| Degenerative disorders of the human spine are associated with exposure to whole body vibration and repeated mechanical shocks. Little is known regarding intervertebral joint forces in response to mechanical shocks experienced in military and industrial vehicles. Spinal acceleration, spinal position, electromyographic activity, and intra-abdominal pressure were measured in volunteers while seated on a multi-axis ride simulator. Subjects were exposed to mechanical shocks ranging in magnitude from 0.5 to 4 g, and 2 to 20 Hz (single sinusoid) wave form in each of the biodynamic axes.; Surface EMG data were used to examine the timing, pattern and magnitude of muscle response to mechanical shocks. The timing and pattern of muscle response relative to acceleration measured at the skin over the spinous processes suggests that the muscle response is due to stretch reflex. Muscle moments estimated from EMG were less than those predicted by the biomechanical model. Despite subjects reporting soreness and fatigue after 4 to 7 hours exposure to repeated mechanical shocks, EMG indices of localized muscle fatigue did not indicate a functional change in muscle status.; A biomechanical model that utilizes measured kinematic data was developed to estimate compressive and shear forces at the L4/L5 intervertebral joint in response to mechanical shock. The model structure includes separate spinal and soft tissue masses, spinal flexion-extension, linear and angular accelerations, flexor and extensor muscle forces, and intra-abdominal pressure. Peak compressive force estimates were greatest for vertical shocks, and ranged from 400 N to 6000 N for 0.5 g to 4 g shocks. Peak shear force estimates were greatest for vertical shocks, and ranged from 50 N to 800 N. Both compressive force and shear force estimates were consistent with material properties of the spine. |
