• Exercise combined with specific exercises to strengthen breathing muscles may not improve breathlessness, physical fitness and life quality. Strength of breathing muscles and endurance increased but not enough to make a difference to patients.
• Specific exercises to strengthen breathing muscles compared to no exercise may improve breathlessness, physical fitness and life quality. Strength of breathing muscles and endurance increased, but we don't know if this benefitted patients.
• We don't know whether exercise or specific exercises to strengthen breathing muscles is better for people with weakened breathing muscles who trained for several weeks.
• Future research should focus on people with weakened breathing muscles and studies should include more people.
What is chronic obstructive pulmonary disease (COPD)?
Chronic obstructive pulmonary disease (COPD) is a lung condition characterized by blockages in the airways, which cause shortness of breath and a cough. It appears after the long-term inhalation of irritating gases like cigarette smoke and chemicals. Training and strengthening the breathing muscles is thought to improve breathing and reduce air obstruction.
What exercise treatments do people with COPD use?
Health professionals use various exercises to help improve people's COPD.
• Some people undertake a program of general exercise and education to help reduce symptoms and improve their exercise capacity and life quality.
• Other people try to improve the strength and endurance of the breathing muscles through a series of breathing exercises using specific devices. This is called 'inspiratory muscle training' (IMT). The devices add resistance to breathing to strengthen the diaphragm and the intercostal muscles between the ribs - the muscles used for breathing. People may then be able to breathe in more air with each breath and be active for longer. The devices are also used by people with healthy lungs to improve their sports performance.
What did we want to find out?
We wanted to find out if exercise combined with IMT compared to exercise alone, and IMT compared to no exercise or sham IMT has a better effect on breathlessness, physical fitness and life quality. (A sham device has no effect on breathing muscles. It allows a fair test of the real devices, because people don't know which they are using.)
We also wanted to check whether IMT was associated with any unwanted effects.
What did we do?
We searched for studies that compared
• exercise combined with IMT with exercise alone; and
• IMT with no exercise or sham IMT.
We compared and summarized the results of the studies and rated our confidence in the evidence, based on factors such as study methods and sizes.
What did we find?
Exercise plus IMT compared with exercise alone
We found 22 studies with 1446 participants, which lasted between 2 and 24 weeks. Exercise ranged from training only on a treadmill, with only a cycle, and a combination of exercises (training with a cycle and treadmill, muscle strengthening, stair climbing, and education). The duration and devices of IMT also varied across the studies.
We found that this combination:
• probably makes little to no difference to breathlessness (measured with different scales);
• has an unknown effect on physical fitness;
• may make little to no difference to life quality (measured with different scales);
• probably makes little to no difference to strength of breathing muscles.
IMT versus no training or sham device
We found 37 studies with 1021 participants, which lasted from 2 weeks to a year. IMT varied across the studies regarding devices, resistance, frequency and supervision.
We found out that IMT alone:
• may reduce breathlessness measured with one scale, but it is unclear if it has an effect when measured with two other scales;
• probably improves physical fitness;
• probably improves life quality when measured with one scale, but it is unclear if it has a benefit when measured with another one;
• may make little to no difference to strength of breathing muscles.
What are the limitations of the evidence?
The studies used different training durations, resistance, devices, number and frequency of sessions, and physical training programs. This makes it hard to draw firm conclusions. Overall our confidence in the conclusions is reduced because the studies were small, some participants may have been aware of which treatment they were receiving, and generally, there was some diversity in the studies.
How up to date is the evidence?
The evidence is up-to-date to 20 October 2022.
IMT may not improve dyspnea, functional exercise capacity and life quality when associated with PR. However, IMT is likely to improve these outcomes when provided alone.
For both interventions, a larger effect in participants with respiratory muscle weakness and with longer training durations is still to be confirmed.
Inspiratory muscle training (IMT) aims to improve respiratory muscle strength and endurance. Clinical trials used various training protocols, devices and respiratory measurements to check the effectiveness of this intervention. The current guidelines reported a possible advantage of IMT, particularly in people with respiratory muscle weakness. However, it remains unclear to what extent IMT is clinically beneficial, especially when associated with pulmonary rehabilitation (PR).
To assess the effect of inspiratory muscle training (IMT) on chronic obstructive pulmonary disease (COPD), as a stand-alone intervention and when combined with pulmonary rehabilitation (PR).
We searched the Cochrane Airways trials register, CENTRAL, MEDLINE, Embase, PsycINFO, Cumulative Index to Nursing and Allied Health Literature (CINAHL) EBSCO, Physiotherapy Evidence Database (PEDro) ClinicalTrials.gov, and the World Health Organization International Clinical Trials Registry Platform on 20 October 2022. We also checked reference lists of all primary studies and review articles.
We included randomized controlled trials (RCTs) that compared IMT in combination with PR versus PR alone and IMT versus control/sham. We included different types of IMT irrespective of the mode of delivery. We excluded trials that used resistive devices without controlling the breathing pattern or a training load of less than 30% of maximal inspiratory pressure (PImax), or both.
We used standard methods recommended by Cochrane including assessment of risk of bias with RoB 2. Our primary outcomes were dyspnea, functional exercise capacity and health-related quality of life.
We included 55 RCTs in this review. Both IMT and PR protocols varied significantly across the trials, especially in training duration, loads, devices, number/ frequency of sessions and the PR programs. Only eight trials were at low risk of bias.
PR+IMT versus PR
We included 22 trials (1446 participants) in this comparison. Based on a minimal clinically important difference (MCID) of −1 unit, we did not find an improvement in dyspnea assessed with the Borg scale at submaximal exercise capacity (mean difference (MD) 0.19, 95% confidence interval (CI) −0.42 to 0.79; 2 RCTs, 202 participants; moderate-certainty evidence).
We also found no improvement in dyspnea assessed with the modified Medical Research Council dyspnea scale (mMRC) according to an MCID between −0.5 and −1 unit (MD −0.12, 95% CI −0.39 to 0.14; 2 RCTs, 204 participants; very low-certainty evidence).
Pooling evidence for the 6-minute walk distance (6MWD) showed an increase of 5.95 meters (95% CI −5.73 to 17.63; 12 RCTs, 1199 participants; very low-certainty evidence) and failed to reach the MCID of 26 meters. In subgroup analysis, we divided the RCTs according to the training duration and mean baseline PImax. The test for subgroup differences was not significant. Trials at low risk of bias (n = 3) demonstrated a larger effect estimate than the overall.
The summary effect of the St George's Respiratory Questionnaire (SGRQ) revealed an overall total score below the MCID of 4 units (MD 0.13, 95% CI −0.93 to 1.20; 7 RCTs, 908 participants; low-certainty evidence).
The summary effect of COPD Assessment Test (CAT) did not show an improvement in the HRQoL (MD 0.13, 95% CI −0.80 to 1.06; 2 RCTs, 657 participants; very low-certainty evidence), according to an MCID of −1.6 units.
Pooling the RCTs that reported PImax showed an increase of 11.46 cmH2O (95% CI 7.42 to 15.50; 17 RCTs, 1329 participants; moderate-certainty evidence) but failed to reach the MCID of 17.2 cmH2O. In subgroup analysis, we did not find a difference between different training durations and between studies judged with and without respiratory muscle weakness.
One abstract reported some adverse effects that were considered "minor and self-limited".
IMT versus control/sham
Thirty-seven RCTs with 1021 participants contributed to our second comparison. There was a trend towards an improvement when Borg was calculated at submaximal exercise capacity (MD −0.94, 95% CI −1.36 to −0.51; 6 RCTs, 144 participants; very low-certainty evidence). Only one trial was at a low risk of bias.
Eight studies (nine arms) used the Baseline Dyspnea Index - Transition Dyspnea Index (BDI-TDI). Based on an MCID of +1 unit, they showed an improvement only with the 'total score' of the TDI (MD 2.98, 95% CI 2.07 to 3.89; 8 RCTs, 238 participants; very low-certainty evidence). We did not find a difference between studies classified as with and without respiratory muscle weakness. Only one trial was at low risk of bias.
Four studies reported the mMRC, revealing a possible improvement in dyspnea in the IMT group (MD −0.59, 95% CI −0.76 to −0.43; 4 RCTs, 150 participants; low-certainty evidence). Two trials were at low risk of bias.
Compared to control/sham, the MD in the 6MWD following IMT was 35.71 (95% CI 25.68 to 45.74; 16 RCTs, 501 participants; moderate-certainty evidence). Two studies were at low risk of bias. In subgroup analysis, we did not find a difference between different training durations and between studies judged with and without respiratory muscle weakness.
Six studies reported the SGRQ total score, showing a larger effect in the IMT group (MD −3.85, 95% CI −8.18 to 0.48; 6 RCTs, 182 participants; very low-certainty evidence). The lower limit of the 95% CI exceeded the MCID of −4 units. Only one study was at low risk of bias.
There was an improvement in life quality with CAT (MD −2.97, 95% CI −3.85 to −2.10; 2 RCTs, 86 participants; moderate-certainty evidence). One trial was at low risk of bias.
Thirty-two RCTs reported PImax, showing an improvement without reaching the MCID (MD 14.57 cmH2O, 95% CI 9.85 to 19.29; 32 RCTs, 916 participants; low-certainty evidence). In subgroup analysis, we did not find a difference between different training durations and between studies judged with and without respiratory muscle weakness.
None of the included RCTs reported adverse events.