Computational biomechanics of the human spine in static lifting tasks

Since mechanical overload and/or overuse are known as major causes of recreational and occupational injuries to the spine, knowledge of spinal tissue loads is essential for design of effective prevention and treatment programs. Infeasibility of experimental methods to measure these loads has persuaded researchers towards the use of biomechanical modeling techniques. In vitro studies show that the spine devoid of muscles buckles under small loads (∼20 N) suggesting that both mechanical stability and equilibrium analyses are needed to investigate the safe load tolerance limits of the spine.

A novel Kinematics-based approach in which the measured kinematics of the spine are prescribed in a nonlinear finite element model to evaluate muscle/spinal loads and stability in upright standing postures has, recently, been developed by our group. An advantage of this approach is in prescribing the in vivo kinematics data that constrain the spinal deformation under external loading and muscle exertions. Each prescribed kinematics generates an additional equilibrium equation between unknown muscle forces and external loads; thus decreasing the degree of redundancy. If the number of equilibrium equations at a spinal level reaches that of unknown muscle forces, the problem is solved deterministically. Such approach satisfies the equilibrium conditions in all directions along the entire length of the spine and yields spinal postures in full accordance with external loads, muscle forces, and passive tissues. To date, application of this method has been limited to static lifting tasks in upright standing postures.

In the current work, the Kinematics-based approach was applied to compute muscle and spinal loads at different spinal levels as well as system stability in static lifting activities involving forward flexion of trunk. Such lifting activities have been indicated as major causes of injuries to the lumbar spine. Posture, pelvic tilt, and load location were recorded in an in vivo study on 15 healthy male volunteers. Surface EMG electrodes were placed symmetrically to record the activity of superficial muscle groups. Upright standing and forward flexion (∼40° and 65°) postures with different lumbar postures (kyphotic, lordotic, and free) with or without 180 N in hands were considered.

A sagittaly-symmetric model of the T1-S1 spine with 56 muscle fascicles was used. Role of intra-abdominal pressure (IAP) and lumbar posture on loading and stabilizing the spine was also studied.

Compared to the kyphotic postures, the lordotic postures increased extensor muscle activities, compression and shear forces at the L5-S1 disc, and (slightly) the spinal stability while decreasing segmental flexion moments. Considering muscle and spinal loads, results support free style posture with slight flexion as the posture of choice in static lifting.

Compared to the case in which linear line of actions were considered for thoracic muscles, muscle forces and spinal compression at all levels substantially decreased as these muscles took curved paths while not allowing any reduction in their lever arm from upright posture. The presented model and methodology guarantees satisfaction of the equilibrium equations in all directions along the entire length of the spine while yielding spinal postures in full accordance with external/gravity loads, muscle forces, and passive tissues with nonlinear properties. (Abstract shortened by UMI.)