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Background: The physical inactivity (PIA) prevalence in adults is not improving. Fitness centres (fitness centres, leisure centres, gyms, and any other semi- or non-structured PA providers) are part of the solution as they are a major contributor to the built environment and are used by thousands of adults to start or maintain a physical activity (PA) behaviour. However, it remains unclear how to effectively promote and maintain PA in these populations. To promote PA at fitness centres issues like the low attendance rate (1.1 to 5.6 times/month), the high dropout levels (>60% within the first 3 months), and the difficulties for monitoring PA need to be addressed. Attendance is a more feasible behaviour to target and monitor, and might increase the members' PA levels, but research in this regard is scarce. Behaviour change models like the COM-B Model might help to identify the barriers for attendance, while the Behaviour Change Wheel (BCW) framework might work to inform the developing of the intervention targeting attendance. Therefore, the aims of this thesis are to explore the PA participation at fitness centres, to identify the main barriers and determinants for attendance fitness centres and to develop a behaviour change intervention according to the BCW to target attendance rate at fitness centres. Methods: Three empirical chapters (Chapters 3 to 5) were conducted. Chapter 3 examined the PA levels from a database with 3,627 members of Spanish fitness centres and the PA levels reported in the Eurobarometer 472 for the Spanish population (n=1,002). This comparison was performed considering factors like the age (18-29 years; 30-44 years; 45-59 years; 60-69 years; ≥70 years), gender (men or women), or the origin of the performed weekly PA (i.e., waking-PA, moderate-PA, or vigorous-PA). The PA levels were analysed using the International Activity Questionnaire short version (IPAQ-SF), and respondents were grouped into Low-PA (Inactive), Moderate-PA, and High-PA according to the criteria set for the IPAQ-SF. Chapter 4 designed and validated a questionnaire for measuring factors influencing attendance at fitness centres using the COM-B Model. Three phases were performed: i) content generation of the questionnaire items through literature searches and focus groups, ii) expert review of the relevance and clarity of the items to determine sufficient construct validity, iii) assessment of content validity and internal reliability through the administration of an online survey to 180 participants from British fitness centres and 430 participants from Spanish fitness centres Chapter 5 developed a general intervention to target attendance at fitness centres. The BCW was used as a framework to inform the intervention development. An iterative approach of four iterations were performed to develop the intervention and the Theoretical Domains Framework (TDF) was used to underpinning the mechanisms of change. The questionnaire developed in chapter 4 was used to inform the main determinants and barriers for increase attendance to fitness centres members. Results: Fitness centres' members showed lower prevalence of PIA and a higher prevalence of High PA than the general population regardless the age and gender (p < 0.05). However, women were less active than men in both studied datasets (p < 0.05). Low-PA group and Moderate-PA group from the Eurobarometer dataset reported >70% of their total MET-minutes/week from walking-PA whilst those fitness centre's members reported significant higher engagement in vigorous-PA. Chapter 4 shows a final version of the questionnaire with 35-items approved in the expert review with acceptable construct validity (e.g., Aikens' V for relevance and clarity ≥ 0.97). However, this questionnaire failed the content validity and internal reliability analysis. A shorter version of 17-item questionnaire showed acceptable validity and internal reliability (e.g., confirmatory factorial analysis: CMIN/DF = 204; CMIN = 175.497; DF = 86; CFI = 0.965; NFI = 0.919; RMSEA = 0.50; PClose = 0.457). Chapter 5 identified low-attendance members (< 2 access/week) that reports low-PA levels as the members who might benefit the most from increasing their attendance rate. On the other hand, each item from the questionnaire developed in Chapter 4 was linked to the most promising intervention functions and behaviour change techniques. Furthermore, a mode of delivery for these behaviour change techniques and an example of how to do it was identified. Discussion and conclusion: Fitness centres may play a key role to address PIA as they engage most of their members in regular PA, including women and older adults. Also, members seem to be more active and perform more vigorous-PA than the general population. However, gender and age differences exist in this population, therefore, attention to these factors should be made. Attendance might be an effective way to address PIA at fitness centres. The 17-items questionnaire developed in Chapter 4 has acceptable reliability and validity, so it might be used to develop future interventions targeting members' attendance. Chapter 5 describes the first structured intervention model to enhance attendance rate in low-attendance-and-inactive members of fitness centres. Further research on the testing of the intervention model defined in Chapter 5 in a fitness centre setting could provide a structure and evidenced-based intervention which can be implemented at scale.

Introduction: The measurement of field-based cardiorespiratory fitness (CRF) has been identified as a feasible way to study the link between physical activity (PA) and health in children and adolescents at a population level (Lang et al., 2018a). Indeed, a former Chief Medical Officer for England called for the measurement of CRF as a surveillance tool almost a decade ago, yet no initiative has been put in place (Department of Health, 2010). Therefore, the overarching aim of this thesis was to determine whether there is a case for CRF testing of children in primary schools in England. Methods: Following institutional ethical approval, the following participants were recruited; children aged 8-11 years from 17 primary schools in the London boroughs of Camden and Islington; children aged 9-11 years from nine elementary schools in Birmingham, Alabama and Oakland, California in the US; and seven teachers from the primary schools in the London boroughs of Camden and Islington. CRF was measured by the 20m Multistage Shuttle Run (20mMSR) as part of an initiative called the My Personal Best Challenge (MPBC). Height and weight were also obtained to calculate body mass index (BMI). Testing was completed by trained coaching staff from Premier Education during a Physical Education (PE) lesson. Results: Study 1 involved performing a systematic review, meta-analysis and meta-regression to investigate whether changes in CRF, as a result of a PA intervention, had a significant impact on executive function or academic performance. Key Findings from Study 1: PA interventions can improve CRF in children and adolescents (effect size (ES) = 0.24 [95% confidence interval (CI) = 0.09, 0.40]), yet do not impact executive function (ES = 0.14 [95% CI = -0.12, 0.41]) or academic performance (ES = 0.11 [95% CI = -0.16, 0.38]). Furthermore, the improvements in CRF produced by PA interventions are not associated with changes in executive function or academic performance, including no negative effect on either executive function or academic performance. Given that most of the PA interventions included in the meta-analysis took place in time allotted for other curricular activities, this finding is particularly noteworthy given the health benefits associated with improving CRF. Study 2 aimed to compare the CRF of children who took part in the MPBC from primary schools in London with other studies in English, as well as to provide an international comparison between children who completed the MPBC in England with a convenience sample of children from the US.Key Findings from Study 2: The current data on children's CRF levels in England is limited, with a lack of raw data available, difference in reporting methods, and data restricted to certain regions. However, comparison of group statistics found that children in the MPBC cohort from London had significantly lower CRF than boys and girls from the East of England (p < 0.001) and boys from Liverpool (p < 0.001). Study 2 also demonstrated how the implementation of a CRF testing programme could be completed successfully in two different countries, with children from the MPBC London cohort having significantly higher CRF than children in the US cohort (p < 0.001). Study 3 analysed the relationship between BMI with CRF, as well as applying a CRF cut-point to identify the percentage of children at an increased risk of cardiometabolic disease (CMD) in the different BMI weight categories. Key Findings from Study 3: There was a significant, but weak, correlation between BMI and CRF in children in England (r = -0.35, p < 0.001). Furthermore, over half of children were at an increased risk of CMD due to low CRF (boys = 51.9%, girls = 55.2%), and approximately a quarter of children (boys = 24.1%, girls = 30.5%) were not being identified by current national measures based upon their BMI as they were "lean but unfit". Study 4 explored how CRF levels of children in primary schools in England change over a 3-year period. Key Findings from Study 4: Children's CRF levels improved during the final years of primary school after accounting for age, sex, socio-economic status, and BMI. Children who attended schools where a higher percentage of children were eligible for the Pupil Premium (PP) had significantly lower CRF (20mMSR z-scores mean difference = 0.64 [95% CI = 0.18, 1.1]), especially younger children, though this difference became less pronounced and disappeared by the end of primary school. Study 5 provided a retrospective examination of the perceptions of primary school teachers, through semi-structured interviews on fitness testing of children and collected feedback on the MPBC programme. Key Findings from Study 5: Teachers had positive perceptions overall about fitness testing in primary schools (n = 6) and believed that children enjoyed participating in fitness tests (n = 5). Conclusions: Overall, this body of work has considered the case for whether CRF testing should be implemented in primary schools in England. This thesis has shown that CRF testing in schools can: be used as a tool to evaluate PA interventions, identify children at an increased risk of CMD, track the CRF of children over time and observe how changes in CRF may differ between groups, and make cohort comparisons nationally and internationally. Further, this thesis has demonstrated that teachers have positive perceptions of fitness testing in schools, and that children enjoy participating. As a result of the included studies, this thesis can conclude that there is indeed evidence for the implementation of standardised CRF testing and that it could help develop policy approaches to children's PA, and be used to evaluate their success.

The current thesis investigated the acute physiological response to exercise in end-stage renal disease (ESRD), with the intention of informing evidence-based exercise prescription guidelines and future research directions. Specifically, cardiovascular haemodynamics, and ventilatory gas exchange were measured during submaximal inter-dialytic (non-haemodialysis days) and intra-dialytic cycling (during haemodialysis). A single group of 20 ESRD patients provided data for three separate experimental chapters: 1) comparison of the acute physiological response to maximal and submaximal exercise in ESRD and healthy participants, 2) comparison of the acute physiological response to standard haemodialysis, and submaximal inter-dialytic and intra-dialytic exercise and, finally 3) comparison of cardiac function during standard haemodialysis and intra-dialytic exercise. A number of novel cardiovascular, haemodynamic and ventilatory gas exchange responses were identified. Most notably, intra-dialytic exercise acutely reduced regional wall motion abnormalities (indicative of reduced cardiac stunning) compared to standard haemodialysis. Secondly, chronotropic incompetence related to deranged cardiovascular responses to submaximal exercise in ESRD compared to healthy participants. Thirdly, hypoxia during haemodialysis, as evidenced by abnormal arterial-venous O₂ difference, minute ventilation, cardiac output and respiratory exchange ratio, contributed to an abnormal physiological response to intra-dialytic exercise compared with inter-dialytic exercise. These data proposea novel therapeutic role of intra-dialytic exercise in the acute reduction ofmyocardial stunning during haemodialysis. Collectively, data from this thesishave identified causes of reduced aerobic capacity in ESRD, and the acuteeffect of haemodialysis on the ability to perform exercise. It is likely thatappropriately prescribed, evidence-based exercise rehabilitation can be safeand effective in ESRD. Based on the current findings, moderate intensityinter-dialytic and intra-dialytic aerobic exercise should be advocated in ESRD.

Exercise referral schemes are a widespread intervention aimed to increase physical activity levels in patients referred with medically diagnosed health conditions or risk factors. However, despite evidence of the benefits of physical activity, the clear evidence of whether exercise referral schemes are an effective approach to increase physical activity behaviour is currently limited. The National Referral Database is a newly formed resource for exercise referral specialists to upload patient data including a variety of health and physical activity outcomes for patients, both pre-and post-exercise referral participation. This database contains data from a large cohort of referral patients to schemes across the United Kingdom. Previous research has never reviewed data from such a large cohort across the United Kingdom. This thesis aims to provide initial observations of what may contribute to an effective exercise referral scheme, by examining key schemes’ characteristics, examining whether exercise referral increases physical activity levels, and to review patients’ characteristics as predictors of the likelihood of dropout in a larger sample than previously analysed. This thesis provides novel, scientifically relevant research, as it contains one of the first large-scale studies based on exercise referral, with robust methodologies. These studies provide insights to the varied design, delivery, implementation and evaluation of schemes at present. Firstly, before this research was conducted, there was little recent evidence delivering insights into what is currently happening on the ground, in terms of actual delivery approaches used within schemes. The first study aims to provide insights which would bridge this gap in the literature. It was found that schemes were typically 12 weeks in length, offering participants bi-weekly (twice a week), unsupervised exercise sessions in a fitness gym, using a combination of cardiovascular, resistance, free weights and body weight exercises. Evaluation techniques varied, along with various methods used to check the fidelity of schemes. Secondly, there was a lack of literature examining the change in physical activity levels of participants upon completion. The second study found that participants did report a statistically significant change in the total physical activity, which was mostly accounted for by increases in moderate-vigorous physical activity. However, participants were classified as moderately active before participation, therefore, participants did not see sufficient changes to move them from the moderately active, to the highly active category. Finally, there was a lack of literature examining patients’ characteristics to predict the likelihood of dropout in a large cohort. Results found that the key predictors of dropout included reporting mixed ethnicity, reporting disability as a mental health condition, being a student or permanently sick/disabled, referred by a specialist doctor or pharmacist or referred from a medical centre. Characteristics which reduced the likelihood of dropout included being male, aged 30 years or over, retired, reporting moderate physical activity levels, referred by an exercise specialist or health improvement services, or referred from an outreach service. Some of these results show an agreement with past literature, validating findings in a larger cohort than previously examined, but also provides new evidence of some characteristics which have not been associated with dropout previously. This study provides initial evidence from the national database, which could be used to develop prescriptions of exercise which focus on key patients’ characteristics to avoid dropout. The findings from this thesis have identified that there are issues which need to be addressed within the design, delivery, implementation and evaluation of schemes and exercise prescriptions to provide consistent and rigorous evidence for the effectiveness of exercise referral schemes. This thesis also identified that there are issues with national guidelines which at present, are broad; however, without the evidence-base and examples of best practice to enhance guidelines, these national guidelines are potentially ineffective. To clarify, there needs to be a more standardised approach to the implementation of schemes, but allowing for variation in the delivery of individualised exercise prescriptions. Thus, being something that needs further exploration and adaptation within guidelines.