Evaluation of Different Methods to Measure Muscle Mass and Strength in a Population With Class II/III Obesity

Rationale: Weight loss therapies should aim to reduce fat mass while preserving both muscle mass and muscle strength. Consequently, there is a need for validated methods to measure muscle mass and strength. Current methods are either expensive and require trained technicians, or have not been validated in populations with class II/III obesity (BMI \> 35 kg/m2). In our previous studies (MUST-MOP and MUSCLE study), we have validated the use of ultrasound (US) for the measurement of lean mass (a proxy for muscle mass) in a population with obesity. These studies showed that the use of US for the measurement of lean mass was feasible, reliable and valid. The aim of the current study is to validate the use of US for the measurement of lean mass after weight loss in a population of bariatric surgery patients. Objective: Primary objective: To assess the validity of US to measure lean mass after weight loss in a population of bariatric surgery patients. Secondary Objectives: * To assess de differences in lean mass between DXA and US in this study compared to the MUSCLE-study. * To assess how the lean mass has changed after bariatric surgery and the effects of lean mass on total weight loss and resolution of comorbidities * To assess whether the change in lean mass has an effect on serum levels of markers for muscle mass. Study design: This study is an observational follow-up study of the MUSCLE study and will take place in the Centre Obesity Northern-Netherlands (CON) at the Medical Centre Leeuwarden (MCL). Study population: The population will consists of participants, who have participated in the MUSCLE study and have received bariatric surgery at the CON / MCL. Main study parameters/endpoints: The main parameters of this study are: lean mass measured by DXA and US. The main endpoint of this study is the validity of the US measurement of lean mass compared to DXA. Nature and extent of the burden and risks associated with participation, benefit and group relatedness: