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Biomedical subjects

A S Jackson

Publications and source records attributed to A S Jackson.

At least 37 records · Page 2Linked to original sources
PubMed1993

Validation of a 20-minute steady-state jog as an estimate of peak oxygen uptake in adolescents.

Distance run tests are often used to estimate peak oxygen uptake (peak VO2) in children. This study examined the concurrent validity of a 20-min steady-state jog (20MSSJ). The sample consisted of 43 boys and 32 girls who performed a 20MSSJ and completed a maximal treadmill test 1 week later. Peak VO2 was measured by indirect calorimetry. Subject characteristics were as follows: boys, mean age = 15.1 +/- 1.0 years, mean peak VO2 = 46.4 +/- 7.7 ml.kg-1 x min-1, mean 20MSSJ = 2.3 +/- 0.5 miles; girls, mean age = 15.3 +/- 1.1 years, mean peak VO2 = 40.4 +/- 5.1 ml.kg-1 x min-1, mean 20MSSJ = 2.1 +/- 0.5 miles. The correlation between 20MSSJ and peak VO2 was .68 across boys and girls. The regression slopes of the boys and girls were common, but the intercepts differed, F (1, 72) = 10.9, p < .05. The influence of weight and skinfold fat was examined. Separate models were developed, and the R for each was increased to .79 (SEE = 4.45 ml.kg-1 x min-1) for the model using weight and .80 (SEE = 4.36 ml.kg-1 x min-1) for the model using sum of skinfolds. The regression equations are as follows: peak VO2 = 35.10 + 6.19 (Run) + 2.96 (Gender) -0.26 (Skinfold fat) peak VO2 = 33.94 + 7.64 (Run) + 5.84 (Gender) -0.18 (Weight) (Run = 20MSSJ performance in miles; Gender; females = 0, males = 1; Skinfold fat = sum of two-site skinfolds in mm; Weight = body weight in kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1990

Prediction of functional aerobic capacity without exercise testing.

The purpose of this study was to develop functional aerobic capacity prediction models without using exercise tests (N-Ex) and to compare the accuracy with Astrand single-stage submaximal prediction methods. The data of 2,009 subjects (9.7% female) were randomly divided into validation (N = 1,543) and cross-validation (N = 466) samples. The validation sample was used to develop two N-Ex models to estimate VO2peak. Gender, age, body composition, and self-report activity were used to develop two N-Ex prediction models. One model estimated percent fat from skinfolds (N-Ex %fat) and the other used body mass index (N-Ex BMI) to represent body composition. The multiple correlations for the developed models were R = 0.81 (SE = 5.3 ml.kg-1.min-1) and R = 0.78 (SE = 5.6 ml.kg-1.min-1). This accuracy was confirmed when applied to the cross-validation sample. The N-Ex models were more accurate than what was obtained from VO2peak estimated from the Astrand prediction models. The SEs of the Astrand models ranged from 5.5-9.7 ml.kg-1.min-1. The N-Ex models were cross-validated on 59 men on hypertensive medication and 71 men who were found to have a positive exercise ECG. The SEs of the N-Ex models ranged from 4.6-5.4 ml.kg-1.min-1 with these subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1989

Enkephalin metabolism: effect of acute exercise stress and cardiovascular fitness.

Inhibition of enkephalin hydrolysis by catecholamines in vitro suggested that local and/or humoral factors released during exercise might facilitate opiate responses by reducing the rate of opiate peptide inactivation. Several measures of enkephalin hydrolysis were determined in blood samples obtained from subjects designated as trained (VO2max, 64.3 +/- 1.6 ml.min-1.kg-1) and un-trained (VO2max, 37.4 +/- ml.min-1.kg-1) both at rest and after maximal exercise stress tests. Enkephalin hydrolyzing activity assessed under optimal conditions was equally distributed between plasma and intact red cells; however, hydrolysis by red cells increased dramatically following osmotic release of red cell contents. There were no apparent differences in enzyme concentration or its distribution between cells and plasma when comparing trained and untrained subjects; P greater than 0.05. There was also no statistical effect of maximal exercise on these measures in either group. However, when the sequential disappearance of enkephalin added to whole blood in vitro was evaluated, blood from trained subjects degraded the enkephalin more slowly than blood from untrained subjects and had half-lives in vitro 30-50% longer both before and after the exercise test; P less than 0.05. Since enzyme concentrations between the groups were similar, the longer half-lives suggest that circulating factors were responsible for moderating the rate of enkephalin metabolism in vivo and that these factors were more concentrated in trained subjects. This would facilitate opiate responses in trained subjects and perhaps provide them with added tolerance for the effort associated with elite performance levels.

Adult↗
PubMed1989

Evaluation of the NASA/JSC Health Related Fitness Program.

This study evaluated the long-term effects of the NASA/Johnson Space Center Health Related Fitness Program (HRFP) which includes a 12-week educational component (EC) and quarterly fitness retests (RT). The groups studied were: Compliers (completed EC and greater than or equal to 75% of RT, N = 64); Non-compliers (completed EC but less than 75% of RT, N = 106); Drop-outs (disenrolled from EC, N = 36) and Controls (randomly selected from eligible program pool, N = 52). Pretest medical examination and maximum stress test data showed the groups did not differ on age, % fat, weight, blood lipids, and VO2max (p greater than 0.05). Multivariate analysis of pre- and posttest change data greater than or equal to 2 years from start showed group differences in blood lipids, body composition and VO2max. Results showed that changes in physical activity were related to program completion and periodic fitness reevaluations, and that these group-related changes were associated with changes in VO2max, percent body fat, body weight, and blood lipids.

Adult↗
PubMed1988

Reliability and validity of bioelectrical impedance in determining body composition.

This study was designed to examine the reliability and validity of the bioelectrical impedance method (BIA) of measuring body composition and compare its accuracy with the results obtained by standard anthropometric methods BIA, skinfold fat, and hydrostatically measured percent fat (% fat) were obtained on 44 women and 24 men. Each subject was tested four times by two testers on two different days. Generalizability theory was used to estimate reliability and measurement error that considered both day-to-day and intertester error. The BIA, skinfold fat, and hydrostatic methods were all found to be reliable (Rxx = 0.957-0.987) with standard errors ranging from 0.9 to 1.5% fat. An additional 26 men (n = 50) and 38 women (n = 82) were tested once and combined with the data used for the reliability analysis to cross-validate BIA estimates of % fat with hydrostatically determined % fat. The cross-validation correlations for the BIA determinations of % fat ranged from 0.71 to 0.76, which were significantly lower than that obtained with the sum of seven (sigma 7) skinfolds equations (rxy = 0.92 for men and 0.88 for women). The correlations between the weight-to-height ratio body mass index (BMI) and hydrostatically determined % fat were 0.75 and 0.74 for men and women, respectively. The standard errors of estimate for the two BIA models ranged from 4.6 to 6.4% fat compared with 2.6 and 3.6% fat for the sigma 7 equations. The BIA method for measuring body composition was comparable to the BMI method, with height and weight accounting for most of the variance in the BIA equation.

Adipose Tissue↗
PubMed1987

The effect of hand-held weights on the physiological responses to walking exercise.

To study the physiological responses to walking with hand-held weights (HWs), 12 untrained men completed three sub-maximal and two maximal treadmill tests. Heart rate, oxygen uptake, respiratory exchange ratio, ventilation, systolic blood pressure (BP), diastolic BP, rate pressure product, and rating of perceived exertion were significantly greater (P less than 0.01) when HWs were added to walking exercise performed at constant treadmill speed and grade. To evaluate whether the evaluated BP response was caused by HWs independent of exercise load, heart rate was held constant at 75% maximum heart rate reserve during the third submaximal test. Systolic BP (151.1 +/- 15.3 mm Hg vs 160.1 +/- 16.9 mm Hg) and rate pressure product (252.1 +/- 27.0 vs 237.3 +/- 25.1) were significantly greater (P less than 0.05) during exercise with HWs. Physiological responses to maximal exercise with and without 3 lb HWs were similar. The time to exhaustion, however, was reduced (P less than 0.01) with HWs. (719.3 +/- 98.1 s with HWs vs 784.4 +/- 118.9 s without HWs). These data indicate that 3-lb HWs can increase the metabolic cost of training (1 MET, 7 to 13 b X min-1) and may be useful in exercise prescription for individuals who do not want to run or are limited in the speed at which they can walk. Due to the exaggerated BP response, caution should be used when prescribing HW exercise for patients where increasing afterload may be a problem.

Adult↗
PubMed1986

Accuracy of measured and predicted residual lung volume on body density measurement.

The effects of measured and predicted residual lung volume on the accuracy of body density and percent fat (%Fat) were investigated. Adult fitness subjects (N = 46) had residual lung volume measured with the oxygen dilution method while those from an athlete sample (N = 134) utilized the nitrogen washout technique. Residual lung volume was also predicted with gender-specific regression equations using height and age and from 24% of vital capacity (%FVC). Residual lung volume alpha reliability for the average of four residual lung volume trials exceeded 0.90 (SEM less than = 161 ml) for the oxygen dilution method and 0.99 (SEM = 30 ml) for the average of two nitrogen washout measures. The standard errors for predicted residual lung volume were 579 and 355 ml, respectively, for the men and women in the adult sample and 288 ml for the trained athlete sample. Estimating residual lung volume from %FVC yielded a SEE of 318 ml for the trained athlete sample. Measured residual lung volume errors resulted in errors of 1.04%Fat, 0.87%Fat, and 0.21%Fat for the men, women, and trained athlete samples, respectively. In contrast, predicted residual lung volume measurement errors resulted in errors of 3.70%Fat, 2.85%Fat, and 1.98%Fat for the respective samples and 2.18%Fat when using %FVC with the trained athletes. Measured residual lung volume introduces little %Fat error while predicted residual lung volume introduces a substantial source of measurement error.

Adipose Tissue↗
PubMed1984

Comparison of exercise blood pressure measured by technician and an automated system.

We evaluated the automated system Blood Pressure Measuring System (BPMS) developed by NASA on 277 adult males who elected to have a treadmill test as part of their annual physical. The BPMS uses acoustic transduction with a computer-assisted ECG gating to detect nonsynchronous noise. The BPMS readings were compared to pressures simultaneously measured by trained technicians. For all stages of work, BPMS readings were higher for systolic and lower for diastolic than technician readings. At peak stages of work, BPMS systolic pressures were about 20 mmHg higher than technician readings. Within each 3-min workstage, BPMS readings were found to be more inconsistent than technician readings. The standard errors of measurement for BPMS were from two to three times higher than technician values. These data showed automated blood pressure readings were significantly different than technician values and subject to more random fluctuations. These findings demonstrate the need to view exercise blood pressure measured by automated systems with caution.

Blood Pressure↗
PubMed1984

Hemodynamic effects of oral smokeless tobacco in dogs and young adults.

Oral smokeless tobacco (snuff) is increasingly used among the young male population. To determine cardiovascular effects of an oral smokeless tobacco product, 10 anesthetized dogs were instrumented to measure blood pressure, heart rate, left ventricular end diastolic pressure, and circumflex coronary, renal, and femoral flows. After a 5-min baseline, a 2.5-g, approximately 1.2% nicotine bolus dose was placed in the buccal space, and measurements were made for 20 min. Significant increases were seen in heart rate, blood pressure, left ventricular pressure, left ventricular end diastolic pressure, and left ventricular dP/dt. Significant decreases in flow were noted in the coronary circumflex, renal, and femoral arteries. The flow reduction was thought to have been mediated by an alpha-adrenergic mechanism. Additionally, 20 human males, mean age 20 years, without nicotine exposure for 72 hr, were given a 2.5-g dose of the same oral smokeless product. From baseline to 20 min, heart rate increased from 69 to 88 beats/min (P less than 0.05), blood pressure from 118/72 to 126/78 mm Hg (P less than 0.05). Thus, oral smokeless tobacco use can produce significant hemodynamic changes in both dogs and normal humans.

Administration, Oral↗
PubMed1984

Research progress in validation of clinical methods of assessing body composition.

Anthropometry is the method of choice for estimating body composition in the clinical setting. The method can be accurate, and requires little time, space, equipment, or financial outlay. Although used extensively in epidemiological research, height/weight indices are not as accurate as skinfold and circumference measures for estimating body composition. The validity of estimating body density is enhanced by using a combination of skin-fold and circumference measures in a multiple-regression model. Some recently developed generalized equations may have a broader application for use in varied populations than several population-specific equations. The newer equations take into account the potential change in ratio of internal to external fat and bone density with age, and the nonlinear relationship between skinfold fat and body density. The validity of using skinfolds for estimating body density can be significantly affected by caliper selection and measurement procedures. Inter-observer errors appear to be the most problematic, with improper skinfold site selection causing the greatest variation among observers. To improve the validity of the anthropometric technique for use in the clinical setting, more precise standards and description of methods need to be developed.

Adult↗
PubMed1984

Research design and analysis of data procedures for predicting body density.

Statistical methods used to develop body composition prediction equations were examined. Equations published in the 1960s and early 1970s have been called "population-specific" equations because they were developed on homogeneous samples. A major limitation of population-specific equations was a low ratio of subjects per variable, which reduces validity. The more recent method has been to develop "generalized" equations with large samples varying greatly in age and body fatness. The statistical models used for generalized equations were nonlinear regression analysis to account for the quadratic relation between body density and skinfold fat. Age was used as an independent variable to adjust for aging. Generalized equations exhibit accuracy consistent with population-specific equations and increase the generalizability of prediction. Future body composition research should focus on the cross-validation of published equations to define the limits of equation accuracy.

Adult↗
PubMed1982

Steps toward the development of generalized equations for predicting body composition of adults.

The estimation of body density from anthropometric variables is often used to measure body composition. The purpose of this report was to review the development of early published equations and follow the transition from population specific equations to generalized equations for adults. The accuracy of population specific equations were improved and refined with the addition of circumference and diameter measures in combination with skinfolds and the use of populations of varied ages and degrees of body fatness. The major weakness of the population specific equations was their inability to account for aging and the non linear relationship between subcutaneous fat and body density. Regression models using the quadratic sum of skinfolds and age have been developed to account for these sources of variation. Cross-validation research has shown that these generalized regression models are accurate for adult males and females varying greatly in age and body fatness.

Adolescent↗
PubMed1981

Evaluation of the NASA/JSC blood pressure screening clinic.

This study evaluated the NASA/Johnson Space Center's blood pressure screening program, which was designed to identify the proportion of NASA employees who were hypertensive and to identify self-report variables related to elevated pressures. In 1978, a random sample of 3892 employees was selected from over 2,100 NASA employees. Twenty-eight percent were classified as hypertensive by the criterion of systolic pressure of 140 or above or diastolic pressure of 90 or greater. Approximately 16% of all employees had a history of hypertension; the remaining 12% of the identified hypertensives were unaware of their potential health problem. Multivariate analysis showed that elevated systolic pressures were more prevalent in the older, heavier employees. Of hypertensives under treatment, about 74% were under poor control as judged from blood pressures randomly taken at their place of employment. These results demonstrated the effectiveness of screening for unsuspecting hypertensives in industry.

Adult↗