Intraabdominal pressure and trunk muscle activity during lifting--effect of abdominal muscle training in healthy subjects.
Isometric training of the abdominal muscles is recommended as a preventive measure against low back complaints in the hope that the increased strength of the abdominal muscles should result in an increased intraabdominal pressure when lifting. There is, however, neither experimental nor clinical proof of this hypothesis. Twenty healthy young men went through intense isometric training of the abdominal muscles for five weeks. Before and after training the subjects were put through a standardized test programme, measuring the strength of abdominal and back muscles, and a series of lifts, 10, 25, and 40 kg, leg lifts and back lifts. The intraabdominal pressure (IAP), and the EMG activity of the oblique abdominal muscles, and of the erector spinae muscle were recorded. It was found that: The strength of the trunk flexors increased markedly after training. The activity of the oblique abdominal muscles when lifting decreased after training, i.e. motor unit recruitment was not improved. The IAP at lifting was not affected by training. The activity of the oblique abdominal muscles was of no decisive importance to the IAP. The strength of the back muscles increased, but the activity of the back muscles at lifts was not affected by training. In back lifts there was no detectable activity of the back muscles during the beginning of the lifting and during a great part of the lowering. In back lifts the maximum activity of the abdominal muscles appeared long before the peak of the IAP which may be of importance with regard to the pathogenesis of inguinal hernia. Further investigation is needed, both to study the effect of training on patients with back complaints, and to study the factors determining the IAP.