When lifting heavy loads, trunk muscle contraction converts the abdominal and thoracic cavities into a nearly rigid-walled cylinder that provides increased extrinsic stability and allows partial transfer of load away from the spine. Because twisting is a common mechanism of low-back injuries, this study was undertaken to determine if trunk rotation results in a decrease in the extrinsic stability of the spine. We studied the effects of changes in trunk posture on intra-abdominal pressure generated during a maximum effort Valsalva's maneuver (IAP max) in eight healthy volunteers. IAP max during standing combined with trunk rotation was found to be significantly lower than IAP max during standing straight (p < 0.05). IAP max during forward flexion combined with trunk rotation was significantly lower than during forward flexion (p < 0.01). The results of our study indicate that trunk rotation adversely effects the ability to perform a Valsalva's maneuver. We conclude that extrinsic stability of the spine is at a biomechanical disadvantage when the trunk is rotated and thus may be a contributing factor to twisting-type back injuries.