Lower-Extremity Force Decrements Identify Early Mobility Decline Among Community-Dwelling Older Adults

Background. Many apparently independent older adults modify daily tasks. Task modifications strongly predict future mobility disability. Clinically intuitive, easily measured "biomarkers" associated with task modifications would offer quantifiable treatment targets for prevention of age-related functional limitations. Objective. The objective of the study was to examine lower-extremity muscle strength deficits and functionally relevant cutoff points associated with daily task modifications in community dwelling older adults living independently. Design. This was a cross-sectional observational study. Methods. Fifty-three participants (mean age=76.4 years, SD=5.2) were tested for task modifications and leg strength. Task modifications were assessed using a previously described tool (summary task modification score). Twenty-six of the participants were classified as task modifiers (TM group), and 27 participants were classified as non-task modifiers (NTM group). A net antigravity leg force in the sagittal plane (NETforce) was calculated by summing the normalized isometric and isokinetic torques from the hip extensors, knee extensors, and ankle plantar flexors. Results. Compared with the NTM group, the TM group exhibited 30.0% and 33.5% reduction in lower-extremity isometric and isokinetic NETforces, respectively. Isometric and isokinetic NETforce cutoff points for task modifications were <= 4.24 and 2.77 N.m/kg body weight, respectively. The isometric and isokinetic models both yielded sensitivity and specificity values of 74.1% and 80.8%, respectively (positive likelihood ratio=3.852, 95% confidence interval=1.699-8.735; negative likelihood ratio=0.321, 95% confidence interval=0.167-0.618). Isometric and isokinetic NETforces were significantly associated with task modifications (odds ratio=2.50 and 2.42, respectively). Limitations. The cross-sectional design of this study does not allow for a test of causal relationships. This study used a modest yet adequate sample size that may limit generalization of the results. Conclusions. Isometric and isokinetic NETforce cutoff points provide quantifiable biomarkers that discriminate community-dwelling older adults who modify daily tasks from those who do not.