Participants were community-dwelling adults recruited from an ongoing study of cardiovascular fitness and brain structure and function. Recruitment took place via local media outlets, including television and print media advertisements. In order to participate in the present study, individuals had to meet the following inclusion criteria: aged 60 to 80 years, have no contraindications to submaximal or maximal exercise testing, and have no medical conditions exacerbated by physical activity participation. Following initial contact by telephone, participants completed a pre-screening interview to determine whether they met inclusion criteria and consented to have their physician contacted for approval to participate in exercise testing. Participants were excluded from participation if they did not meet the above criteria or their physician refused to provide approval for testing participation.
A total of 294 individuals were initially screened to participate in a study of aging and cognitive function. A total of 106 participants were excluded from the original sample due to the presence of medical conditions that may have been exacerbated by physical activity (e.g., knee/hip injuries, serious cardiovascular conditions) or circumstances that interfered with cognitive testing by magnetic resonance imaging (e.g., claustrophobia, metal objects in body). Of those who met the initial inclusion criteria, 16 did not complete the GXT (5 no longer interested, 3 had family problems, 7 could not get approval from their physicians, and 1 could not complete the GXT due to high blood pressure). Thus, 172 individuals completed the GXT and all measures necessary for the equation-predicted CRF. Nineteen of these participants did not attend or finish the Rockport test for a variety of reasons related to scheduling difficulties, inability to complete the test, or injuries unrelated to the GXT.
A brief questionnaire assessed basic demographic information including sex, age, education, income, marital status, and occupational status.
Maximal Graded Exercise Testing (GXT)
A physician-supervised GXT utilizing a modified Balke protocol [18, 19] was used to assess peak oxygen consumption and obtain an objective measure of CRF. Participants walked at a self-selected brisk pace on a treadmill. The incline was increased every two minutes until the participant terminated the test volitionally or the physician stopped the test due to medical concerns. Expired gases were continually sampled and averaged over 30-second intervals throughout the test. A participant's VO2max was the highest value achieved when at least two of the following three criteria were met: a) a plateau of VO2 values, defined as an increase of <1.0 ml/kg despite an increase in power output, b) achieved age predicted maximal heart rate (220-age), and c) respiratory exchange ratio greater than 1.1. The highest MET value recorded by the metabolic measurement system during the GXT was used in subsequent analyses.
Submaximal Exercise Testing
The Rockport 1-mile walk protocol  was used as a submaximal estimate of CRF. This field test was conducted by trained staff with an ACLS certified nurse in attendance. Participants walked in groups on an enclosed, synthetic track, and were instructed to complete the 1-mile walk as quickly as possible without running. Cardiorespiratory fitness was estimated using the following standard Rockport 1-mile walk equations: a) Estimated VO2 (female) = 154.899 - (0.0947*2.2046*weight) - (0.3709*age) - (3.9744*walk time) - (0.1847*exercise heart rate); b) Estimated VO2 (male) = 116.579 - (0.0585*2.2046*weight) - (0.3885*age) - (2.7961*walk time) - (0.1109*exercise heart rate). All values were converted to METs for subsequent analyses.
Predicted Cardiorespiratory Fitness
Predicted CRF was calculated utilizing the original validation regression equation proposed by Jurca and colleagues: Estimated MET Value = Sex (2.77) - Age (0.10) - Body Mass Index (0.17) - Resting Heart Rate (0.03) + Self-reported Physical Activity + 18.07.
Self-Reported Physical Activity Index (SRPA)
SRPA was determined from a single exercise history question, as recommended by Jurca et al. . Participants were asked to choose one of five activity categories that best described their usual pattern of daily physical activity, including activities related to home and family care, transportation, occupation, exercise and wellness, and leisure or recreation from the following: a) Level 1: inactive or little activity other than usual daily activities (Value = 0); b) Level 2: Regularly (≥ 5 d/wk) participate in physical activities requiring low levels of exertion that result in slight increases in breathing and heart rate for at least 10 minutes at a time (Value = 1); c) Level 3: Participate in aerobic exercises such as brisk walking, jogging or running, cycling, swimming, or vigorous sports at a comfortable pace or other activities requiring similar levels of exertion for 20 to 60 minutes per week (Value = 2); d) Level 4: Participate in aerobic exercises such as brisk walking, jogging or running at a comfortable pace, or other activities requiring similar levels of exertion for 1 to 3 hours per week (Value = 3); or e) Level 5: Participate in aerobic exercises such as brisk walking, jogging or running at a comfortable pace, or other activities requiring similar levels of exertion for over 3 hours per week (Value = 4).
Height and Weight
Height and weight were measured utilizing a Seca electronic scale and stadiometer (Model 763 1321139) at the GXT appointment prior to the start of the test. Participants were measured wearing light clothing and no shoes. BMI was calculated using the standard formula of weight (kg)/[height (m)]2.
Resting Heart Rate
RHR was collected at the GXT appointment prior to the start of the test utilizing a supine 12-lead EKG tracing. Participants rested quietly for approximately ten minutes before RHR was measured. Heart rate was recorded for ten seconds and the value utilized was the average of all R-R intervals within this duration.
A personal medical history was used to assess cardiovascular risk factors. Participants were asked to indicate whether or not they had experienced any of 13 conditions associated with an increased risk for cardiovascular disease (e.g., high blood pressure, shortness of breath, pain associated with poor circulation) by answering "yes" or "no". Yes responses were coded as a "1" and no responses were coded as "0". A cardiovascular conditions score was developed by summing all 13 items.
All procedures were approved by a university Institutional Review Board. An initial telephone screening call established criteria for study entry and collected medical history information. After receiving medical clearance from their personal physicians, participants signed an informed consent document and completed the demographic questionnaire at an orientation session prior to any testing. At the first testing session, all participants completed a physician-supervised GXT. Participants completed the Rockport 1-mile walk test at a second appointment, scheduled within 3 to 4 weeks of the GXT. RHR, BMI, and SRPA were determined at the GXT appointment.
Testing the construct validity of the CRF equation was conducted in several stages. First, we examined the descriptive characteristics of the sample, including their SRPA levels, in relation to the original validation samples. We conducted a correlation analysis in which we examined the associations between the equation-estimated CRF MET value and each component of the prediction equation. Next, we conducted a hierarchical regression analysis to determine the independent contribution of each of the CRF model components to overall MET obtained during the GXT. Subsequently, we examined the correlations between the MET values attained from the CRF equation and MET values from the GXT and sub-maximal exercise testing (i.e., Rockport test). We also conducted a repeated measures ANOVA to compare the mean MET levels obtained from the CRF equation, the GXT, and the Rockport test. In addition, we further examined the degree of agreement between the equation method and the GXT method by constructing a Bland-Altman plot . Finally, we correlated the MET values from each of the three methods with the number of self-reported cardiovascular conditions.