Minimal important difference of two methods for assessment of quadriceps femoris strength post exercise program in individuals with COPD

Background: The assessment of quadriceps femoris (QF) strength is recommended by international guidelines as an outcome of exercise training (ET) programs in patients with chronic obstructive pulmonary disease (COPD).

Objectives: To identify the minimal important difference (MID) of two methods for the assessment of improvement of QF strength after a high-intensity ET program in individuals with COPD: 1-repetition maximum (1RM) and maximal voluntary contraction (MVC).

Methods: Individuals with moderate to very-severe stable COPD were submitted to high-intensity ET (3 days/week, 36 sessions). Lung function (spirometry), exercise capacity (6-minute walk test [6MWT]) and QF strength (1RM test and MVC by using a strain-gauge) were assessed before and after the ET program.

Results: Twenty-one individuals were studied (65±8 years, BMI 27±6; FEV₁ 51±16%predicted). At the end of the ET program, QF strength improved significantly (10±4 Nm for MVC and 8 ± 6 kg for 1RM) (P<0.05 for both). The MID for improvement of the QF strength calculated by distribution-based methods ranged from 9.4 to 16 Nm (between 7.4% and 12.6% increase from baseline) for the MVC and from 2.5 to 3 kg (between 12% and 15% increase from baseline) for the 1RM test. Anchor-based MIDs could not be calculated since there was no correlation between the improvement of QF strength and the improvement of the proposed anchor (6MWT).

Conclusions: The MID for improvement of QF strength after a high-intensity ET program in individuals with moderate-to-severe stable COPD ranges between 9.4 and 16 Nm for the MVC assessed with a strain gauge and between 2.5 and 3 kg for the 1RM test.