Intra-abdominal pressure and trunk muscle activity during lifting. IV. The causal factors of the intra-abdominal pressure rise.

The intra-abdominal pressure (IAP) has been regarded as important for stabilization and relief of the lumbar spine when exposed to heavy loads, such as when lifting. Previous trials, however, have failed to increase the IAP by abdominal muscle training. Twenty healthy subjects, 20 low-back patients and 10 weight-lifters, were tested with various breathing techniques in order to elucidate the causal factors of the IAP rise during lifting and the effects respiration. Those with high IAP and low IAP as well as those with great variations in IAP underwent an extended program. The intra-abdominal and intrathoracic pressures and the EMG of the oblique abdominal, the erector spinae and--in some cases--the puborectalis muscles, were recorded. The transdiaphragmatic pressure was calculated both during lifting and during the Mueller manoeuvre. The IAP rise during lifting seems to be correlated to a good coordination between the muscles surrounding the abdominal cavity. Of these, the diaphragm seems to be the most important for the IAP level. Closure of the glottis seems to help the diaphragm to maintain the IAP rise, otherwise the respiration type seems to be less important for the IAP during lifting.