What is the Feasibility of the ISTEP Exercise Test in Boys With Haemophilia

iSTEP 10 minute step test, up and down on a commercial standard 3-height adjustable step Undertaking the 10m-MSWT 20-30 minutes Procedure takes place on testing track at Great Ormond Street Hospital; a 5 minute walk from the Haemophilia Centre. One of the research team accompanies the family with child to the track and will accompany them back to the Centre once testing is finished. Muscle Myometry 10 minutes, carried out by one of the research team at the end of the physiotherapy clinical assessment in the Haemophilia Centre. Wearing of accelerometer (activity/ step counter)7 days Device will be set up and provided at the end of the testing on the day of attendance at clinic. They will be instructed to wear this throughout the week at home when they are awake (at least 8 hours). Haemophilia Joint Health Score (HJHS) - a standardised outcome measure of joint health in people with Haemophilia, undertaken by the Clinical Specialist Physiotherapists in Haemophilia.