Elevation and orientation of external loads influence trunk neuromuscular response and spinal forces despite identical moments at the L5–S1 level Z. El Ouaaid a , A. Shirazi-Adl a,n , A. Plamondon b , N. Arjmand c a Division of Applied Mechanics, Department of Mechanical Engineering, École Polytechnique, Montréal, QC, Canada b Institut de recherche Robert Sauvé en santé et en sécurité du travail,Montréal, QC, Canada c Department of Mechanical Engineering, Sharif University of Technology, Tehran, Iran article info Article history: Accepted 27 June 2014 Keywords: Spine Load height Load orientation Finite element Compression Muscle forces abstract A wide range of loading conditions involving external forces with varying magnitudes, orientations and locations are encountered in daily activities. Here we computed the effect on trunk biomechanics of changes in force location (two levels) and orientation (5 values) in 4 subjects in upright standing while maintaining identical external moment of 15 Nm, 30 N m or 45 Nm at the L5–S1. Driven by measured kinematics and gravity/external loads, the finite element models yielded substantially different trunk neuromuscular response with moderate alterations (up to 24% under 45 Nm moment) in spinal loads as the load orientation varied. Under identical moments, compression and shear forces at the L5–S1 as well as forces in extensor thoracic muscles progressively decreased as orientation of external forces varied from downward gravity (901) all the way to upward ( 251) orientation. In contrast, forces in local lumbar muscles followed reverse trends. Under larger horizontal forces at a lower elevation, lumbar muscles were much more active whereas extensor thoracic muscle forces were greater under smaller forces at a higher elevation. Despite such differences in activity pattern, the spinal forces remained nearly identical ( o6% under 45 Nm moment). The published recorded surface EMG data of extensor muscles trend-wise agreed with computed local muscle forces as horizontal load elevation varied but were overall different from results in both local and global muscles when load orientation altered. Predictions demonstrate the marked effect of external force orientation and elevation on the trunk neuromuscular response and spinal forces and questions attempts to estimate spinal loads based only on consideration of moments at a spinal level. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction In occupational, sportive and recreational activities the human trunk is subject to a wide range of loading conditions with varying magnitudes, orientations and locations. In addition to the gravity and inertial loads, these could include carrying weights in hands, pushing/pulling objects and controlling/resisting loads and motions. Both loading condition and spinal posture are expected to influence the trunk neuromuscular response, spinal loads and hence the risk of injury and back pain. The effect of changes in spinal posture and loading on the trunk response has extensively been investi- gated in vivo by the measurement of L4–L5 intradiscal pressure (Wilke et al., 1999, 2001), instrumented implants (Rohlmann et al., 2013) and muscle electromyography (EMG) activity (Hoozemans et al., 2004; Knapik and Marras, 2009; McGill et al., 1996) as well as by musculoskeletal biomechanical model studies (Arjmand et al., 2006, 2009, 2012; Bazrgari et al., 2008; Cholewicki and McGill, 1996; El Ouaaid et al., 2009; El-Rich et al., 2004; Merryweather et al., 2009). The effect of alterations in a loading parameter on the trunk response was systematically considered however only in few studies. In upright standing, Granata and Orishima (2001) altered the height of a weight held in hands at a constant lever arm to the L5–S1 segment and recorded higher muscle coactivities with elevation. This increase in coactivity was also predicted in a model study only when abdominal antagonistic coactivity and hence stability criterion was introduced (El Ouaaid et al., 2013a) that yielded as a result greater spinal compression forces. Kingma et al. (2007) measured surface EMG under horizontal anterior loads applied in upright standing posture at the T3, T6 and T9 vertebral Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com Journal of Biomechanics http://dx.doi.org/10.1016/j.jbiomech.2014.06.036 0021-9290/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author at: Division of Applied Mechanics, Department of Mechanical Engineering, École Polytechnique, PO Box 6079, Station ‘Centre-ville’, Montréal, QC, Canada H3C 3A7. Tel.: þ1 514 340 4711x4129; fax: þ1 514 340 4176. E-mail address: aboulfazl.shirazi@polymtl.ca (A. Shirazi-Adl). Journal of Biomechanics 47 (2014) 3035–3042