The ability of a partial or full Valsalva maneuver (voluntary pressurization of the intraabdominal cavity) to unload the spine was investigated in four subjects. During the performance of five isometric tasks, intraabdominal and intradiscal pressures and surface myoelectric activities in three lumbar trunk muscle groups were measured. The tasks were carried out without voluntary pressurization of the intraabdominal cavity and then when the subjects performed partial and full Valsalva maneuvers. A biomechanical model analysis of each task was made to help interpret the experimental measurements. Intraabdominal pressure was found not to be an indicator of spine load in these experiments. The Valsalva maneuvers did raise intraabdominal pressure, but in four of the five tasks increased rather than decreased lumbar spine compressions occurred.