The primary objective of the study is to evaluate the effects of exercise on physical function, physical fitness, and body composition, with the main components including muscular strength, cardiorespiratory fitness, muscle mass, fat mass, and fat-free mass. The secondary objectives are to examine exercise adherence and the effects of exercise on health-related quality of life (HRQoL), cancer-related fatigue, and sleep quality. Additionally, the feasibility and safety of the exercise program will be assessed.

Historically, cancer patients, especially those in more advanced stages of the disease, were advised to maintain rest due to concerns about complications associated with tumor progression and the adverse effects of treatment. However, evidence accumulated over the past decades has demonstrated that physical inactivity may accelerate functional decline, reduce physical capacity, and impair quality of life in individuals with cancer. In this context, physical exercise has come to be recognized as an important strategy in cancer care. The regular practice of aerobic and resistance exercises has been associated with the reduction of treatment-related side effects, the preservation of physical function, and improvements in quality of life in different populations of cancer patients. In women with breast cancer, studies have shown that physical exercise can contribute to improvements in cardiorespiratory capacity, muscular strength, physical function, and quality of life, as well as help reduce cancer-related fatigue. However, most of the available evidence focuses on patients in the early stages of the disease or after primary treatment. In the case of women with metastatic breast cancer, scientific knowledge remains limited. Despite therapeutic advances that have increased survival in this population, these patients frequently experience functional decline, severe fatigue, loss of muscle mass and strength, and reduced quality of life. Furthermore, physical exercise is still rarely incorporated into the care of these patients, often due to uncertainties regarding the safety and effectiveness of such interventions. Although emerging evidence suggests that physical exercise may be safe and feasible for patients with metastatic disease, studies investigating structured and long-term exercise interventions in this population remain scarce. Therefore, further research is necessary to expand understanding of the effects of physical exercise in women with metastatic breast cancer, particularly focusing on physical function, physical fitness, body composition, and quality of life.

Inclusion Criteria:

• Women aged ≥ 18 years;
• Confirmed diagnosis of stage IV (metastatic) breast cancer;
• Currently undergoing medical treatment;
• Willingness and functional independence to participate in the exercise program;
• Medical clearance to engage in structured physical exercise.

Exclusion Criteria:

• Presence of unstable bone metastases.
• Untreated or symptomatic brain metastases;
• Evidence of severe cardiovascular disease identified by electrocardiogram (ECG);
• Severe active infection;
• Uncontrolled severe respiratory insufficiency or dependence on supplemental oxygen at rest or during exercise;
• Uncontrolled severe pain;
• Any other medical condition contraindicating participation in physical exercise;
• Conditions that impair adherence to study procedures or the ability to provide informed consent;
• Pregnancy.

Eligibility was based on biological sex characteristics relevant to breast cancer, as this condition is directly related to the presence of breast tissue and exposure to specific hormonal factors that may influence both disease progression and responses to the proposed interventions. Therefore, inclusion criteria were defined based on essential biological characteristics to ensure sample homogeneity and the validity of the study outcomes. Gender identity was respected and recorded; however, eligibility criteria were determined by the biological characteristics required for the study.

On reasonable request

Participants and investigators were not blinded to group assignment, given the nature of the intervention.