Two Trunk Biomechanical Models Estimate Quite Different ... · symmetric lifting tasks in upright...
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Two Trunk Biomechanical Models Estimate Quite Different Muscle Forces and Spine Loads 1,3 Arjmand, N; 2 Gagnon, D; 3 Plamondon, A; + 1 Shirazi-Adl, A; 3 Larivière, C + 1 Ecole Polytechnique, 2 University of Sherbrooke, 3 Robert Sauvé Health and Safety Research Institute, Montreal, QC [email protected] INTRODUCTION Accurate estimation of trunk muscle forces and spinal loads is essential for proper assessment of occupational injuries as well as improved design of prevention and rehabilitation interventions. In absence of non-invasive measurements, a number of biomechanical modeling techniques are commonly employed to tackle trunk complexity and redundancy. For instance, trunk muscle forces and internal loads are often estimated based on the balance of external and internal moments at only one spinal level (e.g., L5-S1). Earlier investigations have however suggested that the negligence of equilibrium requirements at remaining levels leads to gross violation of equilibrium at these levels thus yielding incorrect estimations [1]. Comparative studies on optimisation and EMG methods using single level models have generally revealed substantial differences in estimated muscle and spine forces. Current work aims to perform a comprehensive comparative study between two trunk models under various symmetric/asymmetric tasks; a single-level optimization-assisted EMG- driven approach (EMGAO) [2] that partitions the external moment at a specific level among trunk muscles and the multi-level kinematics- driven (KD) [3] model that exploits measured kinematics to constrain the redundancy separately at different levels. It is hypothesized that estimated muscle forces and spinal loads in these two models differ due mainly to distinct approaches used to resolve the trunk redundancy. METHODS The required kinematics, force plate and EMG data were collected on one healthy male subject under; (1) upright posture holding a weight at different heights (90, 120, 150, and 180 cm with respect to the ground) in front (10.4 kg) or on side (5.2 kg) at a constant lever arm of 30 cm to the S1, (2) holding a 19.8 kg weight close to or away from the chest and finally (3) symmetric forward flexion. The subject performed these tasks slowly to minimize inertial effects. For normalization, EMG data at maximum voluntary contractions (MVC) were also recorded. Identical individualized spinal geometry, passive properties, trunk musculature and wrapping of global (thoracic) extensor muscles were considered in both models. The KD approach [3] employed a nonlinear finite element model subject to gravity/external loads as well as measured segmental rotations. The trunk redundancy was resolved separately at each level using an optimisation algorithm that partitioned the required moment (due to prescribed rotation) among agonist muscles. Estimated muscle forces were applied as additional external loads and iterations were continued till convergence was reached. The EMGAO model [2] partitioned the external moment (evaluated by a 3D upward dynamic link-segment model) among muscles and the spine at the L5-S1 level while using both normalized EMG data and optimisation. To investigate level-dependency of the EMGAO estimations, foregoing analyses were repeated at the L4-L5 and T12-L1 levels as well. RESULTS In forward flexion and compared to EMGAO model, KD model predicted greater total forces in global extensor muscles with differences decreasing towards peak flexion as activity in back muscles disappeared (flexion relaxation). Both models predicted higher abdominal coactivities at larger flexion angles. Estimated axial compression forces at the L5-S1 disc were different by <33% at all angles except at the peak flexion that reached ~45% (Fig. 1). Similarly, higher forces in both left/right global extensor muscles as well as greater spinal loads that increased both with load elevation were predicted in the KD model in asymmetric lifts at all weight heights (Fig. 2). Moreover, generally higher abdominal activities were predicted by the EMGAO model. In symmetric lifting tasks in upright posture, the KD model also yielded much larger forces in global muscles (from 70% to 87% of total muscle forces) whereas in contrast the local (lumbar) and global muscle forces were evenly partitioned in the EMGAO model. Muscle forces differed in the single-level EMGAO approach depending on the level considered. DISCUSSION There is a need to critically compare existing single- and multi-joint models of the spine. Despite identical geometries and passive properties, KD and EMGAO models estimated significantly different net moments, muscle forces and spinal loads in all tasks. The KD model predicted greater activities in extensor global (thoracic) muscles and larger anterior shear forces as compared to the EMGAO model. This latter difference is a direct consequence of the former as extensor global muscles are less effective to counterbalance anterior shear forces in near upright postures. To examine whether or not computed smaller global muscle forces in EMGAO model were sufficient enough to satisfy equilibrium at upper levels, EMGAO calculations were repeated both in forward flexion at 30º and in upright standing with 19.8 kg in hands while examining the equilibrium at the T12-L1 rather than the L5-S1 joint. In these events, required forces in global extensor muscles substantially increased approaching those computed by the KD model. Global muscle forces predicted by the EMGAO model based on the equilibrium at the L5-S1 were hence too small to balance the moment of external load and gravity at the T12 level. This verification indicates the crucial need to consider equilibrium at all spinal levels [1]. In forward flexion, computed L4-L5 disc pressures (IDP, Fig. 1) favourably compared with in vivo data [4] except at peak trunk flexion where the EMGAO model overestimated measurement due to larger abdominal and local lumbar muscle forces. The observed differences between predictions of muscle forces and spinal loads in these models were due primarily to different approaches used to resolve the redundant equations and not as much to differences in net moments or abdominal co activities. Predictions of the single- level EMGAO model were level-dependent; muscle forces estimated at the L5-S1 did not satisfy equilibrium at higher levels. Differences in predictions of such models would vary depending on the task simulated and joint level considered. In future and to improve estimations, the EMGAO model should incorporate a multi-joint strategy to satisfy equilibrium at all levels while the KD model should benefit from recorded EMG activities of the antagonistic muscles to supplement input measured kinematics. ACKNOWELEDGEMENT: IRSST-Québec and NSERC-Canada. REFERENCES: [1] Arjmand et al, ESJ 2007 [2] Gagnon et al, CB 2001 [3] Arjmand & Shirazi-Adl, JB 2006 [4] Wilke et al, CB 2001. Fig. 1 Fig. 2 Poster No. 1485 • 56th Annual Meeting of the Orthopaedic Research Society
Transcript of Two Trunk Biomechanical Models Estimate Quite Different ... · symmetric lifting tasks in upright...
Two Trunk Biomechanical Models Estimate Quite Different Muscle Forces and Spine Loads
1,3Arjmand, N; 2Gagnon, D; 3Plamondon, A; +1Shirazi-Adl, A; 3Larivière, C +1Ecole Polytechnique, 2University of Sherbrooke, 3Robert Sauvé Health and Safety Research Institute, Montreal, QC
INTRODUCTION Accurate estimation of trunk muscle forces and spinal loads is essential for proper assessment of occupational injuries as well as improved design of prevention and rehabilitation interventions. In absence of non-invasive measurements, a number of biomechanical modeling techniques are commonly employed to tackle trunk complexity and redundancy. For instance, trunk muscle forces and internal loads are often estimated based on the balance of external and internal moments at only one spinal level (e.g., L5-S1). Earlier investigations have however suggested that the negligence of equilibrium requirements at remaining levels leads to gross violation of equilibrium at these levels thus yielding incorrect estimations [1]. Comparative studies on optimisation and EMG methods using single level models have generally revealed substantial differences in estimated muscle and spine forces. Current work aims to perform a comprehensive comparative study between two trunk models under various symmetric/asymmetric tasks; a single-level optimization-assisted EMG-driven approach (EMGAO) [2] that partitions the external moment at a specific level among trunk muscles and the multi-level kinematics-driven (KD) [3] model that exploits measured kinematics to constrain the redundancy separately at different levels. It is hypothesized that estimated muscle forces and spinal loads in these two models differ due mainly to distinct approaches used to resolve the trunk redundancy. METHODS The required kinematics, force plate and EMG data were collected on one healthy male subject under; (1) upright posture holding a weight at different heights (90, 120, 150, and 180 cm with respect to the ground) in front (10.4 kg) or on side (5.2 kg) at a constant lever arm of 30 cm to the S1, (2) holding a 19.8 kg weight close to or away from the chest and finally (3) symmetric forward flexion. The subject performed these tasks slowly to minimize inertial effects. For normalization, EMG data at maximum voluntary contractions (MVC) were also recorded. Identical individualized spinal geometry, passive properties, trunk musculature and wrapping of global (thoracic) extensor muscles were considered in both models. The KD approach [3] employed a nonlinear finite element model subject to gravity/external loads as well as measured segmental rotations. The trunk redundancy was resolved separately at each level using an optimisation algorithm that partitioned the required moment (due to prescribed rotation) among agonist muscles. Estimated muscle forces were applied as additional external loads and iterations were continued till convergence was reached. The EMGAO model [2] partitioned the external moment (evaluated by a 3D upward dynamic link-segment model) among muscles and the spine at the L5-S1 level while using both normalized EMG data and optimisation. To investigate level-dependency of the EMGAO estimations, foregoing analyses were repeated at the L4-L5 and T12-L1 levels as well. RESULTS In forward flexion and compared to EMGAO model, KD model predicted greater total forces in global extensor muscles with differences decreasing towards peak flexion as activity in back muscles disappeared (flexion relaxation). Both models predicted higher abdominal coactivities at larger flexion angles. Estimated axial compression forces at the L5-S1 disc were different by <33% at all angles except at the peak flexion that reached ~45% (Fig. 1). Similarly, higher forces in both left/right global extensor muscles as well as greater spinal loads that increased both with load elevation were predicted in the KD model in asymmetric lifts at all weight heights (Fig. 2). Moreover, generally higher abdominal activities were predicted by the EMGAO model. In symmetric lifting tasks in upright posture, the KD model also yielded much larger forces in global muscles (from 70% to 87% of total muscle forces) whereas in contrast the local (lumbar) and global muscle forces were evenly partitioned in the EMGAO model. Muscle forces differed in the single-level EMGAO approach depending on the level considered. DISCUSSION There is a need to critically compare existing single- and multi-joint models of the spine. Despite identical geometries and passive properties, KD and EMGAO models estimated significantly different net moments,
muscle forces and spinal loads in all tasks. The KD model predicted greater activities in extensor global (thoracic) muscles and larger anterior shear forces as compared to the EMGAO model. This latter difference is a direct consequence of the former as extensor global muscles are less effective to counterbalance anterior shear forces in near upright postures. To examine whether or not computed smaller global muscle forces in EMGAO model were sufficient enough to satisfy equilibrium at upper levels, EMGAO calculations were repeated both in forward flexion at 30º and in upright standing with 19.8 kg in hands while examining the equilibrium at the T12-L1 rather than the L5-S1 joint. In these events, required forces in global extensor muscles substantially increased approaching those computed by the KD model. Global muscle forces predicted by the EMGAO model based on the equilibrium at the L5-S1 were hence too small to balance the moment of external load and gravity at the T12 level. This verification indicates the crucial need to consider equilibrium at all spinal levels [1]. In forward flexion, computed L4-L5 disc pressures (IDP, Fig. 1) favourably compared with in vivo data [4] except at peak trunk flexion where the EMGAO model overestimated measurement due to larger abdominal and local lumbar muscle forces. The observed differences between predictions of muscle forces and spinal loads in these models were due primarily to different approaches used to resolve the redundant equations and not as much to differences in net moments or abdominal co activities. Predictions of the single-level EMGAO model were level-dependent; muscle forces estimated at the L5-S1 did not satisfy equilibrium at higher levels. Differences in predictions of such models would vary depending on the task simulated and joint level considered. In future and to improve estimations, the EMGAO model should incorporate a multi-joint strategy to satisfy equilibrium at all levels while the KD model should benefit from recorded EMG activities of the antagonistic muscles to supplement input measured kinematics. ACKNOWELEDGEMENT: IRSST-Québec and NSERC-Canada. REFERENCES: [1] Arjmand et al, ESJ 2007 [2] Gagnon et al, CB 2001 [3] Arjmand & Shirazi-Adl, JB 2006 [4] Wilke et al, CB 2001.
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Poster No. 1485 • 56th Annual Meeting of the Orthopaedic Research Society
