Background: pain lasting for 12 weeks or beyond, which is often referred to as chronic pain, is common for people living with musculoskeletal conditions (e.g. arthritis, low back pain and fibromyalgia). Pain is often not directly related to the degree of muscle or joint damage. Adaptations of the central nervous system (brain, spinal cord and nerves) often occur in chronic musculoskeletal conditions and can influence how we feel pain (central pain hypersensitivity). Pain can impact on muscle activity and movement. Muscle activity is also governed by nerve signals from the central nervous system (neural drive). The goal of this cross-sectional observational study is to investigate whether chronic musculoskeletal pain is associated with altered nerve signalling (neural drive) to skeletal muscles in adults 40 years or over with chronic knee pain, fibromyalgia and healthy pain free volunteers. The main questions it aims to answer are:- * Is central pain hypersensitivity associated with altered nerve signalling to skeletal muscles in adults 40 years or over with chronic pain (knee pain and for fibromyalgia) as well as healthy volunteers? * Is altered nerve signalling to skeletal muscles associated with physical function and disability? Participants will be asked to:- * Have sensory testing to determine how they feel pain * Complete static leg and arm muscle contractions with electrodes on the skin to measure muscle electrical activity * Complete questionnaires * Perform a short set of mobility tasks including walking, rising from a chair and balancing.