Prediction of lactate threshold using the modified Conconi test in distance runners.

This study aimed to examine the validity of the modified Conconi test (CT) to predict lactate threshold (LT) during running. Twelve distance runners randomly performed the modified CT and the incremental test in which LT was determined directly by measuring blood lactate (BLa). Mean values of heart rate (HR) and running speed (RS) at heart rate deflection point (HRDP) obtained through the modified CT were compared with those at LT. Subsequently, the runners who showed a HRDP in the modified CT performed a 30-min prolonged exercise test (PET) at a RS corresponding to HRDP. During this test, the kinetics of BLa and HR were analyzed to determine whether a steady state in these variables could be attained. We succeeded in identifying HRDP in nine of our runners, whereas the remaining three runners showed entirely linear HR response. In those nine runners, no significant difference was found between HR and RS at HRDP and those at LT. Significant correlation was found between HR at HRDP and HR at LT (r = 0.84, p = 0.005), but RS at HRDP was not significantly correlated with RS at LT (r = 0.63, p = 0.07). All nine runners were able to complete the PET with steady state conditions being achieved for both BLa and HR. In conclusion, these findings indicate that the modified CT has a potential to be used as an alternative method for assessment of LT in distance runners presenting a HRDP.

Urban daily life routines and human exposure to environmental discomfort.

This study suggests a shift in focus from studying environmental discomfort in urban strategic stations, from which average results for the city or specific results for selected sites are deduced, and from measuring environmental conditions in fixed monitoring stations to a study in which we monitor, with mobile portable sensors, the exposure of people to environmental sources of discomfort while performing their daily life activities. Significant variations in sense of discomfort were measured in this study, and almost half of this variability was found to be explained while four independent environmental variables were considered: air quality (concentrations of CO), noise level, climatic variables (thermal load), and social loads. The study conducted in the city of Tel Aviv, which suffers from hot, humid summers and cool winters, and noise levels that reach the average levels of 85 dB, and relatively lower levels of exposure to the other potential stressors. These levels of combined exposures result in moderate levels of discomfort for young, healthy people once they experience the more stressing environments in the city. It is shown also that noise from other people is the most salient source of discomfort in Tel Aviv. Levels of discomfort accumulate during the working hours, either due to the impact of social loads or noise, but the subjects showed good coping abilities that enabled them to recover in late afternoons. It seems that thermal load does not have immediate impact, but rather cumulative ones, mainly during transitional seasons when subjects are less adaptive to extreme changes in weather.

The pathopysiology of heat stroke: an integrative view of the final common pathway.

Heat stroke is defined as a condition in which body temperature is elevated to such a level that it becomes a noxious agent causing body tissue dysfunction and damage with a characteristic multi-organ clinical and pathological syndrome. Marked hyperthermia, usually above 40.5°C and associated encephalopathy, occurs after thermoregulation is subordinated to circulatory and metabolic demands and to the associated systemic inflammatory reaction. Exertional heat stroke is a function of both intrinsic and extrinsic modulators. Intrinsic modulators like genetics, fitness, acclimatization, illness, medications, and sleep quality can alter individual risk and outcomes, while extrinsic modulators like exercise intensity and duration, clothing and equipment, ambient temperature, relative humidity, and solar radiation can affect the group risk and outcomes. This review integrates the current theoretical and accepted knowledge of physiological alterations into one model that depicts a common pathway from heat stress to heat stroke.

Acclimation to heat interpreted from the analysis of heart-rate variability by the Multipole Method.

The Multipole Method is a recently developed method to describe time series with highly complex time evolution, such as cardiac rhythm. We hypothesized that applying this method in the analysis of heart rate variability (HRV) could provide valuable physiological information that usually cannot be extracted from the commonly used time and frequency domains analyses. The hypothesis was tested during a 12-day heat acclimation process on four young healthy, male subjects. On the first and the last day of acclimation, HRV was investigated by two different measures: the conventional Standard Deviation of Normal to Normal R-R intervals (SDNN), and the Multipole Method. The analyses revealed that stress on the cardiovascular system was still evident after the 12th day of acclimation, but both methods showed that stress was diminished. Using the Multipole analysis, following acclimation to heat, the autonomic nervous system shows a more effective response. We conclude that the Multipole Method is a very valuable tool for investigating the series of R-R intervals, which are a non-stationary and non-linear complex time series, reflecting a complex regulatory physiological mechanism. Specifically, we showed that stress on the cardiovascular system diminishes following the course of acclimation, but not to its complete relief, which indicates that acclimation to heat is a much longer process than can be judged from the early phenotypic picture.

Physical exercise increases the expression of TNFalpha and GLUT 1 in muscle tissue of diabetes prone Psammomys obesus.

INTRODUCTION: Tumor necrosis factor-alpha (TNFalpha) is a major mediator of insulin resistance. On the other hand, it has been suggested that TNFalpha may facilitate glucose uptake through GLUT 1 expression. We recently found that physical exercise prevented the progression to type 2 diabetes mellitus in diabetes prone Psammomys obesus (sand rat). AIM: The aim of the present study was to characterize the influence of physical exercise on the expression of TNFalpha, its receptor R1 and GLUT 1 in muscle tissue of this animal model. METHODS: Animals were assigned for 4 weeks to four groups: high-energy diet (HC), high-energy diet and exercise (HE), low-energy diet (LC), low-energy diet and exercise (LE). TNFalpha, R1 and GLUT 1 expression were analyzed using Western blot technique. RESULTS: None of the animals in the HE group became diabetic while all the animals in the HC group became diabetic. TNFalpha, its receptor (R1) and GLUT 1 expressions were significantly higher in the two exercising groups (LE and HE) and significantly lower in the HC group compared to the control LC group. CONCLUSIONS: Physical exercise augments the expression of TNFalpha, its receptor R1 and the glucose transporter GLUT 1 in muscle tissue. We suggest that this mechanism may improve glucose uptake through pathways parallel and unrelated to insulin signaling that may include MAPK and/or NO. These biochemical processes contribute to the beneficial effects of physical exercise on the prevention of type 2 diabetes mellitus.

Abnormal thermoregulation in drug-free male schizophrenia patients.

Schizophrenia patients may develop various thermoregulatory disturbances. We hypothesized that a standardized exercise-heat tolerance test [two 50-min bouts of walking a motor-driven treadmill at 40 degrees C (relative humidity=40%)] would reveal abnormal thermoregulation in drug-free schizophrenia patients. Six drug-free schizophrenia outpatients and seven healthy comparison subjects participated in this study. The schizophrenia patients exhibited significantly higher baseline and exertion-related rectal temperature. The relevance of these findings to the pathophysiology of schizophrenia-related thermoregulatory disorders is as yet unclear.

The cumulative heat strain index--a novel approach to assess the physiological strain induced by exercise-heat stress.

The cumulative heat strain index (CHSI) is a new approach for assessing the total physiological strain experienced by subjects exposed to an exercise-heat stress. The index is based on inherent physiological logic that combines the thermoregulatory strain, which is described by the area under the hyperthermic curve, and the circulatory strain, which is characterized by heart-beat count. According to this model, the index reflects the dynamics of changes in the thermoregulatory and cardiovascular components and accounts for the complementary nature of the interaction between them. Mathematically, the index is calculated as follows: CHSI =[ sigma(0-t) hb-fc(0) x t] x l0(-3) x [ integral (0-t) Tre x dt-Tre(0) x t] (units) Where: hb=heart beats, fc(0)=initial lowest heart rate (bpm), Tre = rectal temperature (Tre(0) = baseline Tre) (degrees C) and t = time (min) from the onset of measurements. Four sets of data, from various former studies, have been used to demonstrate the index's applicability and its sensitivity to differentiate between levels of stain under various stressful conditions (e.g. clothing insulation, acclimation to heat and levels of tolerance to heat). In all cases, the index was found to be a sensitive tool for assessing the level of strain. Furthermore, the CHSI can be used to predict potential strain. The index's high sensitivity arises from its nature, which reflects miniature differences in the pattern of changes in the dynamics of physiological responses and therefore is a powerful and practical tool for evaluating even minor changes in strain.

Changes in heart rate variability following acclimation to heat.

We studied the sympatho-vagal balance during acclimation to heat in eight healthy individuals. The subjects, dressed in shorts and tennis shoes, underwent a 10 d procedure of acclimation. Daily exposure lasted 115 min--5 min rest followed by 2 bouts of 50 min exercise (walking on a treadmill at a work load of approximately 350 watt) separated by 10 min rest-at 40 degrees C and 40% relative humidity. We analyzed the time and frequency domains of 256 R-R intervals, toward the end of the second bout of exercise, on the first and tenth days of acclimation. Typical changes for heat acclimation (a reduction in rectal temperature and heart rate, an increase in sweat rate) were observed. Concomitant to a decrease in the final heart rate on the tenth day, sympathetic tone increased. We suggest that, except for the chronotropic response, changes in the sympatho-vagal balance may reflect several adaptive mechanisms of the cardiovascular system. Peripheral factors rather than intrinsic cardiac factors are of relatively greater importance in the direct control of cardiac function.

[Prediction of physiological response from mathematical models].

The ability to predict the physiological responses of workers exposed to extreme environmental conditions, has been a challenge to environmental physiologists for more than 3 decades. Therefore, mathematical models have been developed to predict metabolic rate under various levels of work intensity and dynamic changes in body temperature and heart rate. Based on the effect of exercise on the cardiovascular system, a model was developed to predict mean arterial blood pressure as a function of heart rate. Physiological strain could also be estimated on the basis of thermoregulatory and cardiovascular strains. This paper summarizes knowledge accumulated during 25 years of studies in the field of mathematical modeling of physiological parameters. Besides analyzing the logic underlying each model, it explains the scientific approach in developing a model from its early concept to the model's application in the field.

[Man in a hot climate--early studies of the Institute of Military Physiology].

In the 1950's many IDF soldiers were hospitalized for heat stroke--about 25% of whom died. Analyzing these cases revealed that commanders misinterpreted human ability to perform in the heat and ignored basic concepts of fluid and electrolyte balance and heat load. In the early 1960's a series of studies was conducted with regard to soldiers' performance in the heat. The first study (1959), which later became a classic, was conducted during a 21-day march from Eilat to Metula, crossing all climatic zones of Israel. The study was followed by other investigations which approached the issues of voluntary dehydration, fluid consumption vs sweat loss, salt additives, and the effect of heat load on performance. Based on these early studies, proper regulations were issued to field officers. Over the years, the lessons learned from these studies saved many lives. The number of cases of heat stroke and of other climate-related injuries was dramatically reduced, and performance was enhanced.

Heat intolerance in patients with chronic schizophrenia maintained with antipsychotic drugs.

OBJECTIVE: Schizophrenia may be associated with hyperthermic syndromes such as febrile catatonia, neuroleptic malignant syndrome, and heatstroke. The authors hypothesized that an exercise-heat tolerance test would disclose abnormal thermoregulation in schizophrenic patients. METHOD: Seven male schizophrenic outpatients in remission maintained on depot antipsychotic treatment and eight healthy comparison subjects completed a heat tolerance test that consisted of two 50-minute bouts of walking a motor-driven treadmill at 40xC (relative humidity=40%). RESULTS: A significantly higher rise in rectal and skin temperatures was observed in the patient group. No differences in heart rate, blood pressure, or perspiration were detected. CONCLUSIONS: Schizophrenic patients maintained on antipsychotic drugs exhibit impaired heat tolerance. Possible explanations are a reduced ability to convey heat from the body's core to the periphery with or without excessive heat production. The hyperthermic response to the heat tolerance test may reflect a dysfunction associated with schizophrenia, a neuroleptic-induced side effect, or both.

Exertional heat stroke--the prevention of a medical emergency.

In most cases, exertional heat stroke (EHS) can be prevented in the military setting. The actions taken by the Israel Defense Forces (IDF) and their outcome prove this well. Unfortunately, despite the available information, there are still incidents of failure of command in conducting physical exercise, leading to EHS. In our experience, most incidences are a consequence of disregarding safety regulations. The application of simple and reasonable measures will not only prevent accidents from happening, but will also result in better trained soldiers.

Dantrolene and recovery from heat stroke.

Several authors have shown that dantrolene may be effective in the treatment of heat stroke patients. However, the scant data available are still controversial. The aim of this investigation was to establish an animal experimental model for studying the efficacy of this drug both as a prophylactic agent and as a means of hastening the cooling process after heat stroke. Male albino rats were divided into five groups: Sedentary controls (SC), Sedentary+dantrolene (S+D), Exercise controls (EC), and Exercise+dantrolene (E+D, E+D1). The drug (140 mg x kg(-1) body weight) was administered i.v. either prior to subjection to heat stress (40 degrees C) (S+D, E+D) or upon development of heat stroke syndrome (E+D1). In the S+D group, dantrolene administered prior to heat stress (HS) delayed the development of heat stroke by 70%, although colonic temperature (Tc) at the onset of heat stroke was similar to that in group SC (43.0 +/- 0.1 degrees C and 43.2 +/- 0.4 degrees C for S+D and SC, respectively). In E+D animals, dantrolene shortened exercise endurance in the heat by 17.5%, but concomitantly hindered severe Tc rise (40.9 +/- 0.2 degrees C and 40.0 +/- 0.2 degrees C for EC and E+D, respectively). Administration of dantrolene on the development of heat stroke appeared to improve cooling in the exercise group (0.25 degrees C x min(-1) and 0.18 degrees C x min(-1), for the first 1 5 min of cooling, for E+D1 and EC, respectively). The results suggest that dantrolene is effective as a prophylactic agent in sedentary animals only. It also might have application on development of heat stroke. It is hypothesized that the observed rapid cooling is associated with dantrolene's effect on muscle contraction, thus leading to attenuated heat production and peripheral vascular relaxation.

Exertional heat stroke: a case series.

UNLABELLED: Exertional heat stroke (EHS) is a state of extreme hyperthermia that occurs when excess heat that is generated by muscular exercise exceeds the body's ability to dissipate it at the same rate. EHS is thought to coincide with previously healthy, highly motivated, and relatively untrained individuals exerting in hot environments for long periods. PURPOSE: To establish this notion, the present study was aimed to follow the trends in the incidence of EHS in the period 1988-1996. METHODS: During these years, 150 cases of male soldiers (age = 20+/-3 yr) were reported to our institute as suffering from heat illnesses. According to the files, 82 cases were definitely diagnosed as EHS. RESULTS: More than 50% of the cases occurred during the first 6 months in service. Most of the cases occurred during the summer season (June-September), but 30% of the cases occurred during the spring. EHS was not related to time in the day. Many cases occurred during the night or early morning, even under mild heat load. Forty percent of the cases occurred during very short activities, and about 60% occurred already during the first 2 h of exercise. The results were discussed in view of the regulations which prevail in the Israeli army. CONCLUSIONS: It seems that almost all EHS cases occurred when regulations were not strictly followed.

Fluid-electrolyte balance during labor and exercise: concepts and misconceptions.

Body water and electrolyte balance are essential to optimal physiological function and health. During exercise, work, or high temperatures, a significant level of dehydration can develop, and the ratio of extracellular to intracellular fluid can change, despite an ample supply of water. Physical and cognitive performance are impaired at 1-2% dehydration, and the body can collapse when water loss approaches 7%. Because fluid needs and intakes vary, formulating one general guideline for fluid replacement is difficult. Knowing the amount of water lost in sweat may enable predicting fluid needs via mathematical models for industrial, athletic, and military scenarios. Sodium imbalance might result from excessive Na+ loss or from gross overhydration. In most work or exercise lasting < 3-4 hr, the major concern is that fluid be available to prevent heat-related illnesses, which can be prevented if fluid and electrolyte losses are balanced with intake, using the recommendations presented.