Physical Fitness, Exercise Self-Efficacy, and Quality of Life in Adulthood: A Systematic Review

Background: The aim of the present work 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.


Introduction
Today's developed society is subject to great changes, not always of a positive nature, some of which seem to impact health, well-being, and especially the prolongation of life. Staying healthy is important and has an impact on healthy lifestyle [1].

Quality of Life in Adulthood
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 [2,3]. 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 [4,5]. People are not always aware of these changes [6][7][8][9][10]. 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 [11], the sedentary life continues to affect a wide range of the adult population [12].

Active Life as a Quality of Life Enhancer
A review study [13] 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 [14] 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 [15] 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 [16,17].

Physical Fitness as an Indicator of Quality of Life
Related to active living and physical exercise is the concept of physical fitness, a well-known and powerful health marker [18][19][20] among middle-aged populations, it is even more powerful than physical activity [7] 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 [21]. 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 [22]. 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 [23]. 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 [24].
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 [8,10], it cannot be taken for granted that adults currently incorporate it into their daily routines [8,10].

The Role of Self-Efficacy in Maintaining an Active Life
The self-evaluation that is carried out on one's own activities is called self-efficacy [25]. Expectations of self-efficacy refer to beliefs about personal abilities and the ability to satisfactorily carry out the necessary demands in different situations [26]. 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 [2]. Fortunately, the practice of physical exercise can alleviate these consequences [19]. 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 [27].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 [28]. 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 [29].

The Present Study
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.

Material and Methods
We selected articles in the PubMed, Scopus, Web of Science, PsycINFO, database presenting research results on the relationship between quality of life, physical fitness, and exercise self-efficacy in the adult population. They were chosen because they are the largest and most recognized base of abstracts and bibliographic references in the scientific literature worldwide. This search and analysis was conducted from March to October to July 2020.
We used a pattern of argument follow-up based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) protocol [30], which is recommended for the development of bibliographies, systematic reviews, and meta-analyses, where all works included in the journals in the Journal Citation Report (quartiles 1, 2, 3) and the SCImago Journal Rank (quartiles 1 and 2) were examined.
The exhaustive review of each of the articles was managed according to author, title of article, year of publication, language, URL and/or DOI of publication, indexation of the journal, publication location, city or region of the study, number and type of sample, average age of participants, objective(s), methodology, analyses performed, measuring instruments, and techniques used.
The search terms used were: "Exercise" and "Physical Fitness" and "Self Concept" and "Self Efficacy" and "Quality of Life", and a combination of these with the Boolean operator "AND" under the guidelines of PubMed and with the filter belonging to the PubMed database itself, limiting the age stage to "middle-aged". The following search combinations were used: "Exercise and Physical Fitness and Self Concept and Quality of Life", "Exercise and Physical Fitness and Self Efficacy and Quality of Life", "Exercise and Exercise Test and Self Concept and Quality of Life", "Exercise and Exercise Test and Self Efficacy and Quality of Life". Criteria for the inclusion of articles were: (a) the average age of participants was within the range of 40-70 years (in the case of articles that included this data, the criterion <70 years was accepted); (b) that they were empirical articles, or (c) they were articles from bibliographical reviews. The screening of articles was done manually, and the selected papers were included in a general table (see Table 1).  Figure 1 presents all the articles that were selected for this systematic review. Of the 37 articles reviewed focused on clarifying the relationship between fitness parameters and exercise self-efficacy, 32 articles were focused on populations with some type of pathology [6,. Five articles focused on the pathology-free middle-aged population [62][63][64][65][66]. The results obtained from the articles included in this review are shown in the Table 2
Significant gains in fitness levels are shown for those who maintained exercise for 24 to 36 weeks. The exercise group has higher levels of self-perceived fitness. After the intervention, the group that maintained physical exercise showed significant improvements at 24 weeks in self-perceived physical fitness levels. Higher levels of self-efficacy for the intervention group after the program. These improvements are maintained in the subjects who maintained the exercise. No relationships between physical fitness parameters and self-efficacy and/or quality of life are found.

Physical Fitness Variables
Functional performance: Endurance capacity; muscle strength, muscle function. Psychic Variables Self-perceived Physical Fitness: physical function. Exercise Self-efficacy: Self-efficacy for climbing stairs; Pain; self-perceived physical fitness and other symptoms. Quality of Life: Self-perceived physical fitness; General health; Vitality; Mental health; Physical role; Emotional role; Social function; Body pain. (6MWT). Self-Perceived Physical Fitness: The Arthritis Self-Efficacy Scale (ASES).
The Nordic Walking group gets the most important improvements in terms of physical fitness. Self-efficacy and quality of life also improve the most in this particular group in terms of mental health levels. The improvement in the hours spent in the most vigorous physical activity during the follow-up period is maintained. The strength training group improves functional performance and quality of life factors at 12 months, more than group 3. Quality of life improves more in group 1 and group 2 than in group 3. They do not refer to the relationships between physical fitness parameters and self-efficacy and/or quality of life.
Self-efficacy: Task-Specific Self-Efficacy; The Arthritis Self-Efficacy Scale (ASES). There was a significant improvement in the 6MWT test for both groups. No difference in the comparison between groups. There was no change in both groups in self-efficacy. Physical fitness and the role and physical component of quality of life increased in the exercise group with no difference in treatment. They found no significant correlation between the 6MWT test and self-efficacy and quality of life. Moderate exercise and self-efficacy explained 7.9% of the variation in 6MWT in a multiple linear regression model.

Quality of Life
Exercise Self-efficacy: Exercise Self-Efficacy Scale.

Physical Fitness Variables
Maximal oxygen uptaken Psychic Variables Self-efficacy: Self-perceived physical fitness and general self-efficacy.
The intervention helps to increase exercise levels and reinforce beliefs of self-efficacy. They do not study the changes that occur with 6MWT results. Self-efficacy is the only factor in the intervention that helps reduce depressive symptoms. Exercise and physical endurance are not significant in relation to depression. They do not refer to the relationships between physical fitness parameters and self-efficacy. Group 1 increased their physical exercise practice more than group 2, but there were no significant differences in the aerobic capacity or strength. Exercise Self-efficacy improved for the self-guided group. There are no changes in quality of life associated with body pain. But they do not refer to the relationships between physical fitness parameters and self-efficacy and/or quality of life. The program carried out was effective, increasing adherence to daily physical exercise. The experimental group presented significant improvements in leg and arm strength, self-efficacy and quality of life. They do not refer to the relationships between physical fitness parameters and self-efficacy and/or quality of life.

Physical Fitness Variables
Aerobic capacity Psychic Variables Self-perceived physical fitness: Self-reported mobility. General Self-efficacy: The stable feeling of personal competence to effectively handle a wide variety of stressful situations. Symptom control; Role function; Emotional functioning and communication with physicians. Quality of Life: Mastery; Physical; Psychological; Social; Environmental Physical Fitness: The 10-Min Walk Test (10MWT) Self-perceived physical fitness: Rivermead Mobility Index The experimental group walks faster than the control group and these values are maintained throughout 12 months of follow-up. There are no significant changes in mobility outcomes for any of the groups.
The experimental group obtains better levels of self-efficacy although the values are balanced with the control group over 12 months of follow-up. Quality of life levels are increasing in both groups. No reference is made to the relationships between physical fitness parameters and self-efficacy and/or quality of life.
General Self-Efficacy: The General Self-Efficacy Scale; The Self-efficacy for Chronic Diseases Scales. The results showed a significant increase between groups in the parameters of physical fitness, self-efficacy and quality of life with respect to effect size. There are no appreciable differences between group 1 and group 2 in relation to the variables evaluated. The programs are not effective in their purpose.
No results are presented for the relationships between physical fitness, self-efficacy and quality of life.
Balance Self-efficacy: The Activities-Specific Balance Confidence Scale (ABC Scale).

Quality of Life: Short-Form Health
Questionnaire SF-12.  The results showed significant changes before and after the intervention programs for the two groups. Levels of physical fitness, self-efficacy and quality of life were significantly improved at one year and greater adherence to the training program was shown, resulting in improved perception of health and self-efficacy towards exercise. No reference is made to the relationships between physical fitness parameters and self-efficacy and/or quality of life There are positive correlations between physical fitness through the 6-Min test with the walking and stair-climbing self-efficacy scales. Perceived physical fitness was associated with emotional wellbeing. No correlation was found between self-efficacy and quality of life.
Self-Perceived physical fitness: Individual perception of various aspects of physical condition. Quality of Life: Energy; Fatigue; Wellbeing.
Exercise Self-efficacy:  The experimental group shows improvements after the intervention in the 6MWT test; in walking self-efficacy and in quality of life levels. Follow-up over 1 and 2 years shows that the levels of resistance, self-efficacy and quality of life of the experimental group tend to be balanced with the levels of the control group. They do not refer to the relationships between physical fitness parameters and self-efficacy and/or quality of life.
Exercise Self-Efficacy:  Leg Press. Self-Perceived Physical Fitness: The Arthritis Self-Efficacy Scale (ASES).
No results are presented for the relationships between physical fitness, self-efficacy and quality of life. The group 1 obtains equal or better results in self-efficacy and quality of life after 24 weeks and greater adherence compared to group 2. Psychological benefits may be associated with longer exercise practice affecting mental health and physical fitness.  This program improves perceived barriers to exercise and physical fitness by improving endurance and strength levels, and quality of life. They do not refer to the relationships between physical fitness parameters and self-efficacy and/or quality of life.

Self
Exercise Self-Efficacy: The Self-Efficacy for Exercise Scale (SEE). Quality of Life: The Kidney Disease Quality of Life (KDQOL-36); SF-12 Health Questionnaire. Self-efficacy is positively correlated with quality of life. There is no correlation between self-efficacy and quality of life with the physical fitness parameters studied.
Physical Fitness: The 6-Min Walk Test (6MWT); Cycle Ergometer and Treadmill Based Exercise. Exercise Self-Efficacy: The Physical Activity Self-Efficacy Questionnaire.
The increase in the amount of weekly physical activity, the improvement in physical fitness, self-perceived physical fitness, and self-efficacy due to the intervention process for group 1. They do not refer to the relationships between physical fitness parameters and self-efficacy and/or quality of life. Physical Fitness Variables Aerobic capacity Psychic Variables Self-perceived physical fitness: Physical fitness; Physical strength. Exercise Self-efficacy: Self-perceived physical fitness and self-efficacy to exercise; Perceptions of the ability to overcome barriers to exercise.
Physical Fitness: VO 2 Peak Balke Protocol. Self-perceived physical fitness: The Perceived Importance Profile. Eight-Item Measure of Beliefs in Capabilities. Self-efficacy is inversely related to positive well-being after the implementation of the program. But these do not refer to the relationships between the parameters of physical fitness and self-efficacy. The 6MWTotal and ABC score were each bivariately correlated with steps/d. Self-efficacy score was not significant independent predictor. Self-efficacy: Activities-specific Balance Confidence (ABC Scale)

Discussion
To find the relationship between physical fitness, the role of self-efficacy in physical exercise and physical exercise, and quality of life in the middle-aged population, the systematic review analyzed in detail works published on physical fitness, self-efficacy, and quality of life from 1997 to July 2020. The minimum age of the subjects was 30 years and the maximum age was 80, since there were studies whose age is between these values, even though the average age of the subjects studied was between 40 and 70 years old. A systematic search of the literature was carried out and 37 articles focusing on explaining these relationships were identified. Our results allow us to confirm that there is a relationship between the three explored constructs (physical fitness, quality of life, and self-efficacy in terms of improved health and healthy habits, although the relationship between the three variables in a related way is not entirely clear.
The results have shown that, although there is scientific production that attends to the relationship between the three variables, in most cases the population evaluated is a population with some pathology.
Only in some cases was the evaluated population free of pathologies [62][63][64][65][66] that a variation of the levels of physical fitness affects to the behavior in relation to the barriers towards the physical exercise and of the style of life of the population in consonance as they indicate authors as [50,62]. This is especially relevant since identifying the pathology-free population that regularly exercises and tries to achieve and/or maintain good levels of physical fitness that is one of the main objectives of the current study [67]. All this, together with the novelty of the subject of analysis, means that this subject of study has yet to be clarified and delimited, hence its importance.
On a methodological level, the samples used for the studies was somewhat small: only one study [44] used a sample of 1631 participants, while the others had samples of fewer than 250 subjects. This is due mainly to the fact that these studies were interventions or programs development studies of populations with very specific characteristics; fewer descriptive studies analyze the relationships between the variables under study. This requires us to be cautious when considering the results of the reviewed studies.
The assessment, through evidence, of the capacities that support the physical fitness should be considered as a fundamental aspect in determining the functional capacity of the person. The physical fitness represents a significant influence on the quality of life associated with health, this being a key component in the quality of life [18][19][20]. In relation to the physical fitness variables studied, 30 articles assessed aerobic endurance, and 24 of these used the resistance test called The 6 Min Walk Test. Cardiorespiratory capacity is the main indicator of the subject's state of physical fitness, with maximum oxygen consumption (VO2peak) being the physiological variable that best defines it in terms of cardiovascular capacity. It has been shown that a low level of physical fitness constitutes a major cardiovascular risk factor [67,68] and is a strong and independent factor in all causes of death [69]. In relation to strength, the following were evaluated: general muscle strength; lower body strength; maximum muscle strength of the muscles that mobilize the hand, knee, and elbow; grip strength; maximum strength; maximum grip strength; knee strength; muscle power. It should be noted that various transversal and longitudinal studies have verified that strength decreases with age [70,71], and this decrease is significant starting in the 50s for women and in the 30s or 40s for men [72,73]. It would therefore be advisable to introduce strength exercises into physical activity programs to slow down the process of loss of muscle mass.
On the other hand, given that many of the gestures of daily life require extensive articular paths, this capacity facilitates the functional independence of the person. For this reason, flexibility should be included in recommendations for physical exercise in this phase of life. Flexibility has been evaluated in a small number of studies, although flexibility of the lower and upper body was also assessed [41,[61][62][63]. General mobility, walking and leg mobility, and agility have also been evaluated [34,40,42,43,48,50,54,62]. Static and dynamic equilibrium, which are affected by the progressive loss of sensory-motor function caused by increasing age, were assessed in several studies [42,43,48,53,55,66].
In summary, several studies in this review focused their efforts on understanding what makes a person more consistent in their active exercise behaviors. Many of these, through different types of intervention programs, have shown how increased health perception is linked to increased awareness of personal health status and associated with improved levels of physical fitness [46,50], improved behavior and enhanced adherence [31,39,46,53], and tolerance of sports behavior [6]. Therefore, knowledge of fitness levels can be an important tool in providing specific advice to the population [45]. Being aerobic capacity the most valued capacity and the 6-Min Walk Test the most used.

Self-Efficacy, Fitness, and Quality of Life
Empirical evidence supports the link between exercise self-efficacy and predictions of a variety of health-related behaviors [74,75]. The importance of physical inactivity for public health in the adult population underscores the importance of identifying those physical activity mediators and moderators that can be targeted for interventions to increase physical activity levels [76], being self-efficacy a powerful mediator between physical abilities and physical activity performance [66]. In this review, four articles focused on showing the relationship between physical fitness and exercise self-efficacy, three of which showed a positive relationship between both variables [33,47,55], while on one occasion no relationship was shown between the two [35]. Showing therefore greater tendency that supports the assertions of Bandura [77] that the actual performance of a skill is partially dependent on the perceived ability of the individual to undertake and persist in the achievement of that skill. For example, by limiting the barriers to physical exercise that lead to abandonment or non-participation [50]. These results are consistent with the findings of other studies in which exercise self-efficacy is postulated as a powerful indicator of measures of functional and reflex change in an individual's physical fitness. It is also a determinant in the relationship between physical activity and various aspects of quality of life, including physical and mental health status and life satisfaction [23,66,78,79]. It is therefore desirable to understand in greater depth how to improve self-efficacy towards physical exercise [55].
One's general sense of well-being-being aware of and feeling healthy and adjusted to one's environmental conditions-seems to be an important requirement for developing self-awareness and a satisfying quality of life. Three studies in this review corroborated the relationships between the physical fitness variable and the quality of life variable [33,35,50]. Only Cameron-Tucker's [35] study showed an absence of association between physical fitness and quality of life. These relationships are important because physical condition is a powerful marker of health and quality of life [20] and well-being [24], so it would be very interesting to learn more about these relationships, which have been little studied in the literature. For example, in subjects with chronic stroke, the increase in the number of steps correlates with increases in perceived physical function as a measure of quality of life [50]. On the other hand, if we take into account the importance of the dimensions evaluated for quality of life in middle age and in relation to the other variables analyzed, it should be noted that middle-aged women present more work-family complications and less social support as their perceived benefits of physical fitness increase [80]. It was also found that, among men, low mobility was associated with a lower quality of life in the psychological health domain. This is very important because increased dependence on others and reduced work capacity can be a major challenge for many men [81].
In the studies analyzed, no results have been found that analyze the relationships between the three variables, only the relationships between them two to two, and simply in one [33], the relationships between a measure of exercise self-efficacy and quality of life are analyzed, finding relationships between the 6MWT physical condition test is positively and significantly correlated with walking self-efficacy and with SF-36 physical Subscale but not with mental subscale. But it has not been analyzed, for example, the mediating role that self-efficacy or physical condition can have in relation to quality of life.

Review of Instruments and Measures
In relation to the instruments used in this review, 18 articles evaluated self-efficacy for physical exercise; these focus primarily on evaluating pre-behavioral processes such as change of behavior towards exercise [33,39,49], confidence in designated change towards exercise behavior [6,45,47,54,55], self-perceived capacity to develop sports behavior [45,51], confidence in designated change towards exercise behavior [6,54,55], social support for exercise behavior [46], and self-perceived barriers to exercise behavior [62,65]. Specifically, all of these results are related to Pender contributions, which link healthy behavior to the likelihood of engaging in it and one's sense of self-efficacy. He proposed that self-efficacy for physical exercise has a decisive influence on health behavior, perceived barriers, and commitment to a plan of action [82]. During adulthood there is a slight decline in levels of self-efficacy and mastery, and these influence the perception that there are obstacles to achieving new goals [83]. Therefore, it is essential to improve beliefs about the effectiveness of physical exercise and to promote healthy behavior in the long term.
Quality of life was evaluated with different instruments. Eleven articles used "the SF-36 Health Survey", a questionnaire that provides a clear understanding of what is being measured, how it is used, and the implications for future use. It includes most of the essential concepts for the evaluation of the general health status. It has also proved to be suitable for cross-cultural applications but may be too long for clinical use. In addition, its scoring method is more complicated. The Chronic Respiratory Questionnaire (CRQ), which is one of the available instruments to measure the general health-related quality of life in patients with chronic respiratory condition, and which has been translated into different languages [84]. On 3 occasions the SF-12 Health Questionnaire was evaluated. The SF-12 represents a plausible alternative to the SF-36 for measuring health status, showing only a minimal loss in measurement accuracy in comparation with SF-36 [85]. Other questionnaires analyzed in the results have been used on fewer occasions [46,[54][55][56].
A relevant and conclusive aspect of our review is that a large variety of articles included intervention processes, the results of which focused on checking the possible effects of such interventions on the variables of physical fitness, self-efficacy, and quality of life. These results allow us to assume that, in most cases, the interventions that encourage on physical exercise programs offer benefits for physical fitness, self-efficacy, and quality of life when compared with the control groups, even throughout the follow-up time.

Conclusions
One of the main conclusions of this work is that the important role played by physical fitness and self-efficacy for physical exercise in achieving levels of well-being and quality of life in middle-aged and senior adults. Although one article [33] showed a positive relationship between the three reviewed constructs, the relationships between them are not completely clear. While there is no unanimity on the effects of these variables, it has been found that they are clear predictors of health, they benefit behavioral change, and they have a close relationship that can be mutually influenced. Since current research should try to identify variables that measure and moderate the practice of physical activity in the adult population, these data provide us with vital information that will allow us to deal with the serious problem of physical inactivity in favor of public health [76].
With the objective of promoting integral health, we should raise awareness that prevention should begin before disease appears [86]. However, one of the difficulties among the middle-aged population is lack of time, which undermines this link between personal cultivation and healthy habits. As for the limitations of the study, we should highlight the large age range of the samples examined-a result of the scarcity of studies dealing with this vital period. Likewise, most of the studies we examined referred to subjects with some kind of pathology. Finally, we would add that physical fitness and self-efficacy show a positive relationship, which is important in well-being at this age.