The aging process is accompanied by a decrease in several biological and physical functions, which have an impact on the perception of well-being and quality of life. Among these, the decrease in muscle mass, strength and power are of particular importance as they are associated with poor cardiorespiratory function, functional limitations and mortality. The muscle quality index (MQI) is considered a key determinant of muscle function in adulthood. The main treatment to preserve muscle mass has been strength exercise and in some cases the use of aids that influence protein metabolism such as beta-hydroxy-beta-methyl butyrate (HMB), but the evidence on this topic is not consistent and is inconclusive. Therefore, the main purpose of this study is to investigate if the combination of a power/resistance exercise program and oral supplementation with HMB can improve the quality mass index in adults aged compared with those received a single exercise program after 14 weeks of intervention.

Study design: this is a 14-week, randomized controlled trial. 76 participants will be recruited from the medical services of the National Institute of Rehabilitation (NIR).

Participants: Adults between 65-75 years old, with a body mass index between 25-30 kg/m^2, sedentaries, non-smokers, with compensated comorbidities, and who do not consume nutritional supplements will be recruited. Participants with uncompensated metabolic diseases, with high risk of falls (evaluated with Tinetti scale), frailty syndrome, and b2-adrenergic agonists, glucocorticoids, diuretics or peripheral vasodilators users will not be recruited.

Intervention: Participants will be randomized to 1) power/resistance exercise program (GEx), 2) power/resistance exercise program + HMB group (GExHMB) with computer-generated random numbers. The exercise program will consist of training to improve muscular strength, neuromotor skills, flexibility, endurance, and a greater emphasis on maximum strength and power. Every training session will consist of 15 minutes of warm-up, 40 to 60 minutes of power/resistance training, neuromuscular and aerobic exercises, and 15 minutes of cool-down. The power/resistance and neuromuscular training will be performed 3 days/week, 2-5 sets, 5-15 repetitions for each exercise according to the recommendation of the American College of Sports Medicine training progression models; the aerobic exercise will be performed 2 days/week. All exercises will be performed at moderate intensity and will take place in the NIR Sports Medicine therapeutic gym.

Oral supplementation with HMB will be from a commercial polymeric formula in liquid form (Ensure Advanced®, 230 ml); the supplement intake will be 1 bottle/day. All patients, regardless of the intervention, will be monitored by sports medicine physicians and nutritionists in order to promote adherence to treatment.

The muscle quality index (MQI) will be obtained from the determination of muscular strength by isokinetic dynamometry (at 60°/s) for the lower extremities and by hand dynamometry for the upper extremities; the lean segmental mass in the upper and lower extremities will be determined by 3-frequency electronic bioimpedance analysis. The MQI will be calculated from the muscular strength divided by the muscular mass.

Lower extremities muscular power will be determined by isokinetic dynamometry at 180°/s, physical performance will be assessed by the Short Physical Performance battery (SPPB) and quality of life perception will be evaluated by the EuroQoL-5D questionnaire. As part of the safety measures for the participants, liver function will be monitored by laboratory tests (liver enzyme profile and general urine test) which will be performed at the NIR laboratory.

For each participant the study length will be 14 weeks with 61 exercise sessions for the GEx and GExHMB groups and 98 days of oral supplementation for the GExHMB group. The beginning of the study corresponds to session 1 of exercise and day 1 of supplementation intake. Initial evaluations will be performed 5 days prior to the start of the intervention, while final evaluations will be conducted at the end of 14 weeks of follow-up (2 days after the last day of the intervention). Data will be recorded by study investigators blinded to participant assignment.

Comparison: Comparison group will receive the unique intervention of a strength/resistance exercise program.

Outcomes: The primary outcome is the upper and lower extremities MQI mean change. The secondary outcomes are mean changes in muscular power, physical performance, and health related quality of life.

Inclusion Criteria:

• Body mass index 25-30 km/m^2
• Appendicular skeletal muscle mass index in women ≥ 5.0 kg/m^2 and men ≥ 6.0 kg/m^2
• No regular exercise
• Non-smokers
• Compensated comorbidities
• Without consumption of nutritional supplements

Exclusion Criteria:

• Osteoporosis
• Cardiovascular diseases (recent heart attack, unstable angina, heart failure, complete atrioventricular block)
• Musculoskeletal injuries
• Severe cognitive impairment
• Major depressive disorder
• Thyroid diseases
• Anemia with no response to previous treatment in the last 3 months
• b2-adrenergic agonists, glucocorticoids, diuretics or peripheral vasodilators users
• With amino acid and vitamin D supplementation or suspended with less than 3 months
• Data of renal insufficiency
• Chronic obstructive pulmonary disease
• Malabsorption syndrome associated with malnutrition
• High risk of falls evaluated with the Tinetti scale (score ≤19)
• Frailty syndrome