The purpose of our study was to determine the impact force to the lumbar spine when football players hit a blocking sled. We quantified the loads at the L4-5 motion segment throughout the blocking sequence. Five Division I-A college football linemen were subjects for our study. Kinematic data were obtained while the subjects hit a blocking sled instrumented with a force plate. Three plane forces were then calculated from these data. The average impact force measured at the blocking sled was 3013 +/- 598 N. The average peak compression force at the L4-5 motion segment was 8679 +/- 1965 N. The average peak anteroposterior shear force was 3304 +/- 1116 N, and the average peak lateral shear force was 1709 +/- 411 N. The magnitude of the loads on the L4-5 motion segment during football blocking exceed those determined during fatigue studies to cause pathologic changes in both the lumbar disk and the pars interarticularis. These data suggest that the mechanics of repetitive blocking may be responsible for the increased incidence of lumbar spine injury incurred by football linemen.