.00 ± 0.20) and 50 W (1.28 ± 0.10 vs. 2.32 ± 0.22, respectively) (P < 0.05). Gross and net efficiencies were significantly increased (P = 0.005) in ALP(+) vs. ALP(-) at 10 W. RER was correlated positively with plasma triglyceride during exercise and inversely with high-density lipoprotein (HDL) cholesterol and LDL peak particle diameter during rest and exercise (P < 0.05). These findings suggest that increased muscular efficiency at low exercise intensity and reduced lipid oxidation during rest and exercise may contribute to both dyslipidemia and increased adiposity in individuals with ALP.