The Joint Effects of Sedentary Time and Physical Activity on Mobility Disability Risk in Older People: NIH-AARP Diet and Health Study

Purpose: To determine the level of physical activity necessary to minimize the deleterious effects of large amounts of sitting on risk of mobility loss in an aging cohort. Methods: We analyzed prospective data from 134,269 participants (age 50-71 years) in the NIH-AARP Diet and Health Study between 1995-1996 and 2004-2005, who reported no major chronic diseases and rated their health as fair or better at baseline. Total sitting time (h/day), TV viewing time (h/d) and light- and moderate-to-vigorous-intensity physical activity level (h/week) were self-reported at baseline, and mobility status at follow-up was defined by perceived walking speed. Multivariable logistic regression modeling determined the independent and joint effects of sedentary time and total physical activity on risk of mobility disability (unable to walk or walks at easy speed). Results: At follow-up, 38,798 participants (29%) were defined as having a mobility disability. After adjustment for light- and moderate-to-vigorous intensity physical activity, as well as for important covariables, the independent impact of total sedentary time was almost negligible, whereas disability risk increased in a dose-response manner with increasing category of TV time (p\u3c0.001). Among those in the most active tertile (\u3e7 h/week), sitting 6 or fewer h/day was not related to excess risk of mobility disability, and those in the most active group who reported the highest level of sitting time (≥7 h/day) still had a significantly lower risk (OR=1.11; 95%CI=1.02, 1.20) compared with those reporting the lowest level of sitting (\u3c3 h/day) in the least active group (≤3 h/week; OR=2.07; 95%CI= 1.92-2.23). In contrast, increased TV time was significantly related to increased mobility disability within all levels of reported physical activity –and this risk was accelerated in the least active participants. Conclusion: Reductions in sedentary time, combined with increases in physical activity may be necessary to maintain health and function in older age –particularly among those who are the least active