Study design and participants
This was a cross-sectional, controlled study conducted at the Institut universitaire de cardiologie et de pneumologie de Québec, Québec, Canada. Fourteen people with severe to very severe COPD (forced expiratory volume in 1 s [FEV1] < 50% predicted values) and 12 healthy controls were recruited. People with COPD were eligible if they were at least 40 year of age, had a body mass index (BMI) < 30 kg·m2, were former smokers with a cumulative tobacco exposure > 10 pack-years, and had a sedentary lifestyle defined by a Voorrips score < 922. Exclusion criteria included any cardiovascular, neurological, neuromuscular, or orthopedic diseases that could affect the ability to perform study procedures; current asthma; participation in pulmonary rehabilitation in the past 6 months; and being on > 10 mg daily prednisone equivalent. Apart from lung function criteria, the same inclusion and exclusion criteria were applied to people with COPD and healthy controls to have two groups paired for sex, age, BMI, and level of physical activity. The protocol and the characteristics of the same study participants have already been published elsewhere11, but apart from peak cardiorespiratory response data which are necessary for the understanding of this study, data presented in this manuscript are original.
Procedure
The study consisted in one visit during which anthropometric measurements were assessed by bioelectrical impedance (InBody520, Body Composition Analyzer, Seoul, Korea) and pulmonary function tests, including spirometry, plethysmographic lung volumes, and carbon monoxide diffusion capacity were performed according to standard guidelines23. The 1STS was performed after 30 minutes of rest following the pulmonary function tests as previously described11. Participants were asked to sit with the knees and hips flexed to 90°, feet placed flat on the floor, and hands placed on the hips. Standardized instructions informed study participants to stand completely straight and immediately sitting back as many time as possible in one minute without using hands. Performance of 1STS was reported by the number of repetitions and the associated total work was estimated (number of repetitions*body mass [kg]).
Measurements
Cardiorespiratory monitoring
At rest, during the 1STS, and during four minutes of recovery, oxygen uptake (\(\dot{V}\)O2) was measured breath-by-breath, and heart rate (HR) and pulse oxygen saturation (SpO2) were monitored beat by beat with a portable gas analysis system (Oxycon Mobile; Viasys Healthcare, Jaeger, Germany), as previously described11. The analyzer was calibrated before each test according to the manufacturing recommendations.
Quadriceps oxygenation
A continuous wave multichannel near-infrared spectroscopy (NIRS) system (OxiplexTS, ISS, Champaign, USA) was used at two wavelengths in the near-infrared range (nominal wavelengths of 690 and 830 nm) to detect quadriceps changes in absolute concentration of oxygenated (Δ oxy [Hb-Mb]), deoxygenated (Δ deoxy [Hb-Mb]), total (Δ total [Hb-Mb]) myoglobin–hemoglobin concentration, and muscle saturation index (Δ SmO2) during the 1STS, and during 4 minutes of recovery. The NIRS probe calibration was verified prior to each testing session according to the manufacturer’s recommendations, using a calibration block of known absorption and scattering coefficients. To avoid the influence of room light, probe was covered with an optically black band24 and fixed on the fleshy part of the quadriceps, below the EMG electrode. Prior to testing, the adipose tissue thickness of the leg was measured using skin calipers (Baseline Skinfold Caliper, NexGen Ergonomics, Canada).
Electromyography
Surface electromyography signal (sEMG) from the right quadriceps was recorded throughout the 1STS (FreeEMG300, BTS Bioengineering, Milan, Italy) with a bioelectric signal amplifier, wireless transmission, and bipolar electrodes. The sEMG signal was high-pass-filtered (1 kHz) and preamplified near the recording electrodes. Electrodes were placed on the muscle bellies, longitudinally with respect to the underlying muscle fibers arrangement and were located according to the surface electromyography for the non-invasive assessment of muscles (SENIAM) recommendations25. Before placing the electrodes, the electrical impedance of the skin was reduced by shaving the hair and by cleaning the skin with alcohol. An electrogoniometer (FreeEMG300, BTS Bioengineering, Milan, Italy) was placed on the participant’s left leg to record and dissociate the sitting and standing phases during the 1STS.
Data and statistical analyses
The number of repetitions and the duration of each sit-to-stand were counted using the electrogoniometer signal. The mean duration of two sit-to-stand at 15s, 30s, 45, and 60s of 1STS was calculated. The cumulative total work at 15s, 30s, 45, and 60s of 1STS was calculated by multiplying the cumulative number of repetitions performed at each time by the individual’s body mass.
Breath-by-breath cardiorespiratory data recording was synchronized to muscle oxygenation at 1-s intervals. Therefore, simultaneously recorded data were obtained at rest, and during the 1-minute exercise, and four minutes of recovery. Because of the short duration of the 1STS, end-exercise cardiorespiratory parameters are reported from the mean of the last two breaths.
Changes in Δ oxy [Hb-Mb], Δ deoxy [Hb-Mb], Δ total [Hb-Mb], and Δ SmO2 were calculated with the respective baseline value as reference. On the basis of the Fick principle, Δ deoxy [Hb-Mb] responses reflects the dynamic balance between oxygen delivery and consumption in the investigated muscle, providing an index of fraction oxygen extraction in local muscle, whereas Δ total [Hb-Mb] can be interpreted as an indirect estimation of local blood volume in the tissue26. These data were imported into a personal computer at a sampling frequency of 1 Hz via an analog-to-digital converter (PowerLab, ADInstruments, Australia) allowing synchronization with breath-by-breath cardiorespiratory data. Muscle oxygenation responses were expressed as percent change from resting values. Δ oxy [Hb-Mb], Δ deoxy [Hb-Mb], Δ total [Hb-Mb], and Δ SmO2 were averaged over the last 15 seconds of rest before the test (baseline data) and the last 15 seconds of each 30-second period during the recovery (90, 120, 150, 180, 210, 240, 270, 300 seconds). Because of the 1STS short duration, these data were also averaged over the last 3 seconds at 15, 30, 45, and 60 seconds of 1STS. To take into account difference in the 1STS total work between people with COPD and healthy controls, Δ oxy [Hb-Mb], Δ deoxy [Hb-Mb], Δ total [Hb-Mb], and Δ SmO2 were also expressed by dividing their value by the cumulative work during 1STS and during four minutes of recovery. Results are expressed as mean ± SEM.
All sEMG signals were analysed using custom software written in MATLAB R2018a (The Math Works Inc., Natick, Massachusetts, United States). The signals were digitally filtered off-line with a zero lag fourth order Butterworth filter (band-pass 20–450 Hz) and amplitude analysis was performed using a root mean square-based envelope (root mean square of a non-overlapping 20 ms rectangular window). The signals were cut and time-normalized for each sit-to-stand (one cycle) using the electrogoniometer signals during 1STS. Mean of sEMG envelopes was calculated for each cycle. Frequency analysis was performed on digitally filtered data prior to root mean square envelope and time normalization steps. Power spectral median frequency was calculated for each cycle and time duration of each cycle was extracted.
Sample size calculation was based on Ribeiro et al.13, to detect a quadriceps Δ deoxy [Hb-Mb] difference between people with COPD and healthy controls of 15 ± 18 % at the end of 1STS, with a power of 80% and an alpha of 5%. With this method, the sample size was calculated to be 14 participants in each group. Descriptive data was expressed as mean ± standard deviation, and statistical significance was considered at p < 0.05. All variables were tested for normality using Shapiro-Wilks test. Non-normally distributed data were log-transformed before analysis. Paired t-tests were used to evaluate between-group differences in baseline characteristics, 1STS performance, peak cardiorespiratory responses and symptom perception during 1STS. Two-way repeated measure ANOVA, with Holm-Bonferroni post hoc corrections, were used for within-group quadriceps oxygenation and sEMG data across time points. A mixed model analysis was performed with an interaction term between groups and time. SigmaPlot 11.0 (Systat Software, San Jose, California) was used for statistical analyses.
Ethics approval and consent to participate.
The study was conducted in accordance with the declaration of Helsinki and was approved by the Comité d’éthique de l’Institut universitaire de cardiologie et de pneumologie de Québec, Université Laval (CER: 21539). All participants received written and verbal information about the study and gave their written informed consent before the study commenced.