Health related physical fitness has 3 main subcomponents: body composition, cardiorespiratory fitness (CRF) and muscular fitness. Body composition refers to the percentage 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 3 components of physical fitness are affected by physical activity and are associated with important health outcomes like cardiovascular disease and frailty. Body composition, CRF and muscular fitness develop and change over a lifetime, and, in adulthood, meaningful changes in them are achievable in the general population. Thus, if we want to develop strategies to maintain our health for as long as possible, we need to understand the causes and consequences of these 3 aspects of fitness as well as how they are related to each other.
Behaviours can influence the components of fitness, e.g., higher intensity physical activity is associated with subsequent higher CRF. So, it is unsurprising that government guidelines recommend doing activities to improve/maintain CRF (i.e., moderate-to-vigorous intensity activity) and muscle strength (i.e., strength building activities). However, the general population’s perception of physical activity is skewed towards aerobic activity which mostly benefits CRF. Muscle strengthening activities are often referred to as ‘forgotten’ guidelines. To demonstrate and emphasise the unique added value of the different types of physical activity recommended by the guidelines, we need to understand the independent effects of CRF and muscle strength on health. Moreover, the fitness components are interrelated and can influence each other, but questions remain unanswered, e.g., we do not know whether duration of exposure to obesity is important for CRF or the extent to which CRF influences muscle strength and vice-versa. Understanding interrelationships with obesity in particular is crucial, 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 potentially enormous.
‘Real life’ is complex and outside of a lab or a randomised trial it is difficult to assess cause. However, some extremely important health questions like ‘how much of the effect of obesity on poor health could be avoided if everyone was strong?’ cannot be answered in a lab or by doing a trial. This is where the approach we will adopt is valuable: we will use different analytical methods and different datasets to answer our questions. The datasets and methods we will use have different strengths and weaknesses and taken together they overcome weaknesses of studying cause and effect in a single dataset with a single method. Therefore, our adopted approach is extremely powerful in terms of triangulating evidence for (or against) causality when results from the different datasets and analysis methods are considered collectively. We will use the 5 different national cohorts to address 3 specific knowledge gaps. We aim to better understand the (i) interrelations between components of physical fitness (muscular fitness, CRF, and body composition), how they affect each other and subsequently cause poor health; and to improve understanding of influences over a life-time on, and the development of, (ii) CRF and (iii) muscular fitness. Our work will promote the physical activity guidelines regarding specificity of types of exercise that should be encouraged at the population level and provide evidence for when an ideal life stage might be to intervene to promote maintenance of high levels of CRF and muscular fitness for as long as possible. This work is therefore of relevance, and, will have impact on, health policy. Finally, our work will demonstrate how to make best use of existing data resources.