Being able to participate in games and activities with their friends is one of the things that matters most to boys with haemophilia. At present, there is a lack of robust evidence to determine whether muscle strengthening exercise can improve or negatively affect outcomes for young children with haemophilia. With the help of boys with haemophilia, their parents and physiotherapists the investigators have developed an exercise programme designed to increase muscle strength. Using this intervention the investigators will undertake a single-blinded, two-arm pragmatic randomised controlled trial (RCT) of a 12-week intervention verses usual care of boys with haemophilia aged 6-12 years of age.