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

We aimed to identify the quadriceps muscle strength (QMS) thresholds below which exercise capacity is compromised in men with chronic obstructive pulmonary disease (COPD).

We measured the quadriceps isometric maximum voluntary contraction (QMVC) and calculated the QMVC values normalized to weight (QMVC-BW), height squared (QMVC-H²), 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.

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-H² 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-H² for predicting compromised exercise capacity were 26.2 kg and 9.6 kg/m², respectively.

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.