This study aims to evaluate the effects of an 8-week inspiratory muscle training program on diaphragmatic and respiratory muscle function in frail older adults aged 80 years and above. Frailty is associated with reduced physical capacity, impaired respiratory performance, and a higher risk of disability. Inspiratory muscle training is a simple, low-cost intervention that may improve breathing function, strength, and overall health in very old adults, but evidence in this age group remains limited.

Participants will be randomly assigned to one of two groups: a high-load inspiratory muscle training group or a low-load sham training group. Both groups will use a threshold device and perform 30 breaths once per day, five days per week, for eight weeks. The experimental group will train with progressively increasing resistance (50% to 80% of maximal inspiratory pressure), while the sham group will use a minimal and non-progressive load (15% of maximal inspiratory pressure). All sessions will be supervised and monitored for safety.

The main goal of the study is to determine whether inspiratory muscle training improves maximal inspiratory pressure (MIP), an established measure of global inspiratory muscle strength. Secondary outcomes include ultrasound-based assessments of diaphragmatic structure and function, such as diaphragm thickness, contractile fraction, diaphragmatic excursion, inspiratory time, and inspiratory velocity during deep breathing and sniff maneuvers. These measurements will help clarify the physiological adaptations produced by inspiratory muscle training in this population.

Assessments will be performed before the intervention, immediately after the 8-week program, and again at one and three months after the end of the intervention to examine both immediate and short-term effects. This study may contribute valuable evidence regarding the safety, feasibility, and clinical benefits of inspiratory muscle training in frail very old adults, supporting its potential implementation in rehabilitation and geriatric care.

Inclusion Criteria:

• Adults aged 80 years or older.
• Clinical diagnosis of frailty, defined by a Short Physical Performance Battery (SPPB) score < 9.
• Ability to stand and walk with or without assistive devices.
• Ability to understand and follow instructions for inspiratory muscle training.
• Stable medical condition for at least 3 months prior to enrollment.
• Capacity to provide informed consent or availability of a legal representative.

Exclusion Criteria:

• Acute or unstable cardiovascular, respiratory, or metabolic conditions.
• Severe cognitive impairment that prevents understanding the procedures.
• Diagnosis of neuromuscular diseases affecting respiratory muscles (e.g., ALS, myopathies).
• Severe or uncontrolled hypertension (≥180/110 mmHg).
• Recent thoracic or abdominal surgery (<3 months).
• Severe musculoskeletal disorders limiting participation in training.
• History of recurrent syncope, severe dizziness, or intolerance to respiratory maneuvers.
• Any condition judged by the research team to compromise safety or participation.

At this time, it is unclear whether individual participant data (IPD) will be shared. Data sharing will depend on institutional policies, ethical approvals, participant privacy considerations, and the feasibility of preparing de-identified datasets. A final decision will be made once data collection and analysis are completed.