1. Department of Cellular and Integrative Biology, Division of Biomedical Sciences, Imperial College School of Medicine at St. Mary's, Norfolk Place, London W2 1PG, U.K.
2. Institute of Human Performance, University College London, Royal National Orthopaedic Hospital, Brockley Hill, Stanmore HA7 4LP, U.K., and
3. Department of Medicine for the Elderly, Conquest Hospital, St. Leonards-on-Sea, East Sussex TN37 7RD, U.K.