Effects of covertly measured home exercise adherence on patient outcomes among older adults with chronic knee pain

Background Assessment of home exercise adherence and the degree to which adherence influences changes in patient outcomes is limited by the use of self-reported measures. Objectives To determine the relationship between adherence to a home strengthening program, covertly measured by accelerometers in ankle cuff weights, and changes in self-reported pain, physical function, and knee extensor strength among people with chronic knee pain. Methods This is a secondary analysis of data from a clinical measurement study in 54 adults, aged 45 years or older, with chronic knee pain who completed a 12-week, home-based quadriceps-strengthening program. A triaxial accelerometer was concealed in the ankle cuff weight used for exercises to assess exercise adherence. Associations between exercise adherence and changes in pain and function (measured using the Western Ontario and McMaster Universities Osteoarthritis Index) and peak isometric knee extensor strength were examined using mixed-effects and linear regression models and fractional polynomials. Results Exercise adherence declined from a median of 90% (interquartile range, 70%–100%) in weeks 0 to 2 to 65% (interquartile range, 25%–90%) in weeks 10 to 12. Significant improvements were observed in knee pain (mean change, −3.2 units; 95% confidence interval [CI]: −2.4, −3.9 units), function (mean change, −10.1 units; 95% CI: −7.8, −12.4 units), and knee extensor strength (mean change, 0.34 Nm/kg; 95% CI: 0.26, 0.42 Nm/kg) across the group over the same period. Exercise adherence was not associated with changes in pain, function, and knee extensor strength over 2-week periods or over the entire 12 weeks. Conclusion Covertly measured adherence to a home strengthening program was not associated with changes in patient outcomes. These findings challenge the notion that greater exercise adherence leads to greater improvement in patient outcomes during a short-term intervention.