A lightweight ambient air-cooling unit for use in hazardous environments.

Recent research demonstrated (a) the effectiveness of intermittent conditioned air cooling during rest breaks to significantly reduce cumulative heat storage and (b) that longer work sessions were possible for individuals wearing chemical defense ensembles. To further advance this concept, a strategy for implementing continuous air cooling was conceived; ambient air cooling was added during work cycles and conditioned air cooling was delivered during rest periods. A compact battery-powered beltpack cooling unit (3.9 kg) designed and made at the U.S. Air Force Armstrong Laboratory was used to deliver 5.7 L/sec filtered ambient air during work cycles: 4.7 L/sec to the body and 1 L/sec to the face. Five experimental cycles were conducted in a thermally controlled chamber under warm conditions (32 degrees C, 40% relative humidity) with (1) no cooling-intermittent work, (2) intermittent cooling, (3) continuous cooling during intermittent exercise, and (4) no cooling-continuous work and (5) ambient air cooling during continuous exercise. Intermittent, conditioned, and continuous air cooling resulted in significant reductions in rectal temperature, mean skin temperature, and heart rate as compared with the no-cooling trials. The continuous air-cooling trial significantly improved thermal comfort and sweat evaporation. Results suggest that ambient air delivered during work cycles by a lightweight portable unit (in conjunction with conditioned air delivered during rest periods), can definitely improve personal comfort, reduce skin temperature, and decrease the cumulative fatigue common to repeated work/rest cycles in selected military and industrial applications in which individuals work in chemical defense ensembles.

Continuous and intermittent personal microclimate cooling strategies.

A comparison was made between two personal auxiliary cooling approaches for the relief of thermal stress while wearing the standard USAF Chemical Defense Ensemble (CDE). Subjects exercised at approximately 40% VO2max in either warm (28/24/34 degrees C) or hot (38/26/43 degrees C) environmental conditions, (Tdb/Twb/Tbg degrees C, respectively). During each of three trials, four hours of intermittent work (four work/rest cycles) were attempted. Microclimate air cooling was applied in two different fashions and compared with a control trial during which no cooling was received (NC). In one trial, conditioned air cooling (Tin approximately 20 degrees Cdb) was delivered during rest periods only (intermittent cooling, IC), while during the second trial, ambient air cooling was also applied during the work period in addition to the conditioned air delivered during rest periods (continuous cooling, CC). During the warm condition, exposure cycle time was 45 min work and 15 min rest, while under the hot conditions, exposure cycle time was 30 min work and 30 min rest. Both CC and IC trials resulted in significantly extended work times, lower final rectal temperatures, heart rates, and sweat production (SP) than in the NC trial. Additionally, CC results in significantly lower SP, higher % sweat evaporation, and lower ratings of perceived exertion (RPE) and thermal comfort (TC) than IC at both warm and hot temperatures. Moreover, subjects were better able to maintain thermal equilibrium (i.e., cumulative heat balance) over time using CC compared to IC in the warm environment. The physiological significance of these findings, in some cases, was secondary to the improvement in subjective measures of TC and RPE.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological responses of rhesus monkeys to exercise at varied temperatures.

This research characterizes the effects of selected physiological stressors such as work and various environmental heat loads in rhesus monkeys. Non-human primates (N = 6) were behaviorally conditioned to exercise in a wheel ergometer at approximately 3 METs (1 MET = 3.5 ml O2 uptake.kg-1.min-1). On separate days, each monkey attempted six work/rest cycles (10 min work: 1 min rest) at 15, 25, and 35 degrees C (Tdb), vapor pressure < 10 mm Hg. Core temperature (Tco), body weight (BW) and blood samples were taken immediately before and after exercise. Excessively high heat storage rates dictated that the 35 degrees C trial be limited to three work/rest bouts. The change in Tco during exercise was significantly greater in the 35 degrees C trial than during the 15 and 25 degrees C work bouts. Sweat rate, as determined by the change in BW over time, was also significantly greater during the 35 degrees C trial as compared to the 15 and 25 degrees C trials. Glucose levels (mean +/- SE) declined significantly during exercise from 4.35 +/- 0.1 and 4.58 +/- 0.4 mmol to surprisingly low levels of 1.67 +/- 0.2 and 1.76 +/- 0.2 mmol in the 15 and 25 degrees C trials, respectively. Increases in blood lactate, glycerol, and triglycerides were observed independent of environmental temperature. Free fatty acids increased during exercise in the 15 and 25 degrees C trials but declined slightly during the shorter 35 degrees C trial. The findings of this study indicate that the increased heat storage observed at higher environmental heat loads appears to substantially limit the amount of work these primates can perform, possibly the result of a limited sweat production capacity. Additionally, plasma glucose following exercise decreased to levels not typically seen in humans. The general metabolic profile in these primates was otherwise similar to that observed in humans at this work level.

Intermittent microclimate cooling during rest increases work capacity and reduces heat stress.

Requirements for special protective equipment while working in hazardous environments can present a significantly increased thermal burden and early onset of physical fatigue. Ambulatory (backpack) or tethered personal cooling can accelerate metabolic heat removal, but is often not practical from an ergonomic standpoint. The efficacy of incorporating personal cooling during non-ambulatory rest periods was evaluated in subjects (n = 8) with varying levels of fitness. Treadmill work (approximately 475 W, 40% VO2 max) was alternately performed for 30 min followed by 30 min of rest. Subjects walked and rested under three separate experimental conditions: (1) control (C), in which light clothing was worn; (2) CPE, in which a chemical protective ensemble (CPE) was worn, and (3) CPE plus intermittent microclimate cooling (COOL). The WBGT condition for all trials was 31 degrees C. During the COOL trial the subjects additionally wore a personal cooling vest which allowed for the circulation of chilled liquid over the torso during rest. Under C conditions, relatively modest changes in rectal temperature (Tre) were observed, which stabilized over time. CPE wear resulted in a progressive rise in Tre and early fatigue. The addition of intermittent cooling during each rest cycle (COOL trial) significantly attenuated heat storage such that an oscillating, but equilibrated Tre was established and work capacity was at least doubled. Moreover, the perceived cooling effect was appreciable for all subjects. Therefore, intermittent personal cooling provided a useful means of enhancing work productivity and may have application for certain military and industrial personnel performing heavy work in hot environments. This approach should provide a practical alternative for reducing stress/fatigue when work/rest cycles are employed.

Intermittent cold exposure causes a muscle-specific shift in the fiber type composition in rats.

We examined the effect of long-term intermittent cold exposure on the fiber type composition of the predominantly type I soleus and the predominantly type IIb extensor digitorum longus (EDL) muscles of rats. Cold exposure was accomplished by submerging the rats in shoulder-deep water, maintained at 20 +/- 0.5 degrees C, for 1 h/day, 5 days/wk, for < or = 19 wk. The efficacy of the treatment was tested by subjecting both groups to 20 degrees C water for 45 min while rectal temperature (Tre) and O2 consumption (VO2) were measured. The cold-exposed group displayed a 22% smaller reduction in Tre (P < 0.05) at the end of the exposure and 23% greater VO2 (P < 0.05) during the same period. Fiber type composition was determined using routine histochemical methods for myosin-adenosinetriphosphatase. In the soleus muscle of the cold-exposed rats, the number of type IIa fibers increased 156% (P < 0.05) and the number of type I fibers decreased 24% (P < 0.05). Cold exposure had no significant influence on the fiber type composition of the EDL muscle. Cold exposure resulted in an increase in citrate synthase activity of 20 and 22% in the soleus and EDL muscles, respectively (P < 0.05). The present study demonstrates that intermittent cold exposure induces a type I-to-type IIa transformation in the soleus muscle while having no influence on the EDL muscle.

Comparisons of air and liquid personal cooling for intermittent heavy work in moderate temperatures.

Personal microenvironmental cooling has been used to enhance safety and extend the work capacity of laborers wearing protective clothing. Previous studies of air and liquid cooling have used either very low work rates or high environmental temperatures. Emergency work tasks frequently require high work rates and occur in moderate ambient temperatures. The purpose of this research was to examine the efficacy of intermittent personal cooling during rest and to compare liquid and air cooling systems in subjects engaged in hard work. Fourteen volunteers wearing chemical protective clothing performed treadmill walking at a metabolic rate of 430 W for 45 min followed by a 15-min rest at a wet-bulb globe temperature of 25 degrees C. During rest, volunteers received either no cooling, air cooling, or liquid cooling. Both cooling systems partially alleviated heat strain and increased work time with the air system offering slightly more effective cooling.

Limitations to heavy work at 21 degrees C of personnel wearing the U.S. military chemical defense ensemble.

The purpose of this study was to determine the limitations to work of personnel performing heavy work at 21 degrees C (70 degrees F) while wearing the chemical defense ensemble (CDE) worn by all branches of the U.S. military. Male volunteers (N = 17) wearing the CDE performed an arm and leg work task yielding a time-weighted energy consumption rate of 450 W, under environmental conditions of 21 degrees C and 65-70% relative humidity. Each work bout was continued until one of the following limits occurred: volitional fatigue, rectal temperature (Tre) of 39 degrees C, or calculated heat storage (S) of 140 Wh. Subjects then rested until heat stored due to this work was lost, then repeated the work-rest cycles until refusal, or inability to restart work after a rest cycle. Over a total of 36 work cycles, subjects terminated work cycles for S on only two occasions, with the remainder almost equally divided between cumulative fatigue and high Tre. Intersubject variability was high for work and rest times, S, heat loss, and sweat production. U.S. Air Force Regulation (AFR) 355-8, which provides guidance for heat stress in the form of work/rest cycles, underestimated work tolerance under these study conditions.

An applied model for the evaluation of multiple physiological stressors.

In everyday life, a human is likely to be exposed to the combined effects of a number of different stressors simultaneously. Consequently, if an applied model is to ultimately provide the best 'fit' between the modeling and modeled phenomena, it must be able to accommodate the evaluation of multiple stressors. Therefore, a multidimensional, primate model is described that can fully accommodate a large number of conceivably stressful, real life scenarios that may be encountered by civilian or military workers. A number of physiological measurements were made in female rhesus monkeys in order to validate the model against previous reports. These evaluations were further expanded to include the experimental perturbation of physical work (exercise). Physiological profiles during activity were extended with the incorporation of radio telemetry. In conclusion, this model allows maximal extrapolation of the potential deleterious or ergogenic effects on systemic physiological function under conditions of realistic operational demands and environments.

Glucose transport into rat skeletal muscle: interaction between exercise and insulin.

This study was done to evaluate the effect of insulin on sugar transport into skeletal muscle after exercise. The permeability of rat epitrochlearis muscle to 3-O-methylglucose (3-MG) was measured after exposure to a range of insulin concentrations 30, 60, and 180 min after a bout of exercise. Thirty and 60 min after exercise, the effects of exercise and insulin on 3-MG transport were additive over a wide range of insulin concentrations, with no increase in sensitivity or responsiveness to insulin. After 180 min, when approximately 66% of the exercise-induced increase in sugar transport had worn off, both the responsiveness and sensitivity of the glucose transport process to insulin were increased. These findings appear compatible with the hypothesis that the actions of exercise and insulin result in activation and/or translocation into the plasma membrane of two separate pools of glucose transporters in mammalian skeletal muscle.

Muscle glucose transport: interactions of in vitro contractions, insulin, and exercise.

Exercise increases permeability of muscle to glucose. Normally, the effects of exercise and a maximal insulin stimulus on glucose transport are additive. However, the combined effect on rat epitrochlearis muscle permeability to 3-O-methylglucose (3-MG) of a maximal insulin stimulus followed by in vitro contractile activity of 1.24 +/- 0.06 mumol.10 min-1.ml intracellular water-1 was no greater than that of either stimulus alone. We found that this absence of an additive effect was caused by prolonged exposure to an unphysiologically high insulin concentration (20,000 microU/ml for 60 min), which, in addition to stimulating glucose transport, appears to prevent further increases in permeability to glucose. When the treatments were reversed and muscles were first stimulated to contract and then incubated with 20,000 microU/ml insulin, 3-MG uptake (mumol.10 min-1.ml intracellular water-1) increased from a control value of 0.26 +/- 0.03 to 1.80 +/- 0.15, compared with 1.04 +/- 0.06 for contractile activity alone, 1.21 +/- 0.08 for insulin, and 1.88 +/- 0.11 for exercise (swimming) plus insulin. Swimming plus in vitro contractile activity did not have a greater effect than contractile activity alone. Our results provide evidence that 1) the effect of exercise on muscle permeability to glucose is mediated solely by a process associated with contractile activity, and 2) it is advisable to avoid the use of unphysiologically high insulin concentrations in studies designed to elucidate in vivo actions of insulin.

Energy metabolism in contracting rat skeletal muscle: adaptation to exercise training.

Rats were trained by means of a program of treadmill running. Hindlimb muscles were stimulated to contract in anesthetized rats. Measurements were made on the plantaris and the deep, predominantly fast-twitch red portion of the gastrocnemius. The concentration of ATP plus phosphocreatine (approximately P) decreased less and stabilized at a higher level, whereas inorganic phosphate (Pi) and AMP concentrations increased less and attained lower steady-state levels in trained than in untrained muscles at the same work rate. Similarly, when muscles were stimulated to contract in the perfused rat hindquarter preparation, phosphocreatine (PC) concentration decreased less in trained plantaris muscle during contractile activity that resulted in the same rate of oxygen uptake by trained and untrained muscles. In both preparations, glycogen concentration decreased less and lactate increased less in the trained muscle. From the changes that occurred in the PC-to-creatine ratio during contractile activity and from ATP concentration, it could be estimated that free ADP concentration increased less than one-half as much in trained as in untrained muscles. We conclude that, as a consequence of the adaptive increase in muscle mitochondria, approximately P concentration is higher and Pi, ADP, and AMP concentrations are lower in muscles of exercise-trained compared with untrained rats during the same contractile activity.

Endurance exercise training reduces lactate production.

In situ muscle stimulation in trained and untrained rats was used to reevaluate whether adaptations induced by endurance exercise training result in decreased lactate production by contracting muscles. The gastrocnemius-plantaris-soleus muscle group was stimulated to perform isotonic contractions. After 3 min of stimulation with 100-ms trains at 50 Hz at 60/min, the increases in lactate concentration in the plantaris, soleus, and fast-twitch red muscle (deep portion of lateral head of gastrocnemius) were only approximately 50% as great in trained as in sedentary rats. In the predominantly fast-twitch white superficial portion of the medial head of the gastrocnemius the increase in lactate concentration was 28% less in the trained than in the sedentary group. The decreases in muscle glycogen concentration seen after 3 min of stimulation at 60 trains/min were smaller in the trained than in the untrained group. The reduction in lactate accumulation that occurred in the different muscles in response to training was roughly proportional to the degree of glycogen sparing. These results show that endurance training induces adaptations that result in a slower production of lactate by muscle during contractile activity.

Bradykinin does not mediate activation of glucose transport by muscle contraction.

The purpose of this study was to evaluate the report that bradykinin is the "muscle activity hypoglycemia factor" responsible for the activation of glucose transport that occurs in response to muscle contractile activity. Stimulation of rat epitrochlearis muscles to contract resulted in approximately a fourfold increase in the rate of intracellular accumulation of the nonmetabolizable glucose analog 3-O-methylglucose. Incubation of the muscles with high concentrations of aprotinin (Trasylol), a polypeptide inhibitor of kallikrein which blocks formation of kinins, did not inhibit the activation of sugar transport by contractile activity. Furthermore incubation of muscles with bradykinin did not have a stimulatory effect on the uptake of 3-methylglucose either at a physiological concentration or at high concentrations. These results provide no support for the claims that aprotinin prevents the activation of sugar transport in muscle by contractile activity or that bradykinin is the muscle activity hypoglycemia factor.

Exercise and glycogen depletion: effects on ability to activate muscle phosphorylase.

Phosphorylase activation reverses during prolonged contractile activity. Our first experiment was designed to determine whether this loss of ability to activate phosphorylase by stimulation of muscle contraction persists following exercise. Phosphorylase activation by stimulation of muscle contraction was markedly inhibited in rats 25 min after exhausting exercise. To evaluate the role of glycogen depletion, we accelerated glycogen utilization by nicotinic acid administration. A large difference in muscle glycogen depletion during exercise of the same duration did not influence the blunting of phosphorylase activation. Phosphorylase activation by stimulation of contraction was more severely inhibited following prolonged exercise than after a shorter bout of exercise under conditions that resulted in the same degree of glycogen depletion. A large difference in muscle glycogen repletion during 90 min of recovery was not associated with a significant difference in the ability of muscle stimulation to activate phosphorylase, which was still significantly blunted. Phosphorylase activation by epinephrine was also markedly inhibited in muscle 25 min after strenuous exercise but had recovered completely in glycogen-repleted muscle 90 min after exercise. These results provide evidence that an effect of exercise other than glycogen depletion is involved in causing the inhibition of phosphorylase activation; however, they do not rule out the possibility that glycogen depletion also plays a role in this process.

Ratings of Perceived Exertion, Heart Rate, and Power Output in Predicting Maximal Oxygen Uptake During Submaximal Cycle Ergometry.

In brief: Sixty-two subjects completed a four- stage submaximal cycle ergometer test to volitional fatigue. The purpose was to determine if estimates of V O2 max could be improved by using ratings of perceived exertion singly or in combination with easily obtainable physiological measures during submaximal cycle ergometry. While these procedures may not be acceptable for scientific purposes, clinicians could use them to estimate the aerobic power of their patients and athletes.

Alterations in breast morphology consequent to a 21-day bust developer program.

Normal menstruating females, 19 to 32 years of age, were randomly assigned to either a control (C; N = 17) or an experimental (E; N = 17) group to determine alterations in breast size, shape, and volume consequent to a 21-day bust development program (BDP) using a commercial exercise device as specified by the manufacturer. E and C groups were evaluated on 2 consecutive d prior to initiating the BDP to establish reliability of the test procedures and again at the end of the BDP, 27 to 29 d following the initial measures. Anthropometric measures included the following girths: shoulder, chest, bust with and without brassiere, abdomen, deltoid, and flexed biceps. Breast photography included two 35-mm photographs each from the side and front views, in both full inspiration and full expiration, with the subject position carefully standardized. A total of 10 measurements, corrected for perspective error, were obtained from these photographs. Breast volume was measured using a water displacement technique, with three trials for each breast. Repeated measures analysis of variance procedures indicated no significant differences consequent to the BDP for any of the variables assessed, with the exception of a small decrease in shoulder girth in both E and C and a small decrease in left breast volume in E. It was concluded that the use of a commercial exercise device with a specific BDP does not result in changes in breast size, shape, or volume.

Glycogen resynthesis in leg muscles of rats during exercise.

This study was undertaken to determine whether glycogen resynthesis can occur in glycogen-depleted muscles in response to glucose feeding during prolonged exercise. Rats were exercised for 40 min with a treadmill running program designed to deplete muscle glycogen. One group was studied immediately after the glycogen-depletion exercise. A second group was given 1 g glucose by stomach tube and exercised for an additional 90 min at a running speed of 22 m/min on a treadmill set at an 8 degree incline; they were given additional 1-g glucose feedings after 30 and 60 min of running. The initial 40-min run resulted in liver glycogen depletion, large decreases in plasma glucose and insulin concentrations, and a marked lowering of muscle glycogen. The glucose feedings resulted in greater than twofold increases in the concentrations of glucose and insulin in plasma, and of glycogen in leg muscles, during the 90 min of running. No repletion of liver glycogen occurred. These results provide evidence that glycogen resynthesis can occur in glycogen-depleted muscle despite continued moderate intensity exercise if sufficient glucose is made available.

Effects of the oral hypoglycemic agent methyl palmoxirate on exercise capacity of rats.

The effect of the oral hypoglycemic agent methyl palmoxirate (methyl 2-tetradecylglycidate, McN-3716), a selective inhibitor of long chain fatty acid oxidation, on the exercise capacity of normal rats was evaluated. Daily administration of 2.5 mg/kg for 7 days, or of a single dose of 10 mg/kg, of methyl palmoxirate did not affect the ability of rats to perform strenuous exercise of an intensity that caused exhaustion in less than 30 min. The ability to perform prolonged, moderately strenuous exercise of an intensity that could be maintained for more than 60 min was decreased slightly (17%) in the methyl palmoxirate treated rats. This effect appeared to be mediated by a significant reduction in initial liver glycogen content in the methyl palmoxirate treated rats. As a consequence, the methyl palmoxirate treated rats became hypoglycemic during prolonged exercise. Inhibition of fatty acid oxidation in skeletal muscle was minimal. Treatment with methyl palmoxirate protected against the development of exercise-induced ketosis. It appears that the liver is the major site of action of methyl palmoxirate when given in low dosage.

Maximal exercise and residual lung volume: considerations for body composition analysis.

Pulmonary function was evaluated in 11 female cross country runners before and after maximal treadmill exercise. Hydrostatic weighing was also performed post exercise. Significant increases were found in both residual lung volume (X delta + 210 ml, 20.6%) and total lung capacity (X delta + 170 ml, 3.4%). These changes were not correlated to the intensity of the exercise. The magnitude of the change in residual lung volume resulted in a mean error of 12.1% in body composition prediction. Standardized testing orders are recommended.