Since the publication by Bradford and Spurling in 1945 of The Intervertebral Disc, there has been argument about the figure of 1,600 pounds that they calculated as the load on each lower lumbar intervertebral disc when a person lifts a heavy load with the trunk flexed, especially since experiments have shown that intervertebral discs subjected to increasing pressures yield at values well below this figure. In the author's experiments the discs were destroyed by pressures ranging from 350 to 1,400 pounds, with a mean of 710 pounds.
It occurred to the writer that the spine is not necessarily the only structure in the body that can transmit pressure forces from the shoulder to the pelvis. A raised intra-abdominal pressure impacts a thrust under the diaphragm, which will be transmitted to the thoracic spine and the shoulders by means of the ribs. This thrust can take care of part of the lifted weight and thus decrease the load on the spine.
In experiments in which the intra-abdominal pressure was measured by means of a small balloon in the stomach it was found that the pressure rose proportionally with the amount of weight lifted.
It is suggested that the abdominal fluid ball can exert a longitudinal force only if there is no contraction of the longitudinal muscles (at least anteriorly). Electromyographic studies of the abdominal muscles during weight lifting showed that the transverse and possibly the oblique abdominal muscles contract, but not the recti.
It thus seems that the load on the intervertebral discs is not necessarily so great as Bradford and Spurling calculated, but can remain within safe limits. It is hard to give accurate figures for the amount of load that is taken off the spine in this way, but an estimate would put it at several hundred pounds. The importance of a reflex contraction of the abdominal wall during effort as a protective mechanism for the spine must therefore be appreciated. Voluntary contraction may also be called upon to increase the intra-abdominal pressure and so reduce the load on the discs. This is done by many weight lifters.