aDepartment of Medicine

bDepartment of Physiology

cDepartment of Orthopaedic Surgery

dDepartment of Bioengineering, University of California

eVeterans Affairs Greater Los Angeles Healthcare System, Los Angeles, California, USA

Correspondence to Linda L. Demer, MD, PhD, The David Geffen School of Medicine, University of California, Los Angeles, 10833 Le Conte Avenue, Los Angeles, CA 90095-1679, USA. Tel: + 1310 206 2677; e-mail: [email protected]