Study design

The prospective observational trial conducted between January and October 2021 on outpatients with COPD attending supervised PR at a tertiary referral sub-acute rehabilitation centre. The study was approved in October 2020 by the Ethics Committee of the IRCCS Fondazione Don Carlo Gnocchi and was conducted in accordance with the Declaration of Helsinki. All participants signed an informed consent form before study participation. Trial registration: NCT04767126.


Inclusion criteria were: (1) COPD diagnosis (Global Initiative for Chronic Obstructive Lung Disease (GOLD) stage: II-III-IV)12. (2) Male and female patients, 35–85 years old. Subjects were excluded from the study if they had respiratory diseases other than COPD, unstable clinical conditions, or exacerbations in the last four weeks. Patients with cardiologic, orthopaedic or neurological conditions limiting their ability to perform the exercise training or the assessments planned were also excluded from the study.


Participants attended 15 sessions of supervised PR. During the first session, medical history and baseline measurements were collected (T0). The assessment procedure, except the 6-min walk test (6MWT), was repeated at the conclusion of the PR program (T1) and after three months from the completion of the PR program (T2).


Bioimpedance analysis (RD-545-sv, Tanita, The Netherlands) was used to obtain the fat-free mass index (FFMI; i.e., fat-free mass/height2), where scores < 17 kg/m2 for men and < 15 kg/m2 for women were considered as low13. In addition, muscle quality of the dominant limbs was calculated14.

The total distance covered during the 6MWT was measured for each participant following a standardized procedure15. In addition, the 6-min walk distance (6MWD) was expressed also as a percentage of the predicted value16.

The lower extremity function was evaluated using the Short Physical Performance Battery (SPPB)17. Furthermore, the mean functional muscle power of the lower limbs was estimated using previously reported equation that included time to complete 5STS, subjects’ body mass and height, and the height of the seat (0.43 m) as variables18.

Handgrip strength (HGS) of the dominant arm was assessed with a digital hand dynamometer (Jamar Plus + Dynamometer, Performance Health Supply, USA) by three maximal contractions of 3 s each, separated by 60 s of rest19.

Quadriceps muscle function was measured with a computerized dynamometer (Biodex System 4 Pro, Biodex Medical Systems Inc, USA) in the dominant lower limb. First, the maximal isometric peak torque (PT) was recorded at 65° of knee flexion during a contraction of 4 s, repeated for 3 attempts with 60 s of rest. The percentage of predicted values for quadriceps PT was calculated using a previously reported equation20. Values < 70% of the predicted account for quadriceps muscle weakness21. Then, force steadiness was measured during sub-maximal isometric contractions of the dominant quadriceps, using visual feedback set at 30% of the patient's PT, during 5-s contractions9. Five familiarization trials were performed followed by five attempts, each one separated by 15 s of rest. Quadriceps PT and RFD were normalized to fat-free mass, and force steadiness was computed as the coefficient of variation of force by normalizing the standard deviation (SD) to the mean force.

Muscle activation and kinematic parameters were recorded during strength measurements and the SPPB test using a wireless system (Cometa Wave Plus, Cometa S.r.l., Italy) with surface electromyography (EMG) and inertial measurement units (IMUs)22. After standard skin preparation, EMG electrodes were placed on the rectus femoris of both legs, vastus lateralis and medialis of the dominant leg, and flexor carpi and biceps brachii of both arms. In addition, eight IMUs were attached via elastic bands to the thighs, shanks and feet of both lower limbs, as well as on the pelvis and chest of the patient. The EMG signals from each trial were full-wave rectified and converted into their root mean square (RMS) amplitude. Values of RMS were used to evaluate the maximal muscle activation during tests of quadriceps PT, HGS, and 5STS for both the concentric (i.e., sit-to-stand) and eccentric (i.e., stand-to-sit) phases, as well as to measure the submaximal muscle activation during the test of force steadiness. Results of maximal muscle activation obtained during isometric testing were then normalized to quadriceps PT values, whereas values of maximal muscle activation of 5STS and force steadiness were normalized to the maximum RMS recorded during isometric PT. This ratio (i.e., maximal muscle activation of 5STS/maximal muscle activation during PT) represents indexes of relative muscle activation during the concentric or eccentric phase of 5STS and neuromuscular economy, respectively23,24.

The overall dyspnea was measured with the Barthel index based on dyspnea and with the Medical Research Council (MRC) dyspnea scale25. The risk of death was also assessed with the Body-Mass, Airflow Obstruction, Dyspnea, and Exercise Capacity (BODE) index26. Further description of the assessment procedures and data processing is provided in the Supplementary Information.

Pulmonary rehabilitation program

The conventional PR program consisted of 90 min sessions, 3 days per week, including both training with a cycle ergometer and resistance training with free weights or elastic bands. The workload for the cycle ergometer was calculated using the 6MWD to estimate the peak work rate according to a previously reported equation27. Then, exercise intensity for the first training session was set at 50% of the estimated peak work rate, and increased by 10 W during the following sessions according to the level of dyspnea and fatigue as described elsewhere28,29.

Resistance training started with the maximal load that could be lifted during 20 repetitions for 2 sets, and was weekly increased until achieving the maximal load that could be lifted for 10 repetitions and 3 sets.

Airway clearance techniques were adopted if necessary. Finally, participants received educational support for maintaining regular exercise and physical activities after the conclusion of the PR program. Further details of the PR program are available in the Supplementary Information.

Statistical analysis

The sample size calculation was performed considering the mean functional muscle power estimated during the 5STS test as validated by Alcazar et al.18. Therefore, assuming an increase ≥ 25% in muscle power, with a 25% dropout rate, a sample size of 20 subjects was found (α = 0.05; power = 0.8).

Results were reported as mean and SD, unless otherwise stated. All variables were tested for the normality of the distribution. Variables assessed only pre- and post-PR program were analyzed with paired t-test for normally distributed data, and with Wilcoxon’s signed-rank test in case of not normally distributed data. According to Cohen, the estimated effect size (d) of the treatment was considered small (d = 0.2), moderate (d = 0.5), or large (d = 0.8)30.

The outcome measures assessed at all the three time points were analyzed using mixed-models analysis of variance (ANOVA). The models included time as a fixed effect variable, and participant or participant per time interaction as random effects variables. The Akaike information criterion was used to evaluate the fit of models with different covariance structures31. Post hoc tests compared each of the three time points by using Sidak adjustments to control for Type I error.

Relationships between pre- and post-PR (Δ) changes for variables of muscle function and functional performance were tested using Pearson's correlation coefficient (r). Values of r were considered moderate (0.3 > r < 0.5), large (0.5 > r < 0.7), and very large (0.7 > r < 0.9). Relationships for the same variables at each time point were also tested with linear mixed models. The models were set as for ANOVA analysis, but labelling one variable as dependent variable, and the other as covariate.

Based on significant univariate correlations, multiple linear regression analysis included Δ changes in quadriceps function (i.e., leg muscle quality, PT, RFD, and relative activation during 5STS) as independent variables, and Δ changes in functional performance (i.e., 6MWD, 4mGS, 5STS time and power) as dependent variables. Significant predictors from previous models were then used in a model with baseline independent variables of sex, age in years, and percentage predicted forced expiratory volume in 1 s (FEV1). The stepwise backward method was used to enter data into the models and the assumption of multicollinearity was checked. As general rule, the number of patients analyzed was 5 times the number of variables entered into the multiple regression equation. Statistical analyses were performed using SPSS Statistics software (IBM, Chicago, IL, USA).

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