Circulating osteoprogenitor (COP) cells have gained increasing prominence in musculoskeletal research. This thesis explores the role of COP cells in age-related diseases such as osteoporosis, sarcopenia and frailty. In the process, we developed a new flow cytometric methodology to quantify COP cells and lamin A, an important component of COP cells which showed a strong association with frailty and ageing. This diagnostic method offers a valid and potential tool to diagnose osteosarcopenia and predict frailty and disability in older adults.