1. Hugh Williamson Gait Laboratory, Royal Children’s, Hospital, Flemington Road, Parkville 3052, Victoria, Australia.
2. School of Physiotherapy, University of Melbourne, Parkville 3010, Victoria, Australia.
3. Biostatistics Consulting Service, Department of Epidemiology and Preventive Medicine, Central and Eastern Clinical School, Monash University and the Alfred Hospital, Commercial Road, Prahan 3181, Victoria, Australia.