Effect of exercise, heat stress and dehydration on myocardial performance.

BACKGROUND: Myocardial dysfunction is a well-documented outcome of extended periods of high cardiac output. Whether similar effects occur during firefighting, an occupation characterized by repeated periods of work compounded by dehydration and heat stress, is uncertain. AIMS: To investigate the independent and combined effects of moderate heat stress and dehydration on indicators of myocardial performance following intermittent, submaximal treadmill exercise while wearing personal protective equipment (PPE). METHODS: Twelve aerobically fit young men (age 21.5±2.6 years; maximal oxygen uptake [VO2max] 60.3±4.4ml kg(-1) min(-1)) performed intermittent treadmill walking exercise consisting of three 20min bouts at an intensity of ~40% VO2max separated by two periods of rest in four different conditions in random order: (i) no heat stress-euhydrated, (ii) heat stress-euhydrated (heat stress created by wearing PPE, (iii) no heat stress-dehydrated and (iv) heat stress-dehydrated. We measured core temperature by a telemetric gastrointestinal pill. We determined cardiac variables by standard echocardiographic techniques immediately before and ~30min after exercise. RESULTS: We recorded no significant changes in markers of systolic (ejection fraction, shortening fraction, tissue Doppler-S) or diastolic (mitral peak E velocity, tissue Doppler-E' and E/E') function following exercise in any of the four conditions. CONCLUSIONS: In this model of exercise designed to mimic the work, heat stress and dehydration associated with firefighting activities, we observed no negative effects on myocardial inotropic or lusitropic function.

Skin microvascular reactivity in trained adolescents.

Whilst endothelial dysfunction is associated with a sedentary lifestyle, enhanced endothelial function has been documented in the skin of trained individuals. The purpose of this study was to investigate whether highly trained adolescent males possess enhanced skin microvascular endothelial function compared to their untrained peers. Seventeen highly and predominantly soccer trained boys (V(O)(2)(peak): 55 +/- 6 mL kg(-1) min(-1)) and nine age- and maturation-matched untrained controls (V(O)(2)(peak): 43 +/- 5 mL kg(-1) min(-1)) aged 13-15 years had skin microvascular endothelial function assessed using laser Doppler flowmetry. Baseline and maximal thermally stimulated skin blood flow (SkBF) responses were higher in forearms of trained subjects compared to untrained participants [baseline SkBF: 11 +/- 4 vs. 9 +/- 3 perfusion units (PU), p < 0.05; SkBF(max): 282 +/- 120 vs. 204 +/- 68 PU, p < 0.05]. Similarly, cutaneous vascular conductance (CVC) during local heating was superior in the forearm skin of trained versus untrained individuals (CVC(max): 3 +/- 1 vs. 2 +/- 1 PU mmHg(-1), p < 0.05). Peak hyperaemia following arterial occlusion and area under the reactive hyperaemia curve were also greater in forearm skin of the trained group (peak hyperaemia: 51 +/- 21 vs. 35 +/- 15 PU, p < 0.05; area under curve: 1596 +/- 739 vs. 962 +/- 796 PUs, p < 0.05). These results suggest that chronic exercise training in adolescents is associated with enhanced microvascular endothelial vasodilation in non-glabrous skin.

Exercise tolerance in a hot and humid climate in heat-acclimatized girls and women.

This study compared physiological responses associated with exercise tolerance in girls (G) and women (W) of similar fitness and heat acclimatization level during exercise in a hot and humid outdoor environment (33.4 degrees C and 55.1 % RH; WBGT = 29.9 +/- 0.2 degrees C). Nine pre-menarcheal G (age = 11.3 yr) and nine W (age = 26.8 yr), matched for aerobic capacity and heat acclimatization level, performed a cycling session at 60 % VO2max until fatigue. A sports drink was provided periodically to prevent dehydration. Tolerance time was not different between the groups (G = 56.9 +/- 6.3, W = 76.5 +/- 9.9 min, p > 0.05). During exercise, sweat rate (G = 9.1 +/- 1.1, W = 12.0 +/- 1.1 ml.m(-2).min(-1)), the increase in rectal temperature [T(re)] (G = 0.9 +/- 0.1, W = 1.1 +/- 0.1 degrees C), and heat storage (G = 10.6 +/- 5.3, W = 20.5 +/- 4.5 W.m(-2)) did not differ between the groups. At fatigue, T(re) (G = 38.2 +/- 0.1, W = 38.4 +/- 0.1 degrees C), heart rate (G = 167.3 +/- 7.3, W = 171 +/- 3.3 beats.min(-1)), stroke index (G = 48.3 +/- 1.5, W = 52.4 +/- 1.8 ml.m(-2)), and forearm skin blood flow (G = 9.5 +/- 1.3, W = 11.7 +/- 1.5 ml.100 ml(- 1).min(-1)) did not differ between the groups. Similar to women, the main reasons reported by girls to stop exercising in the heat were localized leg fatigue and gluteus muscle discomfort. We conclude that heat-acclimatized girls exhibit an adequate cardiovascular and thermoregulatory adjustment while exercising in a hot and humid outdoor environment when hypohydration is prevented.

Cardiac responses to progressive upright exercise in adult male cyclists.

AIM: Previous investigations have indicated that maximal stroke volume is the primary factor which differentiates physiological aerobic fitness (VO2max) in endurance athletes from nonathletes. Understanding the pattern of stroke volume response to progressive exercise may provide insights into the mechanisms which are responsible for this difference. METHODS: Doppler echocardiography was used to estimate stroke volume changes with maximal upright cycle exercise in 8 highly trained adult cyclists (mean age 30.5+/-1.6 years) and 16 age-matched nontrained males. Ventricular dimensions were measured during exercise using 2-dimensional echocardiography (parastemal long axis view). Findings were compared to a group of untrained adult male subjects. RESULTS: Mean maximal oxygen uptake was 73.7+/-7.0 and 47.4+/-7.5 ml x kg(-1) x min(-1) in the 2 groups, respectively. Stroke volume rose in both the cyclists and nontrained subjects at the onset of exercise but then plateaued without significant change to exhaustion. Values for stroke index were significantly greater at rest and all levels of exercise in the cyclists (maximal 85+/-13 versus 61+/-13 ml x m(-2)). Two dimensional echocardiograms during exercise in the cyclists revealed a small initial rise in left ventricular end-diastolic dimension and then a small decline, while systolic dimension decreased progressively, resulting in a rise in shortening fraction. CONCLUSION: These findings are consistent with the concept that determinants of stroke volume at rest are most influential in defining differences in maximal stroke volume between athletes and nonathletes.

The circulatory response to exercise: role of the peripheral pump.

The circulation of blood during upright exercise involves a two-pump system, a central cardiac pump and a peripheral pump responsible for systemic venous return. Axiomatically, the function of the two pumps must be equal, and certain evidence suggests that the peripheral pump may "drive" the circulation during exercise. Despite its potential importance to circulatory reserve and aerobic fitness, little is known regarding the determinants of the peripheral pump. Pumping function of skeletal muscle and the suction effect of the left ventricle presumably are principal factors in defining systemic venous return. This review, which focuses on data in humans, examines current information regarding the peripheral pump and its potential role as a critical determinant of maximal cardiac output, maximal oxygen uptake, and endurance fitness.

Organized sports for children and preadolescents.

Participation in organized sports provides an opportunity for young people to increase their physical activity and develop physical and social skills. However, when the demands and expectations of organized sports exceed the maturation and readiness of the participant, the positive aspects of participation can be negated. The nature of parental or adult involvement can also influence the degree to which participation in organized sports is a positive experience for preadolescents. This updates a previous policy statement on athletics for preadolescents and incorporates guidelines for sports participation for preschool children. Recommendations are offered on how pediatricians can help determine a child's readiness to participate, how risks can be minimized, and how child-oriented goals can be maximized.

Strength training by children and adolescents.

Pediatricians are often asked to give advice on the safety and efficacy of strength training programs for children and adolescents. This review, a revision of a previous American Academy of Pediatrics policy statement, defines relevant terminology and provides current information on risks and benefits of strength training for children and adolescents.

The role of physical activity and fitness in children in the prevention of adult cardiovascular disease.

Research data in adults have provided convincing evidence that regular exercise creates a protective effect against the complications of coronary artery disease. Given that the atherosclerotic process begins during the pediatric years, improved physical activity and fitness has been promoted in children and adolescents as a means of preventing cardiovascular disease in adulthood. Although intuitively compelling, direct proof of this rationale for improving exercise habits of children is not available. Scientific data examining the effects of activity and fitness on coronary artery disease risk factors in the growing years do provide some evidence that promoting exercise in children might serve as a means of preventing future cardiovascular disease.

Rowing ergometry for assessment of aerobic fitness in children.

Rowing ergometry has not been utilized as a modality for investigating aerobic fitness in children. This study evaluated the feasibility and reproducibility of measuring peak VO2 during progressive rowing on a Concept II ergometer in 16 healthy boys aged 9-12 years. All subjects were able to row effectively after minimal modeling and verbal instruction. Mean peak VO2 values on two trials were 42.9 (+/-7.9) and 41.6 (+/-8.7) ml x kg(-1) x min(-1) (p>0.05). No significant differences were observed in mean peak heart rate (190+/-11 vs. 191+/-12 bpm), peak ventilation (58.6+/-11.0 vs. 59.1+/-13.9 L x min(-1)), and RER (1.02+/-.07 vs. 1.02+/-.05) on the two trials. The coefficient of variation of peak VO2 was 5.4%. These data suggest that rowing ergometry is an effective method of measuring aerobic fitness in preadolescent boys.

Test-retest reproducibility of maximum cardiac output by Doppler echocardiography.

Test-retest reproducibility of stroke volume and cardiac output using Doppler echocardiography was examined during maximum cycle exercise in 13 young men. A coefficient of variation of 8.5% and 8.1% and intraclass correlation coefficient of 0.90 and 0.91 for maximum stroke volume and cardiac output, respectively, indicate a high degree of reproducibility using this technique.

The biological basis of physical activity.

Successful efforts to improve levels of physical activity in the population are contingent upon an accurate understanding of the determinants of habitual activity. While most research has focused on psychosocial and environmental influences, the potential effect of intrinsic biological control on regular activity has received little attention. This review examines evidence for the existence of such central control, offers a rationale for its function, and suggests implications for preventive health strategies resulting from a biological contribution to habitual activity levels.

Development of ventilatory responses to exercise in normal white children. A longitudinal study.

Cross-sectional studies have indicated that the pattern of ventilatory responses to exercise evolves during the course of childhood. This 5-year study was designed to provide a longitudinal assessment of minute ventilation (VE), tidal volume (VT), and breathing frequency (fR) in 20 children (11 girls, nine boys) between the ages of 9 and 13 years. Subjects performed maximal and identical submaximal steady-state treadmill walking tests annually. No significant gender differences were observed in any of the three variables. At submaximal exercise, VT per kilogram remained stable, with a progressive fall in fR. As a result, submaximal VE per kilogram declined with age. A similar pattern was observed at maximal exercise, but the decrease in VE per kilogram was not statistically significant. Ventilatory equivalent for oxygen (VE/VO2) fell with age at submaximal exercise but declined only in the boys with maximal testing. VE/VO2 at maximal and submaximal exercise was greater in the girls at all ages. These findings support previous data derived from cross-sectional studies.

The influence of short-term aerobic training on blood lipids in healthy 10-12 year old children.

This study was designed to examine the ability an endurance exercise training program to alter blood levels of cholesterol, HDL-cholesterol, LDL-cholesterol, and triglycerides in children. Thirty-one sixth grade students age 10-12 years (20 girls, 11 boys) who were healthy and active volunteered for participation. The training program consisted of 13 weeks of aerobic activities three days a week, 25 minutes per session, with training intensity assessed by heart rate monitors. Serum lipids and maximal oxygen uptake (VO2max) were measured at the beginning of a control period, 13 weeks later at the beginning of the training program, and at the termination of 13 weeks of training. VO2max values for the group improved 5.4%, but no significant changes were observed in any of the blood lipid levels between the control and training periods. These findings suggest that aerobic training of 13-weeks duration is not an effective means of altering blood lipids in healthy normolipemic children.

Submaximal running economy in run-trained pre-pubertal boys.

There is an increasing tendency for young children to participate in training and competitive running. The impact long-term training has upon stimulating functional physiological adaptation has yet to be fully understood. In this study cardio-respiratory and kinematic differences were assessed at submaximal and maximal exercise intensities in run-trained and non-run-trained boys. Thirty three pre-pubertal boys volunteered to take part in the study. The subjects were in two groups: 15 run-trained subjects [age 11.7 +/- 1.06 yrs, mean +/- SD] and 18 non-run-trained (control) subjects [age 11.3 +/- 0.90 yrs]. Two separate (4 x 3 min) submaximal protocols were used for the trained and non-run-trained groups, with two of the speeds overlapping for comparison purposes. In addition, all boys also performed a maximal oxygen consumption test. Mean VO2max value for the run trained group was 60.5 +/- 3.3 ml/kg/min and for the control group 51.1 +/- 4.3 ml/kg/min, (p < 0.001). No significant differences were found for submaximal running economy at either comparison speed. In addition, no significant (p > 0.05) differences were noted between the groups for any of the kinematic variables at the two comparison speeds. However, selected physiological differences did exist at the submaximal running speeds. The source of the differences that did exist between the two groups may be the result of training, genetic pre-selection or developmental differences between the groups.

Plasma norepinephrine responses to cycle exercise in boys and men.

Previous reports have suggested that plasma norepinephrine levels during exercise, an indicator of sympathetic neurologic activity, may be less in children than in adults. This study investigated plasma norepinephrine values at rest, during two submaximal cycle exercise intensities, and at maximal exercise in 11 boys aged 10 to 12 years and 11 men aged 24 to 35 years. Blood specimens were drawn at average submaximal exercise intensities of 58.7% and 73.0% for the boys and 55.3% and 73.3% VO2max for the men. Weight-relative maximal aerobic power was similar in the two groups. No statistically-significant differences were observed in plasma norepinephrine values at rest or during submaximal and maximal exercise between the boys and men. Maximal levels were 1196 (326 SD) and 1385 (612 SD) pg.ml-1 for the boys and men, respectively (p > 0.05). These findings suggest that sympathetic influences during maximal and submaximal exercise are independent of biological maturation.

Physiologic correlates to running performance in pre-pubertal distance runners.

In adults, four major variables have been shown to be associated with success in distance running performance: submaximal oxygen consumption (running economy), peak oxygen consumption (Peak VO2), ventilatory threshold (VT) and fractional utilisation (FU). The primary aim of this study was to describe the relationship between the 3000 m race times of run-trained prepubertal boys to these four variables. Thirteen male run-trained pre-pubertal boys (age 11.7 +/- 1.1 yrs, mean +/- SD), volunteered to take part in a 3000 m time trial and laboratory assessment, consisting of treadmill running at four submaximal speeds (8, 9.6, 11.2 and 12.8 km.h-1) as well as a peak VO2 test. The group demonstrated a heterogeneous array of peak VO2 data. A high level of association (p < 0.05) was found between mass-relative peak VO2 and 3000 m time trial results (r = -0.83). In addition ventilatory threshold expressed as %peak VO2, VO2 at VT and estimated velocity at VT was also highly related to 3000 m time trial (r = -0.78, -0.77 and -0.77) respectively. Fractional utilisation (%peak VO2) was significantly (p < 0.05) associated with race time at the final two submaximal running speeds only (11.2 and 12.8 km.h-1) (r = 0.61 and 0.67, respectively). Respiratory Exchange Ratio (RER) was also found to be significantly (p < 0.05) associated with 3000 m race time at 11.2 and 12.8 km.h-1. Overall peak VO2 appeared to be the single most important factor associated with success at 3000 m.

Aerobic response to endurance exercise training in children.

OBJECTIVE: To investigate the ability of children to improve aerobic fitness, as indicated by changes in maximaal oxygen uptake (VO2max), after a 12-week period of endurance training. DESIGN: Longitudinal prospective training with control period. SETTING: Middle school physical education class. SUBJECTS: Twenty-four girls and 13 boys, ages 10.9 to 12.8 years. INTERVENTION: Three 30-minute sessions of aerobic activity weekly for 12 weeks at an intensity-producing a mean heart rate of 166 beats per minute. Maximal treadmill testing was performed 12 weeks before the training program and again at the start and end because each child served as his/her own control. RESULTS: Mean VO2max did not change significantly during the control period but rose from 44.7 (5.8) to 47.6 (6.4) mL kg-1min-1 (6.5%) with training (P < .05). No differences in training response were observed relative to sex, pretraining VO2max, or sports participation. CONCLUSION: These findings support the concept that VO2max can be improved with endurance training during the childhood years, but the degree of aerobic train-ability is limited in healthy, active children.

Clinical manifestations of the 'athlete's heart' in prepubertal male runners.

Cardiac findings in adult endurance athletes are well characterized, but data regarding the "athlete's heart" in children are limited. This study evaluated cardiovascular features of 10 male prepubertal distance runners ages 11-13 years compared to 18 physically active but untrained boys. Mean VO2max values on treadmill testing for the two groups were 61.2 (3.2) and 51.1 (4.3) ml.kg-1.min-1, respectively. No significant differences in the frequency of carotid bruits, cervical venous hums, heart murmurs, or third and fourth heart sounds were observed between the groups. Mean resting heart rate was 71 (9) bpm for the runners and 73 (8) for the controls (p > 0.05). No significant differences were seen in EKG intervals, axes, or precordial voltages between runners and controls, and echocardiographic chamber sizes, wall thicknesses, and mass indexed to body surface area were also similar (p > 0.05). This study failed to identify clinical features of the "athlete's heart" in competitive child endurance runners compared to non-trained subjects.

Effect of prolonged inactivity on aerobic fitness of children.

Prolonged bed rest diminishes maximal aerobic power in adults, but no information is available regarding the physiologic responses to extended periods of inactivity in children. This study evaluated the impact of 9 weeks of bed rest on recovery peak VO2 in 5 children ages 7-11 years (3 boys, 2 girls) treated for femoral fractures. Beginning 2 weeks after initial ambulation during convalescence, maximal treadmill testing with measurement of gas exchange parameters was performed monthly for 4 months and again at 6 and 9 months. The difference between peak VO2 on the initial test and subsequent plateauing of peak VO2 on subsequent serial tests was considered indicative of alterations resulting from prolonged bed rest. Peak VO2 increased 13.3% to a plateau at 3 months while maximal aerobic power did not change significantly in five healthy control subjects. Between tests 1 and 3 treadmill endurance time improved 98% in the subjects and 1% in controls. These findings suggest (1) habitual activity in children accounts for approximately 13% of aerobic fitness levels, (2) following prolonged inactivity, aerobic fitness is regained by 3 months, and (3) improvements in performance far exceed those accounted for by changes in peak VO2 following extended bed rest in children.