Device-Measured Physical Activity As a Predictor of Disability in Mobility-Limited Older Adults

Loss of the ability to ambulate (mobility) is one of the leading causes of poor quality of life, loss of independence, morbidity, and death in older adults.1 Preventing mobility loss and the onset of mobility disability is an important public health concern given the rapid aging of the population.1 Physical activity (PA) is one of the most-promising interventions to attenuate mobility loss and prevent mobility disability, but individuals at risk of mobility loss are commonly inactive, with evidence indicating that physically inactive individuals tend to be older, less educated, have lower incomes, and have more medical conditions than active individuals.2, 3 We previously reported that a long-term supervised PA program reduced the risk of incident major mobility disability (MMD), defined as loss of the ability to walk 400 m, by 18% in older adults in the Lifestyle Interventions and Independence for Elders (LIFE) Study.4 This work demonstrated that a structured PA intervention was efficacious in preserving mobility and preventing MMD in older adults.

Despite this important finding, data remain sparse regarding how gradations in duration and intensity of PA influence the incidence of MMD. The majority of studies using accelerometry have evaluated PA in younger populations5, 6 or in the evaluation of cardiometabolic health.7 Furthermore, data are needed regarding the potential utility of device-measured monitoring of PA in a nonlaboratory setting for predicting MMD. We aimed to address these gaps in the literature using accelerometry-measured PA, a noninvasive and objective method of tracking and measuring human PA,8 gathered in the LIFE Study. To our knowledge, the LIFE Study is the only Phase 3 PA intervention trial with data on the incidence of MMD in older adults. Thus, this study provides a novel opportunity to address questions related to objectively measured PA and associations with MMD.

Methods

Results

Details of recruitment and baseline characteristics were published previously.12 The mean age ± SD of included participants was 78.9 ± 5.2, 67.2% were women, 23.6% were racial or ethnic minorities, 35.9% were married, and 63.8% reported having a college education. In general, participants were cognitively intact (modified Mini-Mental State Examination score 91.7 ± 5.4), showed low levels of depressive symptoms (Center for Epidemiologic Studies Depression Scale score 8.5 ± 7.8), and had relatively poor sleep quality (Pittsburgh Sleep Quality Index score 5.9 ± 3.8). Details of MMD and PMMD event rates in the LIFE Study were published previously.4 Similar results were observed for the cohort in the present study for MMD (32.6%) and PMMD (17. 9%). Additional details regarding participant characteristics are provided in Table 1. There were no significant interactions between any of these baseline variables and randomized arm (all P > .05).

At baseline, participants wore accelerometers for a mean 7.95 ± 3.24 valid wear days and 837.1 ± 111.1 min/d (≥10 h/d) on valid days. They spent 647 ± 116 min/d of their baseline wear time (77%) being sedentary (<100 cpm). The remaining (nonsedentary) time was spent in activity registering 100 to 499 cpm (137 ± 43 min/d), 500 to 1,039 cpm (38 ± 23 min/d), and 1,040 or more (15 ± 16 min/d). Participants also accrued 2,682 ± 1,475 steps per day, with a 30-minute peak cadence of 34.8 ± 17.2 steps per minute during the baseline measurement period. Participants accumulated 14.1 ± 5.8 bouts of 5 minutes or more (>99 cpm) and 3.6 ± 2.6 bouts of 10 minutes or more (>99 cpm).

The associations between baseline accelerometry variables and MMD and PMMD rate are shown in Table 2. Each 30 minutes of sedentary time was associated with a 10% greater rate of MMD. Conversely, every 500 steps taken was associated with a 15% lower rate of MMD and an 18% lower rate of PMMD. Activity of 1,040 cpm or more was not associated with MMD or PMMD after considering multiple comparisons, although the amount of time spent in this level of activity was small. Peak cadence and number of activity bouts of 5 minutes or more were also associated with lower rates of mobility disability.

When fitting different levels of activities (30 min/d of activity 100–499 cpm, 500–1,039 cpm, and ≥1,040 cpm) in the same model, only moderate PA time activity (100–499 cpm) was significantly associated with MMD (hazard ratio (HR) = 0.89), and no levels of activities were associated with PMMD (Table 3). None of the tested interactions reached significance in the final model, although the interaction between activity of 500 to 1,039 cpm and 1,040 cpm or greater was significantly associated with MMD in Models 1 to 3 (Model 4: P = .10).

Accelerometry data at study visits are shown according to randomization arm in Supplementary Table 3. Across all data collection visits, participants spent 648 ± 114 min/d of their wear time (78.2%) being sedentary. The remaining time was spent in activity registering 100 to 499 cpm (131 ± 44 min/d), 500 to 1039 cpm (32 ± 22 min/d), and 1,040 cpm or more (15 ± 16 min/d). Participants also accrued 2,625 ± 1,545 steps per day, with a 30-minute peak cadence of 35.2 ± 18.9 steps per minute. Participants accumulated 13.3 ± 5.9 bouts of 5 minutes or longer and 3.3 ± 2.5 bouts of 10 minutes or longer.

Associations between time-dependent accelerometry (collected at 0, 6, 12, and 24 months) variables and incident MMD and PMMD are shown in Supplementary Tables S2 and S3. The results are consistent with the results for baseline associations. In particular, every 30 minutes spent sedentary was associated with 17% greater rates of MMD and PMMD. Most indices of PA were also significantly associated with rates of MMD and PMMD. When fitting differing levels of activity jointly, minutes per day at 100 to 499 cpm PA level was negatively associated with MMD and PMMD (both HR = 0.81) after adjusting for 500 to 1,039 cpm, 1,040 cpm ore more, and covariates. Minutes per day of 1,040 cpm or more was also negatively associated with MMD (HR = 0.52) and PMMD (HR = 0.48), after adjusting for lower levels of activities (100–499 and 500–1,039 cpm).

Discussion

The primary finding of this study is that accelerometry-based indices of PA and sedentary behavior were associated with rates of MMD and PMMD in mobility-limited older adults. Specifically, every 30-minute increase in sedentary time was significantly associated with a higher rate of MMD. These findings were consistent when using a single baseline assessment or serial longitudinal assessments over a 24-month period.

The findings of the present study expand upon the main LIFE Study results by demonstrating that a single baseline assessment of PA can be predictive of subsequent MMD events. This shows the potential added value of accelerometry in this older population as a clinical tool for assessing PA levels. Individual counseling based on accelerometry-derived PA levels may reduce inactivity levels and further reduce risk of MMD. For example, average number of steps per day was associated with a 15% lower event rate of MMD and an 18% lower rate of PMMD in LIFE Study participants. Increasing daily number of steps may be a motivating and easily achievable outcome that could decrease risk of MMD in older adults with mobility impairments. These results are particularly notable given some concerns that researchers in the field have expressed related to the reliability and sensitivity of such accelerometry-derived information in adults with slowed or uneven gait or similar mobility impairments.13 The present results showed that PA levels of 100 to 499 cpm were associated with a significantly lower (11%) MMD event rate. Higher levels of PA (500–1,039, ≥1,040) were not associated with lower MMD and PMMD event rates. Although the current PA guidelines suggest that higher levels of PA (moderate to vigorous PA (MVPA)) be achieved for health benefits (760–1,952 cpm),14 these findings suggest that more-achievable light PA (100–499 cpm) can have important effects on MMD.

The accelerometry cut-points used in the present study differ somewhat from some others previously reported in the literature. For instance, one report of adults aged 65 and older from the National Health and Nutrition Examination Survey (NHANES) used more than 1,952 cpm for MVPA, 760 to 1,951 cpm for light PA, and 101 to 759 cpm for very light PA.15 Other authors used a threshold of 760 cpm to characterize MVPA,16 although they authors that the 1,952-cpm threshold used for MVPA determination resulted in fewer participants meeting these levels of PA than with a 760-cpm MVPA threshold.16 Another study reported that 40% of older adults were unable to engage in MVPA using a 1,041-cpm threshold to characterize MVPA.17 There is a lack of clarity regarding appropriate count thresholds that will not over- or underestimate MVPA in older adults of varying age groups and mobility impairment levels. Based on our results, for individuals who are not able to achieve MVPA intensities, recommending lower levels of PA intensity (up to 500 cpm) that are more achievable and effective in lowering incident risk of MMD may increase involvement of moderately functioning older adults in PA interventions and further lower the risk of physical disability.

In addition to tracking activity, accelerometry provides important information regarding inactivity and sedentary behavior. Higher levels of sedentary time are independently associated with greater metabolic dysfunction, physical disability, and mortality.16, 18 LIFE Study participants spent 77% of their waking hours being sedentary (Supplementary Table 3). This is in agreement with other studies of older adults.19-21 The present analysis showed that lower levels of sedentary time were associated with lower (10%) MMD event rates. Similarly, another study showed that participants in NHANES with mobility disability accumulated not only the highest volumes of sedentary time per day (69%), but also more sedentary time in longer sedentary bouts and shorter active bouts than those without mobility disability.15 It has been shown that regular interruption of sedentary time, even with light-intensity activity, is associated with health benefits in some populations.16 Therefore, tracking of PA and sedentary time, in the form of bouts and total time, may have relevance for predicting MMD. Moreover, PA guidelines for older adults should focus not merely on increasing PA levels, but also on reducing total daily sedentary time in older adults to prevent incident MMD. It is important that health professionals and patients understand these relative benefits so that the older adults can be counseled according to mobility and health status.

This study had a few limitations worth noting. First, the lack of general agreement on PA intensity thresholds in this population is a limiting factor in generalizing these results to other groups of older adults. Additionally, causal interferences may not be drawn from this longitudinal study. Another aspect of the study that warrants discussion is the inclusion of the PA group in our primary analysis. This approach incorporates a group that was specifically given an intervention designed to affect the primary risk factor. Our rationale for this approach was to capitalize upon all of the data available in the trial and to evaluate the effects of accelerometry in individuals who did and did not engage regularly in PA as might be seen in everyday settings. Analyses including an interaction term for randomized arm and analyses conducted separately within groups did not reveal an indication of any differences in the utility of accelerometry between groups. Still, the potential confounding of this approach should be noted.

In conclusion, this study has several important strengths, including the multisite clinical trial study design, large sample size, and multimodal PA data collection (according to accelerometry and self-report). These results indicate that accelerometry-based baseline and longitudinal assessment of PA and sedentary time levels are useful for assessing event rates of MMD and PMMD in mobility-limited older adults. Number of steps per day and time spent in lower levels of activity (e.g., 100–499 cpm) were significantly associated with lower MMD event rates. Therefore, public health messages encouraging increases in low-level activity and decreases in sedentary time may be fruitful in reducing mobility disability in this population. Moreover, accelerometry may be a good clinical tool for assessing and preventing MMD risk in older adults, which may help in better counseling older individuals on PA levels needed to prevent MMD. Further studies are warranted to support these findings are further refine recommendations for accelerometry activity thresholds.