text: Loads transmitted during lumbosacral spinal manipulative therapy.
  An in vivo biomechanical study of three separate manipulation procedures administered in random order. A biomechanical computer model estimated the loads passing through the spine at the level of interest. The difference in loading effects from manipulation were contrasted for all six degrees of freedom based on treatment method. Quantitative information was compared with loads predicted by existing biomechanical models for work tasks in materials handling jobs. Benefits to patients with low back pain from manipulation have been reported. Little is known about the biomechanics for any of the several types of procedures available or about how the loads that are applied affect the spine. Studies of isolated forces applied during high-velocity and low-amplitude procedures in constrained conditions have been made. Values have ranged from 20 N to 550 N at rates up to 7101 N/sec. Vertebral motions arising from these forces have been estimated to be up to 0.1 cm and 1.8 degrees. Complex loads that pass from common lumbar procedures through the spine have not been studied. A total of 54 samples from procedures were administered in random order to 11 volunteers. The amplitude of loads was controlled, by intent of the procedure, to be as large as deemed clinically safe. Volunteers were positioned on a specially constructed treatment table capable of sensing forces and moments about three axes. Myoelectric measures of trunk muscles were obtained by surface electrode recordings. Measurements served as input to a biomechanical model that estimated the loads passing through the spine during the procedures. Statistical descriptions of the procedures and their component loads were constructed, and comparisons were made based on the high-velocity and low-amplitude procedure applied and variation in the volunteer's initial posture. Values were contrasted with spinal loads during materials handling activities. Validity and fidelity of the table reaction loads were confirmed. Muscular response during the procedures was negligible and did not enter into the estimates of loads transmitted through the spine. Statistical significance (0.000 < P < 0.180) was found for comparisons of transmitted load components based on high-velocity and low-amplitude procedures and initial posture. Effects on the spine were comparable with those encountered by airline baggage handlers, with 92% of men and 83% of women estimated to be of sufficient strength to sustain them. None of the volunteers experienced any discomfort or complications as a result of the tests. The data reported here support the hypothesis that transmitted loads may be influenced by patient posture and the procedure selected. These loads, however, were more complex than clinically assumed. Estimates of the loads transmitted were consistent with those observed in common tasks requiring lifting and twisting movements.