A dynamic biomechanical model to determine loads (joint forces) attained at the lumbosacral joint-centre during the stance phase of normal level walking was developed. The biomechanical model was based on rigid free body-segments; namely, the foot, shank, thigh, pelvis and head, arms and trunk (HAT) segments. In this biomechanical model, the forces and moments acting on the lumbar spine are derived from body-segment mass and movements of the trunk. These external forces and their moments must be equilibrated by internal forces, that is, contributions from the erector spinae or rectus abdominus muscle group, and abdominal pressures from within the trunk together with spine forces. Three-dimensional co-ordinate and ground-reaction force data were captured from five healthy young male subjects performing normal level walking in the motion analysis laboratory of the Department of Orthopaedic Surgery, National University of Singapore. These data were captured using the VICON motion analysis system. Both of these data sets were pre-processed, and together with relevant anthropometric parameters and physical measurements of the subject, were passed into the biomechanical model to compute the resultant loads at the lumbosacral joint-centre. The results of this study showed that the peak resultant loads at the lmbosacral joint-centre were between 1.45 and 2.07 times body-weight.NOMENCLATUREa L instanteneous linear acceleration at centre-of-mass of body-segment (L)D moment arm of abdominal forcee muscle group moment armf(t 0 ) first differential at frame t 0 f(t 0 -t) kinetic parameter, one frame before t 0 f(t 0 +t) kinetic parameter, one frame after t 0 F A abdominal forceF c joint compression force component at the lumbosacral joint-centreF j inter-segment force at proximal joint-centre (j)F L S X x-component of inter-segment force at lumbosacral joint-centreF L S Z z-component of inter-segment force at lumbosacral joint-centreF M muscle forceF s joint shear force component at the lumbosacral joint-centreg gravitational accelerationI L moments of inertia of body-segment (L) about an axis through proximal joint-centre (j)k L radius of gyrationl 1 horizontal distance between centre-of-mass of HAT and lumbosacral joint-centrem b mass of HATm L mass of body-segment (L)M H moment in the sagittal plane about hip joint-centreM j moments about proximal joint-centre (j)M T moment in the sagittal plane about lumbosacral joint-centreP A abdominal pressurer L length of body-segment (L)r L C M length of body-segment (L) from the proximal joint-centre (j) to the centre-of-massF R e s resultant load (joint force) at lumbosacral joint-centret time interval between framesx h x-component of instantaneous linear acceleration at centre-of-mass of HATz h z-component of instantaneous linear acceleration at centre-of-mass of HATα angle (degrees) between horizontal and line joining lumbosacral joint-centre and mid-point of shoulder markersθ H trunk angle (degrees) measured relative to vertical axisθ T thigh angle (degrees) measured relative to vertical axis L instantaneous angular acceleration at centre-of-mass of body-segment (L)