Our data were collected as part of the Cerebrovascular Disease and its Consequences in American Indians (CDCAI) Study also known as the Strong Heart Stroke Study. The CDCAI Study was a cross-sectional study of cerebrovascular disease conducted with 1033 Strong Heart Study surviving participants who were aged ≥60 years , but 215 of the participants were removed from analyses due to one tribal community that subsequently withdrew study consent. We recruited participants from three study locations, including the Northern Plains, the Southern Plains, and the Southwestern United States. Participants were recruited for the study with trained field staff making the initial contact by telephone or during a planned home visit. The study’s purpose was explained by the field staff who would then invite the individual to participate in the study. If the individual was willing, field staff would screen them for study eligibility. Study exclusion criteria included prior surgery for a cerebral aneurysm; an implanted cardiac pacemaker, defibrillator, or artificial heart; contraindicating metal prostheses; a cochlear implant, spinal cord stimulator, or other implanted electrical device(s); history as a metal worker given the possibility of retained metal fragments; body weight of ≥350 pounds; and/or physical or cognitive inability to complete study procedures. Between 2010 and 2013, all of the participants received clinical, cognitive, and functioning assessments. Study procedures were approved by 13 organizations that included five Tribal Review Boards or Tribal Councils, five academic or medical institutional review boards (IRBs), and three regional Indian Health Service IRBs. The primary institution that provided institutional review board approval for this study was the University of Washington. As stipulated in the tribal approvals, we are unable to identify the other approving entities for this study in order to maintain the anonymity of the participating tribes. The design and recruitment methods for the CDCAI Study are described in more detail elsewhere .
The Short Physical Performance Battery (SPPB) measured lower body functioning using standing balance, gait speed, and chair stands . SPPB scores are associated with falls , disability [17, 19], nursing home admission , and mortality [17, 20]. The validity and reliability of the SPPB have been established in large, community-based, geographically and racially diverse samples of older adults [17, 21].
Standing balance was assessed by asking each participant to attempt three increasingly difficult positions without the use of assistance devices and to hold each position for 10 s. Participants first had their standing balance examined with feet side-by-side, then in a semi-tandem position with the heel of one foot beside the big toe of the other, and finally in a tandem position with the heel of one foot directly in front of the toes of the other. For the side-by-side position standing task, participants were scored as 1 if they held the position for 10 s and 0 if they did not attempt or were unable to hold the position for 10 s. For the semi-tandem position standing task, participants were scored as 1 if they held the position for 10 s and 0 if they did not attempt or were unable to hold the position for 10 s. For the tandem standing task, participants were scored as 2 if they held the position for 10 s, 1 if they held the position for 3–9.99 s, and 0 if they did not attempt or held the position for <3 s. The three standing balance positions were summed to generate a total balance score that ranged from 0 to 4.
For gait speed, participants were asked to walk a 15-ft straight course at their usual pace continuing beyond the end of the course if they felt they could do so safely. Participants were permitted to use a cane or other walking aid as needed. Scores for the 15-ft walk were adapted from the 3- and 4-m walks by using the same pace requirements and extrapolating time limits to account for the additional distance covered. Participants were scored as 0 if they were unable to perform the walk, 1 if they completed the walk in more than 9.94 s, 2 if they completed in ≤9.94 s, 3 if they completed in ≤7.09 s, and 4 if they completed in ≤5.52 s.
For chair stands, study site field staff determined whether each participant could safely stand up from sitting in a chair without assistance. Those who could were then asked to stand up from the chair five times as quickly as possible without using their arms. Participants were scored as 0 if they were unable to complete 5 chair stands or completed 5 chair stands in >60 s, 1 if they completed in 16.7–60.0 s, 2 if they completed in 13.7–16.69 s, 3 if they completed in 11.2–13.69 s, and 4 if they completed in <11.2 s.
All SPPB task scores range from 0 to 4 and the total SPPB score is the sum of all three task scores, which ranges from 0 to 12 where higher scores are reflect better performance. We examined the individual SPPB tasks and total score as continuous measures as well as examined the total SPPB score as a binary measure such that ≥10 denoted “good” performance and ≤9 denoted “poor” performance .
Independent variables included demographic characteristics, study site, anthropometrics, cognitive functioning, depressive symptomatology, grip strength, hypertension, diabetes mellitus, heart disease, prior stroke, smoking, alcohol use, and over-the-counter arthritis or pain medication use. Demographic characteristics included age, gender, marital status, educational attainment, and annual household income. Anthropometrics included body mass index (BMI) and waist-hip ratio, which were directly measured during CDCAI assessments. BMI was calculated as measured weight in kilograms divided by measured height in meters squared; waist circumference was measured at the umbilicus with the participant in a supine position; and hip circumference was measured at the widest portion of the buttocks with the participant standing.
Cognitive functioning was measured with the Modified Mini-Mental State (3MS) examination, which has possible scores ranging from 0 to 100 where higher scores are reflective of better cognitive functioning. The 3MS includes screening items on temporal and spatial orientation, immediate and delayed memory, attention and concentration, language and naming, verbal fluency, and executive functioning . Depressive symptomatology was measured with the Centers for Epidemiologic Studies Depression Scale (CES-D) , a 20-item instrument that describes the frequency of symptoms within the last week by using a 4-point scale ranging from 0 (rarely or none of the time) to 3 (most or all of the time). When scored, the CES-D ranges from 0 to 60, with higher scores indicating more depressive symptomatology. Scores were analyzed as a binary variable by using the standard cutoff score of ≥16 to reflect a clinically significant level of symptoms .
Grip strength was ascertained three times in kilograms for both hands by using a calibrated dynamometer. Measures of the participant’s dominant hand were averaged for use in our analyses. Hypertension was assessed with measured blood pressure (systolic ≥140 or diastolic ≥90) or self-reported use of antihypertensive medication. Diabetes mellitus was defined as fasting glucose of at least 126 mg/dL or self-reported use of insulin or an oral hypoglycemic. Heart disease was determined with a “yes” answer to the question, “Has a medical person ever told you that you have (or had): congestive heart failure, a heart attack, any other heart trouble, a bypass, a valvular repair or replacement, and/or a pacemaker installed?” Stroke was also determined with a “yes” answer to the question, “Has a medical person ever told you that you have had a stroke?”
Participants were asked about current tobacco smoking and alcohol consumption in the past 30 days, with “yes” or “no” response options. Lastly, over-the-counter medication use for arthritis or pain was determined with a “yes” answer to the question, “Do you take over-the-counter medicines for arthritis or pain, like Advil, Motrin, or Aleve?”
We used mean, standard deviation (SD), count, and percent to describe participant characteristics, including total SPPB scores and scores on the three individual SPPB tasks. To assess associations of demographic characteristics, anthropometrics, cognitive functioning, depressive symptoms, grip strength, clinical conditions, lifestyle behaviors, and over-the-counter medication use for arthritis or pain with both individual SPPB task and total SPPB scores, linear regression models were used with adjustment for age, gender, and study site. Our result estimates were reported using 95% confidence intervals (95% CI) for each independent factor individually. Estimates for continuous independent variables were given per clinically meaningful change in unit, which tended to be close to the estimate of standard deviation for the entire sample population. To assess associations between selected independent variables and the binary total SPPB score (good versus poor), generalized linear models with Poisson distribution and log-link were used, with inclusion of all independent factors in the same model. Continuous variables were transformed based on their standard deviations in order to facilitate direct comparison of degree of effect from each of the different independent variables. Exponentiated Poisson coefficients were interpreted as relative risk under assumption of reasonably rare outcome (25%). All of our analyses were performed with Stata 13 .