Reliability of quadriceps muscle power and explosive force, and relationship to physical function in people with chronic obstructive pulmonary disease: an observational prospective multicenter study

Background: Muscle power declines with age and is a stronger determinant of physical function than strength. Muscle power using computerized dynamometry has not been investigated in COPD. Objectives: To determine: 1) test-retest reliability of quadriceps power using a standardized protocol with computerized dynamometry; and 2) associations between quadriceps strength and power, and functional capacity. Design/Setting: Prospective observational study in four Canadian research labs. Participants: People with mild to very severe COPD. Methods: Tests were conducted on two days. Quadriceps muscle maximal strength was evaluated during a static maneuver using maximal voluntary isometric contractions (MVIC). Rate of torque development (RTD) during MVIC was used to assess explosive force. Muscle power was measured using a dynamic, isotonic protocol from which peak and average power and peak velocity were derived. Functional capacity was assessed with the Short Physical Performance Battery (SPPB). Reliability was assessed using intraclass correlation coefficients (ICC), standard error of measurements (SEM), and Bland Altman plots. Spearman and Pearson correlation coefficients were used for associations. Results: 65 patients (age 69 +/- 8 years; FEV1 48 +/- 21% of predicted) were included. ICC was 0.77 for RTD and 0.87-0.98 for isotonic power measures (95%CI 0.63-0.99, p < .001); SEM < 10% for average/peak power and peak velocity, and > 30% for RTD. SPPB had moderate correlation with average power, but not with MVIC or RTD. Conclusion: The standardized isotonic protocol with computerized dynamometry was reliable in assessing quadriceps power in COPD. Our data highlights that average power correlates best with functional capacity, indicating higher relevance than static measures when investigating determinants of function.