Prediction of maximal oxygen uptake from submaximal exercise testing in aerobically fit and nonfit men.

Aerobic physical fitness, as determined by the body's maximal capacity to utilize oxygen (VO2max) during demanding work, is an important determinant of a person's ability to perform many military tasks. The present 2.4 km (1.5 mi) run has not proven itself capable of accurately estimating this important factor on a periodic basis. This paper reviews prior studies of heart rate response to known workloads on a cycle ergometer to estimate VO2max. This submaximal test, as revised by scientists at the USAF Armstrong Laboratory at Brooks AFB, TX, was validated on 22 male subjects by comparing the test results with laboratory measurements of VO2max obtained by analysis of expired air during maximal treadmill exercise. Two groups of subjects were selected; one consisting of highly trained runners and the other of inactive subjects who did not perform regular aerobic exercise. The cycle ergometry prediction underestimated measured VO2max by 8.1 ml.kg-1 x min-1 (SEE = 4.25) in all subjects, but there was a correlation of 0.95 between the estimated and measured values. Both estimated and measured VO2max were significantly higher in the group of trained runners than in the inactive subjects.

Hormone and energy substrate changes during prolonged exercise in the heat.

Eleven trained men (aged 34.5 +/- 2 yrs) were studied during a 16.1 km run in the heat (Ta = 30.2 degrees C). Fasting blood samples were taken prior to the run and at 6.4, 12.9, and 16.1 km, and 3 h recovery. Serum or plasma glucose, insulin, glucagon, glycerol, and catecholamines were measured. Mean values were: exercise intensity, 80% of VO2max; final rectal temperature, 39.9 degrees C; and weight loss, 4.0%. Glucose increased 61% by 6.4 km, then decreased significantly by 16.1 km. Glycerol increased by 415% at 6.4 km, and continued to increase throughout the run. Epinephrine increased progressively during the run, but norepinephrine increased at 6.4 km, and did not change further during the exercise. Insulin increased slightly at 6.4 km, then decreased significantly from 6.4-16.1 km. Glucagon increased from 6.4-12.9 km and remained elevated at 3 h recovery. Hormone and substrate measurements obtained only before and after prolonged exercise may not reflect changes that occur during the course of the exercise. The observed insulin-glucagon relationships vary from previous findings in nontrained subjects at lower exercise intensities.

Validation and application of single breath cardiac output determinations in man.

Cardiac outputs by single breath (Qsb) and Fick (Qf) procedures were compared in five healthy males during supine rest and exercise with Qf ranging from 6-19 L X min-1. The prolonged exhalation (SB) was not controlled. The Qsb calculations incorporated an equation of the CO2 dissociation curve and a "moving spline" sequential curve-fitting technique to calculate the instantaneous R from points on the original expirogram. The resulting linear regression equation for all 38 comparisons obtained (r = +0.76, p less than 0.001, mean difference +/- S.D. = 2.93 +/- 2.72 L X min-1) indicated a 24% underestimation of Qf. A substantial portion of the variability during exercise (n = 28) was due to a difference in alveolar ventilation between the time of the mixed expired (E) gas collection and the SB maneuver. When Qsb was corrected (Qsb) by a linear regression based on the difference between Re and Rsb during exercise and by adding 2.44 L X min-1 at rest (the mean difference), the relationship was greatly improved (Qsb = 0.14 + 0.99 Qf, r = +0.93, mean difference +/- S.D. = 0 +/- 1.47 L X min-1). A subsequent study during upright rest and exercise to 80% of VO2max in 6 subjects indicated a close linear relationship between Q'sb and VO2 for all 95 values obtained (r = +0.94), with slope and intercept close to published studies utilizing invasive cardiac output measurements. Considerations of measured blood gases in relation to estimated values suggested that underestimates of Qf arose, at least in part, from arterial desaturation during the SB maneuver. Detailed computational procedures are provided for implementing this improved Qsb procedure.

Plasma volume changes in middle-aged male and female subjects during marathon running.

Circulatory fluid shifts were studied in middle-aged runners (6 males and 5 females, ages 32-58 yr) during a 42.2-km marathon race run in mild weather (dry-bulb temperature = 17.5-20.4 degrees C). Running times for the subjects were 3:12-4:40 (mean values were 3:34 for males and 4:10 for females). Venous blood samples were taken without stasis in all subjects seated at rest before the start of the race and within 3 min of finishing; eight of the subjects also paused for samples at 6 and 27 km during the race. At 6 km, body weight loss averaged less than 1%, whereas plasma volume (PV) had decreased by 6.5% in male subjects and 8.6% in female subjects. By the end of the race, hypohydration had reached 3.2% in male subjects and 2.9% in female subjects, but PV in both groups remained stable. Sweat rates during the race averaged 545 and 429 g X m-2 X h-1 for male and female subjects, respectively, with ad lib. water intake replacing 21-72% of fluid loss. Increases in plasma protein concentration throughout the race reflected the observed initial decrease in PV. The interpretation of PV responses to exercise and/or hypohydration is critically dependent on selection of base-line conditions; we were able to control for posture-exercise effects by treating the early exercise (6 km) sample as the base line for examining the effects of later fluid loss. Under these conditions, the vascular compartment resisted volume depletion. The ability to maintain stable PV can be explained in part by relationships among oncotic and hydrostatic pressures in the intra- and extravascular fluid compartments.

Plasma substrate response in men and women during marathon running.

Venous blood samples from three male and four female subjects were taken in conjunction with a 26.2-mi marathon race prior to the start (control), at 17 mi, and immediately after the finish. Plasma concentrations of glucose and free fatty acids (FFA) were determined, and the ratio of unsaturated to saturated FFA (U/S FFA) was calculated. Compared to the control value, glucose was unchanged at the finish despite a slight increase at 17 mi (not significant). FFA increased significantly (p less than 0.05) from control to finish and from 17 mi to finish in both groups. The U/S FFA ratio also increased progressively from control to finish. No significant differences were found between male and female groups for any of the measured values. We conclude that blood glucose is maintained at normal levels during prolonged heavy exercise in trained women as in men, and that FFA availability increases with time during strenuous exercise in both sexes.

Plasma volume and blood metabolites in middle-aged runners during a warm-weather marathon.

Plasma volume (PV), blood-borne substrate, and electrolyte responses to a warm weather (WBGT 15.5-24.5 degrees C) marathon race were observed in four male and two female runners averaging 48 years of age. Additionally, continuous recordings were made of heart rate (HR) in three, and of rectal temperature (Tre) in two of the men. The race was finished by three of the men, with an average time of 3.61 h, a running pace estimated to require from 60-66% of the runner's VO2 max. Near steady state levels for HR and Tre, ranging from 140-165 beats . min-1 and from 38.5-39.3 degrees C, respectively, were reached early in the race. Total and percentage weight losses for the finishers were 2.52 kg and 3.9%, 2.18 kg and 3.4%, 4.77 kg and 6.7%, respectively; corresponding reductions in PV for these runners were 5.4%, 13.2%, and 27.4%. Pre-race control and immediate post-race serum glucose concentrations averaged 109 and 154 mg . dl-1 respectively. Final blood lactate values ranged from 11-42 mg . dl-1. Thus, it may be concluded that middle-aged runners who were successful in completing the marathon in a warm environment did so while maintaining steady state levels of HR and Tre. Although only water was ingested during the race, serum glucose for the finishing runners remained above, while serum [Na+] and [Cl-] remained within +/- 8% of, control values throughout the run. Complete data obtained from only one of the runners suggest that the initial exercise-induced reduction in PV is not augmented by subsequent cumulative dehydration provided water intake during the race is sufficient to limit weight loss to less than 4%.

Physiological effects of cold air inhalation during exercise.

Selected physiological responses of six normal subjects were oberved, during rest and exercise, while they breathed a) ambient, and b) cold (-35 degrees C) air. All experiments were 10 min in duration, and the exercise experiments consisted of pedaling a bicycle ergometer at loads requiring approximately 60% and 75% of each subject's VO2 max. Heart rates and minute ventilations during the most strenous exercise averaged approximately 170 bpm and 70 l, respectively. Diastolic blood pressure was significantly lower, during cold air inhalation. Oxygen uptake, respiration rate, and rectal temperature were not affected by cold air breathing; and no subject complaints were attributable to cold air inhalation. Recent studies in the literature suggest that cold air is not fully warmed in the upper respiratory passages; however, the present study observed only slight changes in measured physiological responses to rest and exercise with cold air breathing.

Cardiac output during rest and exercise in desert heat.

Cardiac output and stroke volume were measured in two environments and at metabolic rates ranging from rest to the maximum rate that could be sustained for 25 minutes. One environment was indoors at about 23 degrees C, the other outdoors in desert sunshine and low water vapor pressure. The age range of the one female and four of the male subjects was from 19 to 40; the fifth male subject was 85 years old. Cardiac output was the same in the two environments; stroke volume was less at higher metabolic rates in the heat. The cardiac output for the old man was about one-tenth less and stroke volume about 20 ml less than that observed for the same work 50 years earlier.

Physiological and psychological effects of heat stress simulating cockpit conditions.

It is generally accepted that physiologically tolerable heat stress can adversely affect human performance, but it is difficult to predict the impact of a specific environment. Experiments were designed to determine the effects of thermal conditions similar to those occurring in aircraft cockpits in warm climates, where both high air temperatures and radiant heat play important roles. Subjects (n = 13) were exposed to heat for 2 h, had a 30-min break, then repeated the exposure. Conditions were Tdb = 35 degrees C and Twhb = 26 degrees C, with or without use of infrared lamps which raised globe temperature to 47 degrees C. Measurements included skin, rectal, and esophageal temperatures, heart rate, weight loss, and hematocrit. Subjective Fatigue Estimates (SFE) and Repetitive Psychometric Measures (RPM) were performed before, during, and after each heat stress. Both thermal conditions were physiologically compensable but induced marked subjective fatigue and altered the learning curve for some subtests of the RPM. Similar conditions in aircraft can be associated with impaired performance, particularly in new or emergency situations.

Total body water and lean body mass estimated by ethanol dilution.

Total body water (TBW) was determined on 35 subjects with a tritium (HTO) and an ethanol (ETH) dilution method, the latter using breath analyses for blood ethanol content. Lean body mass (LBM) was estimated by hydrostatic weighing. Mean values for water fraction (TBW/wt) were 0.618 +/- 0.05 with HTO and 0.603 +/- 0.06 with ETH. The difference was not significant. The correlation between the two methods was highly significant (r = 0.90, SEE = 3.5 liters, P less than 0.0001). High correlations were also found between TBW and LBM with either method for TBW (r = 0.94 with HTO, r = 0.91 with ETH). Mean values for TBW/LBM were 0.735 with HTO and 0.717 with ETH. The ETH method compares favorably with the HTO, it has the advantage that it is nonradioactive and can be repeated daily if necessary.

Fluid shifts during thermal stress with and without fluid replacement.

Six unacclimatized men rested for 4 h in a hot, dry environment (Ta 50 degrees C, Tw 26 degrees C; wind speed 0.5 m/s) without fluid replacement (DH). Another group of six men were exposed to the same thermal stress, replacing evaporative fluid loss with warm 0.1% NaCl solution (FRP). Total grams of circulating hemoglobin, determined by CO immediately prior to and again during the last minutes of heat exposure, increased an insignificant 1.6 and 1.3% during DH and FRP, respectively. With DH, body weight loss of 2.6% was accompanied by a 7.8% reduction in calculated plasma volume (PV). Even when body weight was maintained (FRP), PV decreased 2.9% during the heat exposure. Total circulating serum protein did not change as a result of the heat stress with either DH or FRP. In a test-retest series of experiments on four men, DH was not detrimental to sweat rate which averaged 251 g/m2-h compared with 239 g/m2-h with FRP. It was concluded that hemodilution is not a general response to acute heat exposure, and the disproportionately large reduction in PV during thermal dehydration was confirmed.

Physiological effects of solar heat load in a fighter cockpit.

The use of bubble canopies to improve vision in fighter aircraft exposes the cockpit to a high radiant heat load. Incoming sunlight increases the heat stress on crewmembers, both by raising air temperature and by directly heating exposed skin and clothing. An F-15 aircraft at Edwards AFB was modified to permit cockpit ventilation by external ground carts. Eight volunteers from the Test Pilot School were studied during 1-h periods in the closed cockpit, in sun and in shade. Mean cockpit air temperatures were 35.2 degrees C in shade and 51.9 degrees C in sun with PH2O less than 10 torr. The corresponding WBGT's were 22.6 and 36.4 degrees C. Sunlight added significantly to overall heat stress, as indicated by a rising heart rate and evaporative weight loss of 284 g/m2 - h (shade value was 109 g/m2 - hr). Mean skin temperatures were 34.3 degrees C in shade and 35.8 degrees C in sun. Particularly high skin temperatures were observed on the chest, the forehead and the top of the head under the helmet. The legs remained cool due to the flow of conditioned air, and this may explain why rectal temperature showed no meaningful change. Heat stress, which alone poses no physiological hazard, may cause crew performance decrements as well as diminishing acceleration tolerance. Possible means of eliminating or ameliorating these effects are discussed.