The aim of the entry is the elaboration of a systematic review of existing research on physical fitness, self-efficacy for physical exercise, and quality of life in adulthood. Method: Using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement guidelines, and based on the findings in 493 articles, the final sample was composed of 37 articles, which were reviewed to show whether self-efficacy has previously been studied as a mediator in the relationship between physical fitness and quality of life in adulthood. Results: The results indicate that little research exists in relation to healthy, populations with the majority being people with pathology. Physical fitness should be considered as a fundamental aspect in determining the functional capacity of the person. Aerobic capacity was the most evaluated and the 6-min walk test was the most used. Only one article shows the joint relationship between the three variables. Conclusions: We discuss the need to investigate the mediation of self-efficacy in relation to the value of physical activity on quality of life and well-being in the healthy adult population in adult life.
Adulthood is a period of the life cycle that differs widely due to socio-economic, labor, and cultural conditions. Although it can cover a wide range of ages, current scientific convention specifies an age span that begins between the ages of 40–45 and ends between the ages of 60–65, at which point we can speak of the beginning of old age . During the process of adult maturity, important body changes take place or have already taken place, such as menopause and andropause, which involve diverse psychological impacts and, frequently, physiological changes. A loss of bone mass, for example, reduces the strength of the body, making it more vulnerable an injury or disease in daily life . People are not always aware of these changes . Recently, although there seems to be some interest among the population in understanding the keys to maintaining health and quality of life and to face the decline or deterioration that occurs in old age with better physical and mental health , the sedentary life continues to affect a wide range of the adult population .
A review study  indicated that moderate and systematic physical activity is one of the factors that most affects quality of life. During childhood and adolescence, physical activity is academically programmed, and the habit of physical activity is regulated by schooling, with varying degrees of effectiveness and quality. In old age, health systems and community medicine usually incorporate guidelines that recommend moderate physical activity, with advice on the value of walking, swimming, or going to gyms and social health centers. These efforts, sometimes, are not always successful. However, during the mature adult years  that precede old age, the adult population seems to be under pressure from work and family responsibilities, leaving little time for personal attention to preventive health and well-being needs. Some research  has revealed the challenge of practicing physical activity or sport in this period of the life cycle. The responsibilities of early adulthood are self-regulated by the experience and years of mature adulthood, and it is at this stage that the practice of physical activity and/or sport for optimal fitness becomes a challenge, because it is known to benefit the individual’s overall health .
Related to active living and physical exercise is the concept of physical fitness, a well-known and powerful health marker  among middle-aged populations, it is even more powerful than physical activity but we must understand physical fitness as a concept broader than one related exclusively to biological health; it can be defined as the ability to carry out daily tasks with vigor and liveliness, without excessive fatigue, and with enough energy remaining to enjoy leisure time or to cope with unexpected emergencies . Therefore, in addition to being related to biological health, physical fitness is also closely related to psychosocial factors on the human spectrum and has been found to influence fitness parameters . However, few studies present data associating physical fitness in adults with it is psychosocial benefits. It is known that, as a method of achieving general well-being, physical fitness has a large regulated role in the negative relationship between the sedentary life and quality of life . Thus, knowing the levels of physical fitness can be an important tool in providing specific advice to the population in relation to their well-being .
However, although it is known that physical activity and improved physical fitness generate benefits and play a fundamental role in both biological and psychological well-being , it cannot be taken for granted that adults currently incorporate it into their daily routines .
The self-evaluation that is carried out on one´s own activities is called self-efficacy . Expectations of self-efficacy refer to beliefs about personal abilities and the ability to satisfactorily carry out the necessary demands in different situations. Losses inherent to the aging process, such as those related to physical functioning, can affect how one believes in one´s control, or loss of control of self-efficacy . Fortunately, the practice of physical exercise can alleviate these consequences . However, even though people understand the beneficial effects of healthy habits on their own bodies and on their overall well-being and health, we are not sure if there is reciprocity between this knowledge and the integration of physical exercise into their life routines .This may seem a paradox in relation to classical theories of motivation towards physical exercise, which emphasize the role of rationality in the decision-making process . It is here that the concept of self-efficacy for physical exercise becomes important, since it determines in part one’s motivation to practice physical activity and is one of its most powerful predictors .
As a result of these considerations, empirical evidence suggests the important role that the relationship between self-efficacy and the practice of physical activity and exercise performance can play; however, the relationship and influence between self-efficacy and quality of life in terms of physical fitness during mid-life remains relatively limited and therefore does not provide clarifying results. Furthermore, this relationship appears to be very important if we consider that physical fitness is a factor intimately related to well-being and quality of life, as well as a quantitative aspect of each person’s physical functioning—functioning that declines as one ages, therefore, analysis of the relationship between these constructs appears to be an interesting hypothesis for a systematic review. To this end, the general objective of this study was to carry out an exhaustive review of the existing literature delve deeper into this topic. In particular, a specific objective that was established, review the measurement instruments for the specific variables.