The goal of this clinical trial is to learn how muscle capillaries influence the response to resistance exercise training in older adults. The study will include older men and women who may experience age-related declines in muscle mass and function. The main questions it aims to answer are:

• Does baseline muscle capillarization influence the increase in muscle size and function following resistance exercise training?
• Can resistance exercise stimulate the formation of new capillaries in older adults with lower baseline capillarization, and how is this related to muscle growth?

Participants will:

• Undergo assessments of muscle structure, capillarization, and physical function before and after the intervention.
• Participate in a supervised resistance exercise training program.
• Provide muscle samples and other physiological measurements to evaluate changes in muscle growth and blood vessel formation.

Researchers will analyze the relationship between baseline muscle capillarization, exercise-induced angiogenesis, and improvements in muscle mass and function to better understand why some older adults respond more effectively to resistance exercise than others.

This randomized clinical trial will investigate the effects of different exercise training strategies on skeletal muscle adaptations in older adults. Participants will be randomly assigned to one of three training groups and will complete a supervised 12-week exercise program, consisting of three sessions per week.

The interventions will include different combinations and sequencing of resistance and aerobic exercise during the first 8 weeks of training, followed by a standardized resistance training program during the final 4 weeks. The study is designed to determine whether the timing and type of exercise influence muscle growth, vascular adaptations, and functional outcomes.

Participants will undergo assessments before the intervention and after 4, 8, and 12 weeks of training. Measurements will include body composition, muscle size and architecture, muscle strength, aerobic fitness, and physical function. A subgroup of participants may also undergo optional muscle biopsies to evaluate changes in muscle capillarization and biological markers associated with muscle adaptation.

The results will help identify exercise strategies that optimize muscle and vascular adaptations in older adults and improve understanding of the mechanisms underlying individual variability in training responsiveness.

Inclusion Criteria:

• Age ≥ 65 years
• Body Mass Index (BMI) >18 and <30 kg/m²
• Stable body weight for at least 3 months prior to enrollment

Exclusion Criteria:

• Prediabetes or diabetes according to ADA criteria
• Chronic diseases, including cardiovascular, liver, respiratory diseases, or cancer
• Acute inflammatory conditions, infections, or other clinically significant illnesses
• Regular participation in vigorous physical activity (≥2 sessions per week)
• Current or recent (within the previous 3 months) treatment with steroid medications
• Regular use of protein supplements