Effects of variation in external pulling force magnitude, elevation, and orientation on trunk muscle forces, spinal loads and stability.

Nowadays in various daily, occupational and training activities, there are many occasions with forces supported in hands acting at various magnitudes, elevations, and orientations with substantial horizontal components. In this work, we aim to compute trunk muscle forces, stability, and spinal loads under pulling external forces applied at 3 elevations and 13 orientations. Under an identical upright standing posture and upper body weight, the trunk active-passive response is computed using a validated iterative finite element kinematics-driven model. Pulling forces of 80, 120, and 160N are resisted symmetrically in both hands held at 20, 40, and 60 cm elevations above the L5-S1 and oriented each in upward (-90°), inclined upward (-75°, -60°, -45°, -30°, and -15°), horizontal (0°), inclined downward (15°, 30°, 45°, 60°, and 75°) and finally downward in gravity direction (90°). In addition, in all analyses, an antagonist moment of 10Nm is applied in order to generate rather small antagonist coactivity and intra-abdominal pressures of 8-12 kPa are considered when abdominal muscles are active under upward pulling forces. Results demonstrated substantial differences in muscular response, spinal loads, and stability margin as the pulling force elevation, orientation, and magnitude altered. Compression and shear forces at lower lumbar levels peaked under forces at higher elevations acting with downward inclinations. Minimum spinal forces were computed at all elevations under pulling forces in the upward direction. Trunk stability was also maximum under these latter forces pulling upward. These findings have important consequences in rehabilitation, training, and design of safer occupational activities.