Abstract: Authors identified 19 studies, of which nine studies with 202 participants met the review's inclusion criteria. There was wide variation in the methodological and written quality of the included studies. Four of the nine included studies were published as abstracts only and lacking concise details, thus limiting the information available. Seven studies were parallel studies and two of a cross-over design. Respiratory muscle training interventions varied dramatically, with frequency, intensity and duration ranging from thrice weekly to twice daily, 20% to 80% of maximal effort, and 10 to 30 minutes, respectively. Participant numbers ranged from 11 to 39 participants in the included studies; five studies were in adults only and four in a combination of children and adults. No significant improvement was reported in the primary outcome of pulmonary function (forced expiratory volume in one second and forced vital capacity) (very low-quality evidence). Although no change was reported in exercise capacity as assessed by the maximum rate of oxygen use, a 10% improvement in exercise duration was found when working at 60% of maximal effort in one study (n = 20) (very low-quality evidence). In a further study (n = 18), when working at 80% of maximal effort, health-related quality of life improved in the mastery and emotion domains (very low-quality evidence). With regards to the review's secondary outcomes, one study (n = 11) found a significant change in intramural pressure, functional residual capacity and maximal inspiratory pressure following training (low-quality evidence). A further study (n = 22) reported that respiratory muscle endurance was significantly longer in the training group (P < 0.01). No studies reported on any other secondary outcomes. Meta-analyses could not be performed due to a lack of consistency and insufficient detail in reported outcome measures. There is insufficient evidence to suggest whether this intervention is beneficial or not. Healthcare practitioners should consider the use of respiratory muscle training on a case-by-case basis. Further research of reputable methodological quality is needed to determine the effectiveness of respiratory muscle training in people with cystic fibrosis. Researchers should consider the following clinical outcomes in future studies; respiratory muscle function, pulmonary function, exercise capacity, hospital admissions, and health-related quality of life. Sensory-perceptual changes, such as respiratory effort sensation (e.g. rating of perceived breathlessness) and peripheral effort sensation (e.g. rating of perceived exertion) may also help to elucidate mechanisms underpinning the effectiveness of respiratory muscle training.

Plain-language summary: We searched for studies where people with cystic fibrosis were put into either a group for respiratory muscle training or a control group at random. We included nine studies with 202 people which used a wide variety of training methods and levels. In seven of the studies, the treatment group and the control group each only received either respiratory muscle training or a control treatment (one study had three groups in total: one receiving control treatment and two receiving different levels of training). In one study the participants received both types of treatment, but in a random order. Lastly, one study compared training with usual care. The studies lasted for a maximum of 12 weeks and all were quite small; the largest only had 29 people taking part. The studies included people with a range of ages over six years old, but most seemed to be adults. The studies reported a variety of outcomes. All reported some measure of respiratory muscle strength, and most reported at least one measure of lung function, however only three studies reported on quality of life. Results could not be combined to answer the review question, because the studies either did not publish enough details or did not use the same standard measurements. No study found any difference in lung function after training, but one of the studies reported an improvement in exercise duration when training at 60% of maximal effort and a further study which trained participants at 80% of maximal effort reported some improvements in quality of life judgements. There was some evidence of an improvement in respiratory muscle function in one study. Given this lack of information, a recommendation for or against respiratory muscle training cannot be made. Future studies should look to improve upon the methods of those previously conducted, and should report using standardised measurements. It was generally unclear how people were split into groups for treatment and whether this would have affected the results. Two studies stated that the people assessing the outcomes did not know which treatment those taking part had received, but this was unclear in other studies. Individuals dropped out of three of the studies for reasons which may be directly related to the treatment and therefore may introduce a risk of bias to the results. Other studies did not state how many people dropped out of them. We assessed the quality of the evidence and judged the evidence for lung function, exercise capacity and health-related quality of life to be very low quality, but the evidence for respiratory muscle function to be low quality.