Background: Individuals with PD commonly have a significant reduction in respiratory muscle strength and inspiratory muscle endurance, and it can intensify with the disease progression. Respiratory muscle training has shown to increase respiratory muscle strength in individuals with Parkinson's Disease (PD). However, the effect size on other functional outcomes has not been determined and/or investigated. In addition, no studies have investigated the effects of high-intensity respiratory muscle training (inspiratory and expiratory) in this population.

Objectives: The primary aim of this study will be to investigate the effects of high-intensity respiratory muscle training on inspiratory and expiratory muscle strength in individuals with PD. The secondary aim of this study will be to investigate the efficacy of high-intensity respiratory muscle training in improving inspiratory muscle endurance, peak cough flow, dyspnea, fatigue, exercise capacity, and quality of life in this population.

Design: A randomized controlled trial with blinded assessment will assign eligible participants to either: high-intensity respiratory muscle training (experimental group) or sham training (control group). Individuals will perform a home-based intervention, not directly supervised, consisted of two daily 20-min sessions (morning and afternoon), seven times a week, during eight weeks.

Study Outcomes: Primary outcomes are inspiratory and expiratory muscle strength (MIP and MEP). Secondary outcomes are respiratory muscle endurance, peak cough flow, dyspnea, fatigue, exercise capacity, and quality of life.The outcomes will be measured at baseline, post-intervention (after the 8-week intervention), and one month after the cessation of the intervention (12-week follow-up).

Conclusion: The results of this trial will provide valuable new information on the efficacy of high-intensity respiratory muscle training in improving muscle strength and other functional outcomes in individuals with PD.

Sample size calculation: The sample size calculation was performed considering the primary outcome measures (inspiratory and expiratory muscle strength). The effect size for inspiratory muscle training was derived from a RCT with a similar population and intervention. Considering a significance level (α) of 5% and a power of 0.80, thirteen participants per group are required (a total of 26 participants). The effect size for expiratory muscle training was also derived from a RCT with a similar population and intervention. Considering a significance level (α) of 5% and a power of 0.80, fourteen participants per group are required (a total of 28 participants). Therefore, a sample size of 28 individuals (14 in each group) was defined (largest sample size calculated). Assuming an expected dropout rate of 20%, a total sample size of 34 individuals was set (17 in each group).

Statistical analyzes: All statistical analyzes will be performed by an independent examiner, blinded to the group allocation. Each participant will assigned a unique code. All analyzes will be performed using SPSS (SPSS Inc., Chicago, IL, USA). The normality of data distribution will be for all continuous numeric variables. Descriptive statistics will be calculated for all outcomes.The effects of the interventions will be analyzed from the collected data using intention-to-treat. Data from the last available assessment will be used for missed sessions. Two-way ANOVA with repeated measures (2*3) will be used to evaluate the differences between groups, considering the time factor (considering baseline, post-intervention, and 4-week follow-up), for the variables: inspiratory and expiratory muscle strength, respiratory muscle endurance, peak cough flow and exercise capacity. The level of significance will be set at 5% and adjusted for multiple comparisons. Data distribution and equality of variance will also be analyzed, to ensure the parametric analysis has been applied correctly. The effect sizes will be calculated to determine the magnitude of the differences between the groups. The differences between the two mean values will be expressed in units of their SD, expressed as Cohen's d, or mean results for the experimental group minus the mean results for the control group, divided by the SD of the control group. Effect sizes between 0.2 and 0.5 will be considered small; between 0.5 and 0.8, medium; and above 0.8, large.

Inclusion Criteria:

• Parkinson's disease diagnosed by a neurologist
• Ability to walk independently, with or without assistive devices
• Taking anti parkinsonian medication, and who have been medically stable for at least six months
• Classified between stages 1-3 of the modified Hoehn & Yahr Scale (Schenkman et al., 2001)
• Maximum inspiratory pressure less or equal to -80 cmH2O or maximum expiratory pressure less or equal to 90 cmH2O

Exclusion Criteria:

• Possible cognitive impairment as determined by cutoff scores (in points) of the Mini-Mental Status Examination
• Use deep brain stimulation (DBS)
• Smokers or who stopped smoking less than six months ago; have been affected by respiratory or cardiac infections in the last month
• Had any other neurological, musculoskeletal, cardiovascular or respiratory disorders that could affect their ability to perform the tests

Data collection and analysis will be carried-out by a researcher, blinded to the group allocation. In order to maintain the participants blinded to the training load, the device will be covered with opaque material.