SJB will be compared while on standard (weight based, 20-40 IU/kg) and PK tailored prophylaxis(maintaining trough FVIII > 0.015 IU/mL). This is a crossover study will participants spending months 0-6 on standard prophylaxis and then changing over to PK tailored dosing for months 7-18. A comprehensive joint assessment involving bleed history, clinical examination, physical activity, specialist physiotherapy review, X-rays and MRI scanning of bilateral ankles, knees and elbow will be performed at months 0,6 and 18. Haemophilia Joint Health Score (HJHS), International Physical Activity (IPAQ) and EuroQoL 5-Dimensions (EQ5D) Questionnaires will also be performed at these three timepoints.