Minimal important difference of quadriceps maximal voluntary contraction (QMVC) in COPD: a prospective cohort study

BACKGROUND: Quadriceps maximal voluntary contraction (QMVC) reliably measures quadriceps muscle force and predicts mortality in chronic obstructive pulmonary disease (COPD). However, the minimal important difference (MID) of QMVC is not well-established. AIM: To estimate the MID of QMVC parameters in people with COPD following pulmonary rehabilitation (PR). METHODS: QMVC was measured before and after 8 weeks of outpatient PR in people with COPD. Absolute and % change in QMVC, and change in normalised QMVC were calculated using paired t-tests. Anchor and distribution-based methods (0.5×SD change, SEM, minimal detectable change at 95% confidence, effect size and 1.96 SEM) were used to estimate the MID. RESULTS: Of 903 participants, 383 were excluded due to PR non-completion or missing QMVC data with 520 included in the analysis (37% female; mean (SD) age 70.2 (8.4) years; forced expiratory volume in 1 s 51.4 (21.4)% predicted). QMVC parameters increased with PR; mean (95% CI) or mean (SD) change: QMVC 2.0 kg (1.5 kg to 2.5 kg), 10.6% (27.7%) and normalised QMVC 5.0% predicted (3.9% to 6.2%). Anchor-based MID estimates were precluded due to weak/no correlation with external anchors. Using distribution-based methods, the MID for QMVC change, QMVC % change and normalised QMVC change were estimated as mean (range) 3.55 kg (1.84 kg to 5.11 kg), 18.34% (9.60% to 26.60%) and 7.78% (3.78% to 12.48%) for all participants. However, MID estimates for absolute and % change in QMVC differed markedly between men and women. Normalised QMVC estimates demonstrated smaller sex-based discrepancies. CONCLUSION: We provide MID estimates for QMVC parameters. Sex-specific or normalised MID estimates for QMVC should be used to facilitate the interpretation of change.