SPINE Volume 29, Number 23, pp 2633–2642 ©2004, Lippincott Williams & Wilkins, Inc. Muscle Activity, Internal Loads, and Stability of the Human Spine in Standing Postures: Combined Model and In Vivo Studies Marwan El-Rich, MSc, Aboulfazl Shirazi-Adl, PhD, and Navid Arjmand, MSc Study Design. The load in active and passive spinal components as well as the stability margin in standing postures  load in hands are studied using both compu- tational model and in vivo studies. Objective. To investigate muscle activity, spinal loads, and system stability in standing postures. Summary of Background Data. Study of the human trunk yields a redundant system, the satisfactory solution of which remains yet to be done. Existing biomechanical models are often oversimplified or attempt to solve the problem by equilibrium of loads at only one cross section along the spine. Methods. In vivo measurements are performed to ob- tain kinematics (by skin markers) as input data into model and EMG activity (by surface electrodes) for validation of predictions. A thoracolumbar model, while accounting for nonlinear ligamentous properties and trunk musculature, solved the redundant active-passive system by a novel kinematics-based approach that used both the posture and gravity/external loads as input data. In both studies, neutral standing posture was considered with weights up to 380 N held in hands with arms extended close to the body either in front or on sides. Results. Predicted muscle forces were in satisfactory agreement with measured EMG activities. The activity in extensor muscles significantly increased with the load magnitude when held in front, a trend that disappeared as loads were held on sides. Abdominal muscles remained relatively silent. Large compression forces of 2000 N were computed in lower lumbar levels when 380 N was held in front. Coactivity in abdominal muscles markedly increased internal loads and stability margin. Conclusion. A tradeoff exists between lower loads in passive tissues (i.e., tissue risk of failure) and higher sta- bility margins as both increase with greater muscle coac- tivation. Greater muscle activity observed under load held in front did not necessarily yield larger stability margin as the position of load appeared to play an important role as well. The strength of the proposed model is in realistic consideration of both passive-active structures under postures and gravity/external loads, yielding results that satisfy kinematics, equilibrium, and stability require- ments in all directions along the spine. Key words: muscle, posture, load, finite element method, stability, coactivity, EMG. Spine 2004;29: 2633–2642 The kinematics redundancy in biomechanical models of complex joints, such as the human spine, has presented an obstacle in estimating the muscle forces as well as joint reaction loads. Accurate determination of load dis- tribution among passive and active components of the human trunk in various recreational and occupational physical activities is of prime importance for the deter- mination of optimal postures and exercises, design of implants, and effective prevention, evaluation, and treat- ment of spinal disorders. In vivo studies have been car- ried out to estimate spinal muscle forces and internal loads indirectly by measuring intradiscal pressure 1–3 or loads on fixation systems. 4,5 Because of the absence of noninvasive techniques, biomechanical models have be- come indispensable tools to partition net applied mo- ments to determine muscle forces and internal loads on passive tissues. To overcome the presence of kinetic re- dundancy, various approaches as reduction method, op- timization, EMG-assisted models, or a combination of these have been proposed. 6–8 The reduction method, by neglecting some muscles, grouping them, or assuming a priori relations between them, reduces the problem to a deterministic set of static/ dynamic equations of motion written at a specific level along the spine (e.g., L3–L4 disc midplane). 9 –12 Optimi- zation methods, on the other hand, solve the foregoing equilibrium equations by optimization of some cost (ob- jective) functions constrained with inequality equations on muscle stresses. 8,13–16 Finally, in EMG-assisted mod- els, the measured EMG data of selected muscles are used to partition the net external moments in between passive and active components. The muscle forces are modulated by a Gain factor in an attempt to satisfy the equilibrium of moment equation at the level considered. 7,17,18 Hy- brid optimization-EMG-assisted models have also been developed to minimize the variation in Gains while sat- isfying the moment equations of equilibrium in different planes. 6,19 One major shortcoming in all foregoing biomechani- cal models is in the attempt to satisfy the static or dy- namic balance of net external moments only at a single cross section along the spine (often taken at lower lum- bar levels) and not along the entire length of the spine. In this case, the calculated muscle forces may indeed not satisfy the same equations when written at another level From the Department of Mechanical Engineering, Ecole Polytech- nique, Montre ´al, Que ´bec, Canada. Supported by grants from the IRSST-Que ´ bec and the NSERC-Canada. The manuscript submitted does not contain information about medical device(s)/drug(s). Federal and Professional Organization funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and reprint requests to Aboulfazl Shirazi-Adl, PhD, Department of Mechanical Engineering, Ecole Polytechnique, P.O. Box 6079, Station ‘centre-ville,’ Montreal, Quebec, Canada H3C 3A7; E-mail: abshir@meca.polymtl.ca 2633