The superiority of supervised center-based training programs compared with unsupervised home-based ones in older adults remains unclear, and no evidence exists on whether including a motivational component could moderate these differences. The present randomized controlled trial aims to determine the role of supervision and motivational strategies on the safety, adherence, efficacy, and cost-effectiveness of different training programs for improving physical and mental health in older adults. Participants (n=120, aged 60-75 years old) will be randomly divided into five groups: 1- Control group, 2- Unsupervised home-based exercise group without motivational intervention (UNSUP), 3- Unsupervised home-based exercise group with motivational intervention (UNSUP+), 4- Supervised center-based exercise group without motivational intervention (SUP) and 5- Supervised center-based exercise group with motivational intervention (SUP+). Participants assigned to the exercise groups will participate in a 24-week multicomponent exercise program (3 sessions/week, 60 min/session), while participants in the control group will be asked to maintain their usual lifestyle. Physical and mental health outcomes will be assessed, including lower and upper-body muscular function, physical function, cardiorespiratory function, anthropometry and body composition, health-related quality of life, cognitive performance, anxiety and depression status, physical activity and sedentary behavior, sleep, biochemical markers, motivators and barriers to exercise, individual's psychological needs, and level of self-determination. Assessments will be conducted at baseline (week 0), mid-intervention (week 12), at the end of the intervention period (week 25), and 24 weeks after the exercise intervention (week 48).

Life expectancy for the Spanish population has increased to 85 years for women and 79 years for men, and it is estimated that Spain will be the second oldest country in the world by 2050. This fact contains an inevitable economic and public health challenge. Physical exercise is an effective intervention to attenuate the aging-related decline in physical function and well-being as well as for reducing morbidity and mortality risk. Current guidelines on physical activity (PA) by the World Health Organization (WHO) recommend that older adults perform at least 150-300 min/week of moderate-intensity aerobic PA, 75-150 min/week of vigorous-intensity aerobic PA, or a combination of both, as well as at least 3 days of strength and balance exercise. However, a large part of the older adult population does not meet the general recommendations for PA. This could be at least partly due to the fact that this type of population may have some limitations when starting a supervised center-based physical training program. In this regard, evidence overall shows that supervised center-based training programs are usually limited by issues such as economic constraints, convenience, and access or time commitments. Moreover, there are unpopulated areas comprising a large percentage of older people without direct access to facilities and qualified professionals to engage in effective physical activity. Besides that, the COVID-19 pandemic has opened an opportunity to carry out training programs from home without the supervision of a professional. If effective and safe, these interventions would represent an alternative to increasing the accessibility of physical exercise for older adults.

To date, there is a lack of consensus on the effectiveness, safety, and adherence of unsupervised home-based training programs. Several investigations have shown greater effectiveness on different health variables in exercise programs conducted under the supervision of a professional compared to those performed autonomously at home. However, some studies suggest that a home-based exercise intervention could be as effective as a supervised one. Furthermore, meta-analytical evidence recently published by the research team of this project found that the adherence rate to unsupervised physical exercise programs was low, and the intensity applied was not adequately prescribed. It is important to note that the lack of motivation in unsupervised programs might play an important role in these findings. Thus, the use of motivational strategies that foster autonomous motivation might help increase adherence to unsupervised exercise programs, with this adherence being a key factor for achieving health adaptations.

Therefore, studies analyzing how to cover all these limitations of unsupervised physical exercise are warranted to answer this research question. Along these lines, an adequate prescription of exercise dose and the implementation of motivational techniques could compensate for the lack of physiological stimulus and low adherence typically observed with this type of training program. Furthermore, there is no evidence comparing the cost-effectiveness, safety, and adherence of supervised face-to-face intervention vs online supervised intervention with and without motivational strategies. Under this context, the aim of this randomized controlled trial (RCT) will be to determine the efficacy, cost-effectiveness, safety, and adherence of different exercise training programs with or without supervision and the inclusion of motivational techniques in older adults.

Participants (n= 120; men and women aged 60-75 years old) will be divided into 5 groups: 1- Control group, 2- Unsupervised home-based exercise group without motivational intervention (UNSUP), 3- Unsupervised home-based exercise group with motivational intervention (UNSUP+), 4- Supervised center-based exercise group without motivational intervention (SUP) and 5- Supervised center-based exercise group with motivational intervention (SUP+). Exercise groups will perform physical exercise 3 days a week (1-hour sessions) for 24 weeks, with these sessions being performed from home (with the help of a mobile application that will be developed for this purpose) or at a center with the direct supervision of an exercise professional, according to the assigned group. The primary outcome will be lower-body muscular function. Secondary outcomes will be upper-body muscular function, physical function, cardiorespiratory function, anthropometry and body composition, health-related quality of life, cognitive performance, anxiety and depression status, physical activity and sedentary behavior, sleep, biochemical markers, motivators and barriers to exercise, individual's psychological needs, and level of self-determination. Further, an analysis of the costs of the programs (cost-effectiveness and cost-utility), adherence (rate of adherence to the exercise program), and safety (falls and adverse events) will be carried out.

Inclusion Criteria:

• People aged between 60 and 75 years old.
• Being able to speak and read fluent Spanish.
• Being able to walk independently.
• Being physically able to participate in an exercise program.
• Having a smartphone and being able to understand and use mobile applications autonomously.

Exclusion Criteria:

• Acute or terminal illness.
• Myocardial infarction, coronary artery bypass grafting, angioplasty, angina, or other cardiac condition in the past year.
• Uncontrolled medical problems that the general practitioner considers would preclude patients from undertaking the exercise program (e.g., acute systemic illness such as pneumonia, acute rheumatoid arthritis, and acute or unstable heart failure).
• Conditions requiring a specialized physical exercise program (e.g., uncontrolled epilepsy, significant neurological disease or impairment, inability to maintain an upright seated position or unable to move independently, multiple sclerosis, cancer, Parkinson's, Alzheimer's, or chronic obstructive pulmonary disease).
• General practitioner-diagnosed hypertension that has not been controlled.
• Type I Diabetes or uncontrolled Type II Diabetes.
• History of major psychiatric illness including schizophrenia, generalized anxiety disorder, or depression according to the DSM-5.
• Morbid obesity (body mass index >39)
• Three or more self-reported falls in the last year.
• Not living in the community (e.g., living in nursing homes).
• Having participated in an exercise program during the 6 months prior to the study.
• Diagnosis of COVID-19 with hospitalization in intensive care unit.
• Any other consideration that interferes with the study aims and could be a risk to the participant, at the discretion of the researcher and the general practitioner.