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.