Functionally Relevant Threshold of Quadriceps Muscle Strength in Patients with Chronic Obstructive Pulmonary Disease

OBJECTIVES: We aimed to identify the quadriceps muscle strength (QMS) thresholds below which exercise capacity is compromised in men with chronic obstructive pulmonary disease (COPD). METHODS: We measured the quadriceps isometric maximum voluntary contraction (QMVC) and calculated the QMVC values normalized to weight (QMVC-BW), height squared (QMVC-H(2)), and body mass index (QMVC-BMI) in 113 patients with COPD. The functional exercise capacity was evaluated using the 6-minute walk distance (6MWD), and 6MWD <350 m was defined as functional exercise intolerance. Thresholds were determined for QMVC and its normalized values to achieve high specificity (>0.90) with maximal sensitivity. P-values <0.01 were considered statistically significant. RESULTS: Data from 99 male patients (age, 74 ± 6 years; percentages of predicted forced expiratory volume in 1-s, 56.9 ± 26.4%) were analyzed; 3 women and 11 participants with the missing data were excluded. Multivariate logistic regression models identified significant associations of QMVC and QMVC-H(2) with 6MWD, after adjustment for age and dyspnea. C-statistics showed that the area under the curves of all QMVC parameters were comparable. The thresholds of QMVC and QMVC-H(2) for predicting compromised exercise capacity were 26.2 kg and 9.6 kg/m(2), respectively. CONCLUSIONS: QMS thresholds in men with COPD could help clinicians evaluate whether QMS is insufficient to achieve 6MWD ≥350 m and thereby identify patients who should be specifically targeted for muscle strengthening training during their pulmonary rehabilitation program.