Understanding causes and consequences of variation in body composition, cardiorespiratory and muscular fitness

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Physical fitness is essential for disease prevention, maintaining mobility and physical independence. So, to live long and healthy lives, we need to understand how to maximise and maintain our physical fitness for as long as possible. This is the main aim of my work. Physical fitness can be broken down into 3 main subcomponents: body composition, cardiorespiratory fitness (CRF) and muscular fitness. Body composition refers to the amount of muscle, fat, bone and water in the body. CRF refers to the ability of the heart and lungs to supply oxygen to muscles during physical activity. Muscular fitness refers to the ability to do work against a load (and is usually judged by muscle strength). The components of fitness are affected by physical activity and they are associated with major diseases like cardiovascular disease and frailty. Body composition, CRF and muscular fitness develop and change over a lifetime, and, importantly, as adults, we can improve them to benefit our health.Our behaviours can affect our fitness levels, e.g., higher intensity physical activity is associated with better CRF. It is therefore unsurprising that government guidelines recommend doing moderate-to-vigorous intensity activities and strength building activities to improve/maintain our CRF and muscular fitness respectively. However, as a population when we think of physical activity, we tend to think about aerobic activities which mostly benefits CRF. Muscle strengthening activities are often forgotten. My work will quantify the extent to which CRF and muscle strength reduce health and economic burdens and will therefore help us understand the independent effects of these aspects of fitness on health.Body composition, CRF and muscular fitness are interrelated and can influence each other. But, there is still lots we do not know about these complex relationships and how they may change over a lifetime. For example, we do not know whether how long a person is obese is important for CRF. Understanding relationships with obesity in particular is very important and urgent, because compared with older generations, younger generations are accumulating greater exposure to obesity throughout their lives, and the impact of living longer with obesity is unknown. My work will allow us to quantify how, and at what lifestage, obesity is associated with CRF and muscular fitness. It could provide support for interventions earlier in life when behaviour changes (e.g., resistance training to increase muscular fitness), might be easier to implement.'Real life' is complex, and traditional research methods are not able to answer every important health related question. For example, 'how much of the effect of obesity on poor health could be avoided if everybody was strong?' is not a question that can be answered in a lab or by doing a trial. To be able to answer these complex but important questions, this study will use a mix of different methods and datasets. This approach is powerful because there is no single 'best' method or dataset to answer my questions and all the different methods and datasets have different strengths and weaknesses. By using a mix of methods and data, I will be able to more accurately 'triangulate' evidence, by collectively considering results, to identify causality.Using 5 different datasets I will address 3 knowledge gaps: (i) to understand the interrelations between components of physical fitness (muscular fitness, CRF, and body composition), how they affect each other and subsequently cause poor health; (ii) to improve understanding of influences over a lifetime on, and the development of, CRF and (iii) likewise, muscular fitness. My work will impact health policy by, e.g., promoting activity guidelines for specific types of exercise that should be encouraged at the population level and providing evidence for when an ideal life stage might be to promote maintaining high levels of CRF and muscular fitness for as long as possible.