Of the Shuttle crewmembers, 519 had available data from the annual examination closest to, and before, their missions. Their gender distribution shows that 86.4% are males. Almost half of these crewmembers were between 38 and 43 years of age, making this age group the largest subset. About 70% of the entire group was younger than 44 years of age. Means of the following variables were measured for the Shuttle crewmembers: age (at time of examination), body mass index (BMI), percentage of body fat, mean VO2max, and maximum heart rate (MHR). BMI and percentage of body fat provide measures of fatness for the group. BMI, which is more accurate for a group of people than for an individual, is calculated by dividing the weight (in kilograms) by the squared height (in meters). A BMI between 22-26 for both men and women is about standard. Percentage of body fat is an estimation of fatness based on the sum of three skin-fold measurements and age at last birthday. Values of 10%-15% for men, and 20%-25% for women, are considered to be the norm for the general population. VO2max and MHR are estimations of cardiovascular fitness that are measured during an Exercise Tolerance Test. VO2max is measured by the volume of oxygen, in milliliters per kilogram of body weight, expired per minute, and is an indicator of an individual's max aerobic capacity. A higher number indicates a higher aerobic capacity. The normative values of VO2max for men and women in good aerobic condition in the 30-49 years age group are 41 ml/kg/min and 37 ml/kg/min respectively. The MHR is measured in beats per minute, and lower values are more desirable. For individuals in the 38-49 years of age group, the expected MHR is 179.7 beats per minute.
Each EVA per mission is accounted for, so that an astronaut is counted for every EVA mission and the age group s/he has achieved. All 79 EVA participants had data available from examinations closest to and before their missions. The majority (70.9%) of the EVA participants were under the age of 44, and 91.1% of them were male. The small number of female participants precludes data analysis of this subset.
As expected, the crewmembers, those in the 30-43 year age group, had higher VO2max and lower MHR values than the previously mentioned norms.
All of the parameters varied only slightly between EVA participants and Shuttle crewmembers. The largest age group representation for both EVA participants and crewmembers is in the 38-43 year age group (48.1% and 47.2%, respectively). In comparing the majority of each group's subjects (38-43 year age group), there is very little difference in the BMI, average percentage of body fat, or mean MHR between the two age groups. When these values are compared in terms of aging trend, EVA participants, unlike Shuttle crewmembers, did not show the expected trend of increasing BMI, increasing percentage of body fat, and decreasing VO2max in progressing from the younger to older age groups. This may mean that EVA participants maintain better overall fitness within the older age group than crewmembers as a whole. However, the small age group subsets of this sample may also affect their mean values.
There is little difference in the age makeup between the Shuttle crewmembers and EVA participants. The majority of crewmembers and EVA participants also have very similar fitness levels, as shown in the small difference in the physiological variables values between the two age groups. The difference in aging trend between the two groups may indicate that EVA training facilities maintenance of aerobic fitness for the crewmembers. As the entire astronaut corps ages, and more EVA data accumulate from the maintenance and assembly of the ISS, a more comprehensive analysis can be performed to substantiate this observation.
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