The aim of this project is to understand the impact of a four week preoperative exercise programme on fitness and metabolic rate and weight before surgery. A multi-site randomised trial sponsored by UCL, recruiting patients awaiting bariatric surgery for obesity at University College London Hospital (UCLH), Whittington Health NHS Trust, or Homerton University Hospital NHS Foundation Trust. The investigators will measure fitness, metabolic rate, body weight/composition and grip strength in all participants, before and after a four-week supervised physical activity intervention. The investigators will also interview participants about their participation in the study in regard to the planning of future studies.