Life expectancy has been increasing for the last 150 years, but the maintenance of health has not kept pace with increased lifespan, and on average, UK adults spend the last decade of life in poor-health, with major consequences for society and the individual. Persistent physical inactivity is thought to be a key contributing factor to the risk of poor health and functional decline occurring in middle-aged and older adults. It is therefore concerning that most middle-aged adults spend \>8hrs/day being sedentary, with average step count of 3000-4000 steps/day. To be able to holistically assess the effectiveness of future strategies to address age-related decline in health, and devise public health messages to help individuals reach older age in better health, it is essential that the complex physiological effects that activity and inactivity have across biological systems are characterised. The goal of this intervention study is to compare the impact of physical activity and inactivity on body functioning. Twenty moderately active participants will decrease their physical activity for three months to match the average amount carried out by middle-aged people in the UK. They will then undertake 3-months of reconditioning training to restore their fitness. In addition, twenty sedentary participants will increase their physical activity to UK recommended levels for six months. Before and at points during the intervention period, participants will be asked to make some measurements at home and attend the University of Nottingham to have multiple assessments made. These include; * fitness, muscle strength and function tests, * completion of questionnaires and computer-based brain puzzles * having muscle and fat tissue biopsies and blood samples taken. * The study also involves having MRI scans. This 5-year study will commence in January 2024, with participant recruitment starting in March 2024 and finishing in May 2027.