[1] Univ Aberdeen, Inst Appl Hlth Sci, Epidemiol Grp, Aberdeen AB9 1FX, Scotland
[2] South London & Maudsley NHS Fdn Trust, Physiotherapy Dept, London, England
[3] Keele Univ, Keele Cardiovasc Res Grp, Stoke On Trent, Staffs, England
[4] Univ E Anglia, Fac Med & Hlth Sci, Queens Bldg,Norwich Res Pk, Norwich NR4 7TJ, Norfolk, England