Abstract
Objective To investigate the association of stair climbing difficulty and stair climbing frequency with the risk of all-cause mortality over 13 years in adults with or at high risk for knee OA.
Methods We used data from the Osteoarthritis Initiative (OAI), a prospective cohort study of community-dwelling adults with or at high risk for symptomatic knee OA. The exposures were stair climbing difficulty and frequency, assessed at baseline using self-report questionnaires. The outcome was all-cause mortality, assessed from baseline through 13 years of follow-up. Kaplan-Meier survival curves and Cox proportional hazards regression were used to investigate the association between stair climbing exposures and all-cause mortality.
Results Three hundred seven (6.81%) and 310 (6.84%) participants in the difficulty and frequency samples, respectively, died during 13 years of follow-up. Those who were limited in any capacity in terms of their stair climbing ability had 54% to 84% greater hazard of all-cause mortality, and those who climbed at least 7 flights of stairs per week had 38% lower hazard of all-cause mortality.
Conclusion Adults with or at high risk for knee OA who report difficulty with climbing stairs or who infrequently use stairs are at greater hazard of all-cause mortality. Stair climbing difficulty and frequency are simple to collect and changes may occur early in OA progression, allowing for early intervention. Brief questions about stair climbing behaviors can serve as a functional vital sign within the clinician’s toolbox.
Knee osteoarthritis (OA) is a leading cause of functional limitation1 (eg, difficulty with climbing stairs) and is the 11th most burdensome disease globally.2 There is a major public health need to effectively manage this chronic disease, given the high burden—more than 350 million adults worldwide3 and over one-third of adults over the age of 60 in the US have arthritis—and the fact that there is no cure.4 The downstream consequences of knee OA that is not well-managed include cardiovascular events and premature death.5-8 This is of particular concern, given the high prevalence of physical inactivity9 and overweight/obesity10 in this population, both of which are also strong risk factors for these consequences.
One important clinical skill that helps with chronic disease management is the ability to identify subsets of patients who are at highest risk of poor health outcomes from those with the least risk. Previous studies have shown a slow walking speed11 and difficulty with walking7 to be powerful predictors of all-cause mortality in older adults generally and in those with knee OA. However, walking speed is challenging to assess in some clinical environments due to constraints with space and the time needed to perform the walk test. A promising alternative to measured walking speed is patient-reported stair climbing. Changes in stair climbing, driven by pain, are often one of the first reported limitations for those with knee OA.12 Stair climbing, like walking speed,13-15 challenges several body systems, such as the cardiovascular and muscular systems. As such, greater difficulty with stair climbing or less frequent stair climbing may serve as early functional vital signs for possible decline in these systems and the future onset of poor health in patients with knee OA. Within the general population, a limited number of studies have shown limitations in stair climbing to be related to increased risk of all-cause mortality.16,17 However, little is known about this relationship in knee OA. Addressing this gap would allow for the risk-stratification of adults with knee OA into those who need advanced care, such as rehabilitation, knee injections, and joint replacement.
The purpose of this study was to investigate the associations of self-reported stair climbing difficulty and stair climbing frequency with the risk of all-cause mortality over 13 years in adults with or at high risk for knee OA. We hypothesized that those reporting greater stair climbing difficulty and less frequent stair climbing would have greater hazard of all-cause mortality compared to those without difficulty and/or with higher frequency.
METHODS
Study participants. We used data from the Osteoarthritis Initiative (OAI), a publicly available dataset from a longitudinal, observational cohort study of 4796 community-dwelling adults with or at high risk for symptomatic knee OA (SxOA), aged 45 to 79 years old, conducted at 4 different clinical sites (Baltimore, Maryland; Columbus, Ohio; Pawtucket, Rhode Island; and Pittsburgh, Pennysylvania). Participants were considered to have SxOA if they had both (1) frequent knee symptoms, defined as “pain, aching, or stiffness in or around the knee on most days for at least 1 month during the past 12 months,” and (2) radiographic evidence of knee OA (ROA) at the tibiofemoral joint, defined as tibiofemoral osteophytes of an Osteoarthritis Research Society International (OARSI) atlas grades 1-3 (referred to as the progression cohort).18 Participants were considered at high risk for SxOA if they did not have SxOA but had characteristics that put them at increased risk of developing SxOA at baseline (referred to as the incidence cohort). These characteristics included (1) knee symptoms in the past 12 months; (2) being overweight; (3) history of knee injury or surgery; (4) family history of knee OA, defined as a total knee replacement in a parent or sibling; (5) Heberden nodes; and (6) repetitive knee bending.18 Eligibility based on these characteristics was determined by age-specific criteria and is detailed in the full OAI protocol.18 Additionally, there was a small control cohort of participants who did not meet either set of criteria for the progression or incidence cohorts. For this study, we used the following datasets (version number): allclinical00 (0.2.3), allclinical01 (v1.2.2), allclinical02 (v2.2.2), allclinical03 (v3.2.1), allclinical04 (v4.2.1), allclinical05 (v5.2.1), allclinical06 (v6.2.1), allclinical07 (v7.2.1), allclinical08 (v8.2.2), allclinical09 (v9.2.1), allclinical10 (v10.2.2), allclinical11 (v11.2.1), kxr_sq_bu00 (v0.8), measinventory (v11), and outcomes99 (v10). Data are available at: https://nda.nih.gov/oai.
The OAI was funded by the National Institutes of Health. Each site received institutional review board (IRB) approval, and participants completed written informed consent prior to participation. This study was submitted to the University of Delaware’s IRB and determined to be exempt (IRB #1125357).
Study exposures. The exposures of interest were stair climbing difficulty and frequency, which were assessed at the baseline enrollment visit using patient-reported questionnaires.
Stair climbing difficulty was assessed with the question, “Does your health now limit you in climbing several flights of stairs?” which was taken from the 12-item Short Form Health Survey (SF-12). The SF-12 is a patient-reported questionnaire that measures general physical and mental health and is a reliable and valid measure of overall health status.19 Answer choices included “yes, limited a lot,” “yes, limited a little,” and “no, not limited at all.” The answer, “no, not limited at all,” served as the reference group.
Stair climbing frequency was assessed with the question, “During the past 7 days, how many flights of stairs have you climbed up?” Answer choices included “less than one flight,” “1 to 2 flights,” “3 to 4 flights,” “5 to 6 flights,” and “more than 6 flights.” We collapsed answer choices to create 3 groups: “0 to 2 flights/week,” “3 to 6 flights/week,” and “7 or more flights/week.” As it is unknown how many flights of stairs per week are considered beneficial, we used the lowest frequency exposure group (0-2 flights/wk) as the reference group.
Study outcome. The outcome of interest was all-cause mortality. All-cause mortality refers to death from any cause and was assessed from baseline through 13 years of follow-up. Time to death was measured in days and was calculated as the difference between a participant’s enrollment date and their death date. Death dates were able to be adjudicated using a death certificate or obituary in approximately 93% of deaths.
Potential confounders. Potential confounders were assessed based on their potential association with stair climbing and all-cause mortality and included age20-23 (continuous), gender5,24-26 (male vs female), BMI20,27-29 (calculated as weight in kilograms divided by height in meters squared; continuous), race30-32 (White vs non-White), presence of ROA7,20,33,34 (defined as Kellgren-Lawrence grade ≥ 2 in at least 1 knee; present vs absent), numerical rating scale pain20,22,35-38 over the last 30 days (0-10, knee with worst pain score was used; continuous), and presence of comorbidity5,39,40 (measured using the modified Charlson Comorbidity Index41,42; ≥ 1 vs none).
Statistical analysis. The analytic sample was restricted to those who answered the stair climbing difficulty and frequency questions at baseline. Given that it is possible that participants may not have answered both stair climbing behavior questions, there were separate analytic samples for each stair climbing question. We first described each analytic sample using standard sample characteristics, where continuous metrics were reported as means and SDs and categorical metrics were reported as proportions and percentages.
To investigate the relationship between stair climbing and all-cause mortality, we produced Kaplan-Meier survival curves and used Cox proportional hazards regression. Prior to performing the Cox regression analysis, we ensured the survival curves had hazard functions that were proportional over time by testing the proportional hazards assumption and we assessed whether the log hazards and each covariate were linear using the plotted residuals. We censored participants when they either (1) died (had the event), (2) were lost to follow-up at a later study visit (did not have the event), or (3) reached the end of follow-up without dying (did not have the event). Person-time was measured in days and calculated as the difference between the enrollment date and censor date. For participants who completed the final follow-up visit, their censor date was recorded as the last date that death dates were obtained. For the Kaplan-Meier survival curves, cumulative incidence of all-cause mortality for each stair climbing exposure was plotted, and a log-rank test was performed to test for a difference in survival between stair climbing exposure groups. For the Cox proportional hazards regression, we calculated hazard ratios (HRs) and 95% CIs, adjusted for potential confounders. CIs that did not include the null value were interpreted as indicating a statistically significant difference in the absolute hazard between the compared exposure groups.
To determine if the association of stair climbing with all-cause mortality differed within specific subgroups, we repeated our analyses in separate models stratified by subgroups, adjusted for the remaining potential confounders. Next, in separate models, we included an interaction term of each stair climbing exposure with the following subgroups, adjusted for the remaining potential confounders. The subgroups were age (≥ 65 vs < 65 yrs), gender (female vs male), BMI (< 25.0 vs 25.0-29.9 vs ≥ 30.0), comorbidity (0 vs ≥ 1), SxOA (defined as having both ROA and pain on most days of the month for the past 12 months; presence vs absence), ROA (Kellgren-Lawrence grade ≥ 2, with or without symptoms; presence vs absence). Interaction terms were evaluated for significance individually, with an a priori significance level set to 0.05.
Last, we performed a sensitivity analysis excluding participants who walked < 0.8 meters per second on a 20-meter walk test to remove those who were at greater risk of all-cause mortality.11
RESULTS
From the overall OAI sample of 4796 participants, 40 participants were excluded that were missing the stair climbing difficulty question and 14 participants were excluded that were missing the stair climbing frequency question. For both samples, a further 127 participants were excluded that did not complete the first follow-up visit at year 1, and 122 participants were excluded that were in the control cohort. This left 4507 participants in the stair climbing difficulty sample and 4533 participants in the stair climbing frequency sample (Figure 1). Both samples were approximately 61 years old, 58% female, and had a BMI of approximately 28 (Table 1). During the 13 years of follow-up, approximately 6.8% of participants within each sample died.
Participant flow diagram for stair climbing difficulty and frequency analytic samples. Freq: frequency; OAI: Osteoarthritis Initiative.
Descriptive characteristics of each stair climbing exposure group at baseline.
For stair climbing difficulty, average time to death, survival probability, and incidence rates over 13 years were the worst for the “limited a lot” group (6.5 [SD 3.1] yrs, 86.3%, 10.9 deaths/1000 person-yrs [PYs], respectively), followed by the “limited a little” group (6.4 [SD 2.9] yrs, 89.9%, 8.5 deaths/1000 PYs, respectively), and the best in the “not limited at all” group (6.9 [SD 3.0] yrs, 93.4%, 5.3 deaths/1000 PYs, respectively; log-rank test P < 0.001; Figure 2). Compared to participants who were not limited at all, those who were limited a lot had 84% greater hazard of all-cause mortality (adjusted HR [aHR] 1.84, 95% CI 1.21-2.81) and those who were limited a little had 54% greater hazard of all-cause mortality (aHR 1.54, 95% CI 1.18-2.02; Table 2).
Kaplan-Meier survival curves for all-cause mortality by stair climbing difficulty. The solid dark gray line represents the “limited a lot” stair climbing difficulty group. The dashed dark gray line represents the “limited a little” stair climbing difficulty group. The dotted light gray line represents the “not limited at all” stair climbing difficulty group. Percentages next to each curve represent the survival curve probabilities.
HR and 95% CI for all-cause mortality by stair climbing difficulty and frequency.
For stair climbing frequency, average time to death, survival probability, and incidence rates over 13 years were the worst for the 0-2 flights/week group (6.4 [SD 3.2] yrs, 84.5%, 13.0 deaths/1000 PYs, respectively), followed by the 3-6 flights/week group (6.0 [SD 2.9] yrs, 90.6%, 8.1 deaths/1000 PYs, respectively), and the best in the 7+ flights/week group (6.9 [SD 2.9] yrs, 92.7%, and 5.9 deaths/1000 PYs, respectively; log-rank test P < 0.001; Figure 3). Compared to participants who climbed 0-2 flights/week, those who climbed 3-6 flights/week had 27% lower hazard of all-cause mortality (aHR 0.73, 95% CI 0.47-1.15) and those who climbed 7+ flights/week had 38% lower hazard of all-cause mortality (aHR 0.62, 95% CI 0.44-0.87; Table 2). Only the 7+ flights/week group reached statistical significance in the fully adjusted model.
Kaplan-Meier survival curves for all-cause mortality by stair climbing frequency. The solid dark gray line represents the 0-2 flights/week stair climbing frequency group. The dashed light gray line represents the 3-6 flights/week stair climbing frequency group. The dotted dark gray line represents the 7+ flights/week stair climbing frequency group. Percentages next to each curve represent the survival curve probabilities.
From the stratified analyses, we found that age, gender, BMI, presence of comorbidity, ROA, and SxOA were effect measure modifiers in the relationship between stair climbing difficulty and all-cause mortality; however, none of the interaction terms reached statistical significance (Table 3). The results of the sensitivity analysis excluding participants with low functioning at baseline (walking speed < 0.8 m/s) did not differ from the main analysis (Supplementary Table S1, available from the authors upon request).
HR and 95% CI for all-cause mortality by stair climbing difficulty and frequency, stratified by age, gender, BMI, comorbidity, ROA, and SxOA.
DISCUSSION
We found that both difficulty with stair climbing and less frequent stair climbing were related with greater all-cause mortality hazard over 13 years in adults with or at high risk for knee OA. These findings highlight that trouble with stair climbing and/or infrequent stair use may serve as important early risk factors of future poor health outcomes in OA. In addition, these findings add to what is known about the predictive value of measures of physical function for future health. Namely, our findings support clinicians further evaluating patients with knee OA who report trouble climbing stairs or low stair climbing frequency.
Our findings are similar to previous studies with adults from the general population. Two previous studies explored the relationship between stair climbing frequency and all-cause mortality. Rey-Lopez and colleagues16 found that men in the Harvard Alumni Health Study who climbed 10 or more floors of stairs per week saw an 8% to 16% lower risk of all-cause mortality compared to those who climbed fewer than 10 floors per week. Sanchez-Lastra and colleagues17 found that adults in the UK Biobank cohort who reported climbing a flight of stairs more than 5 times a day had a 7% to 9% lower risk of all-cause mortality compared to those who did not climb any flights during a typical day. Two older studies from the same Harvard Alumni Health Study found similar results. Paffenbarger and Lee found that relative to those who climbed fewer than 20 floors per week, those who climbed greater than 20 floors per week had at least an 11% decrease in risk of death after 15 years.43 Lee and Paffenbarger separately found that those who climbed greater than 10 stories per week had at least an 11% lower risk of all-cause mortality after 15 years compared to those who climbed fewer than 10 stories per week.44 Our study findings add to these previous studies as we extend the association of stair climbing with mortality to adults with knee OA.
Why would stair climbing behaviors be related to future mortality? The ability to climb stairs requires contributions from multiple body systems that deteriorate with age,45 and difficulty with and/or less frequent stair climbing could signal a problem in 1 or more systems. For example, stair climbing taxes the cardiovascular system, requiring anywhere from 3.5 to 8.8 metabolic equivalents.46 Further, it challenges the musculoskeletal system to produce force that exceeds that required for level walking,47,48 to the point it is employed as a strengthening exercise for older adults.49 Stair climbing is even more difficult for adults with knee OA, as they show altered mechanics and muscle activation during stair use compared to their healthy peers.50 Stair climbing requires use of the visual and vestibular systems to successfully navigate up and down steps using motor control and coordination of the lower and upper body motor systems. As such, negative changes to stair climbing behaviors may reflect early deficits or declines in these important body systems. However, these changes do not necessarily indicate impairment in any specific body system but rather signals that problems may exist on the whole.
Because of the high demand that stair climbing places on multiple body systems and that stair climbing can represent overall body health and function, stair climbing behaviors may serve as an additional functional vital sign in a manner similar to walking speed.13-15 Like walking speed, declines or deficits in stair climbing may indicate that there is some impairment in a given body system, prompting further evaluation and testing to determine the causes—whether they be cardiovascular, muscular, or related to balance. However, changes to stair climbing may occur at a much earlier stage than changes to walking speed or other less demanding functional tasks, as it places a heavier demand on body systems. Further, early knowledge of declines in stair climbing may be a powerful clinical tool to predict future health status, serving as an early indicator of possible health decline. Stair climbing may have several advantages over walking speed as a functional vital sign. First, it is simple to collect and can be assessed with a few self-reported questions. Second, it is possible that changes to stair climbing may occur earlier than perceived or actual decline in walking speed, namely because of the high demand this task has on multiple body systems beyond walking on a flat surface.
The present study has several strengths. First, we used the OAI, a large, rich dataset with a lengthy follow-up time, which is ideal for survival analysis. Further, survival analysis accounts for time-to-event, which highlights how the time horizon for developing the outcome varies across groups. Second, to our knowledge, we are the first to explore the relationship of stair climbing with all-cause mortality within those with knee OA. This is particularly important as adults with knee OA are already at greater risk for premature health decline and death compared to the general population. Third, we explored stair climbing difficulty as a risk factor for all-cause mortality, which, to our knowledge, has not been previously examined in any capacity. Fourth, we performed a stratified analysis to determine if any additional factors modified the relationship between stair climbing and all-cause mortality to better speak to subsets of participants who are at increased hazard.
This study also had several limitations. First, it is possible that the underlying reasons for stair climbing changes are different for each participant. For example, the driving factor for difficulty may be knee pain for some, whereas for others, it may be strength or balance. Misclassification is also possible, since some participants may climb stairs infrequently because of difficulty with this task, whereas others may simply not encounter stairs in their daily lives (eg, those living in single-story homes). Regardless of these underlying reasons, we still observed a relationship between stair climbing and all-cause mortality that is consistent with prior cohort studies. Second, we used single, patient-reported questions to measure stair climbing, and the reliability and validity of these single questions has yet to be investigated. For example, the stair climbing questions do not determine the full extent of stair climbing ability nor do they distinguish between difficulty with ascending or descending stairs. As well, there is likely a ceiling effect for frequency as a large portion of the sample fell into the 7+ flights per week group. Further, there is the potential for recall bias, as the stair climbing exposures are self-reported. Last, the OAI primarily consists of participants who are relatively highly educated and mostly White, so these findings may not be generalizable to other populations. Future research might explore the joint association between stair climbing difficulty and frequency and determine how changes in stair climbing affect the relationship found in the present study.
Adults with knee OA who have difficulty climbing stairs or those who seldom use the stairs are at greater hazard of all-cause mortality. Although these relationships are complex, they are consistent with prior research and thus may have implications for clinical practice. Stair climbing behaviors are simple for patients to recall and relatively quick for clinicians to collect. Changes to stair climbing may occur early in the course of knee OA and are often the first reported limitation, potentially serving as an early indicator for future decline. Therefore, these data would give clinicians ample time to monitor, treat, or refer the patient for intervention to prevent or delay negative health outcomes. In summary, quick, simple questions about stair climbing can serve as a functional vital sign within the clinician’s toolbox.
Footnotes
The authors declare no conflicts of interest relevant to this article.
- Accepted for publication December 20, 2023.
- Copyright © 2024 by the Journal of Rheumatology
