Chronic low back pain (LBP) affects 50–80% of adults in their lifetimes, yet the injury etiology is unknown. Those exposed to repeated flexion-compression are at a higher risk for LBP, such as helicopter pilots and motor vehicle operators. Live animal lumbar injury models offer insight into in vivo injury mechanisms, but interspecies scaling is needed to relate live animal results to human. Human (n=16) and porcine (n=16) lumbar functional spinal units (FSUs) were loaded in repeated flexion-compression to establish endplate fracture risk. Applied flexion oscillated from 0 to 6° and applied peak stress ranged from 0.65 to 2.38 MPa for human and 0.64 to 4.68 MPa for porcine specimens. Six human and nine porcine specimens experienced endplate fracture. The 95% confidence intervals for human and porcine 50% injury risk curves in terms of stress and cycles overlapped, but the mean porcine injury risk was lower for a given stress level than that for human. An optimised constant stress scale factor of 0.742 on the 50% porcine risk results in a risk curve similar in shape and injury tolerance to human 50% risk. Further studies are needed to establish injury thresholds for tissue failure that may occur before endplate fracture.