Observations on intra‐abdominal pressure and patterns of abdominal intra‐muscular activity in man

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Abstract
Cresswell, A. G., Grundström, H. & Thorstensson, A. 1992. Observations on intra‐abdominal pressure and patterns of abdominal intra‐muscular activity in man.Actri Physiol Scand144, 409418. Received 16 July 1991, accepted 11 October 1991. ISSN 0001–6772. Department of Physiology III, Karolinska Institute, and Department of Radiology, Danderyd Hospital, Stockholm, Sweden.The aim was to investigate possible relationships between activities of the individual muscles of the ventrolateral abdominal wall and the development of pressure within the abdominal cavity. Intra‐muscular activity was recorded bilaterally from transversus abdominis, obliquus internus, obliquus externus and rectus abdominis with fine‐wire electrodes guided into place using real‐time ultrasound. Intra‐abdominal pressure was measured intragastrically using a micro tip pressure transducer. Six males were studied during loading and movement tasks with varied levels of intra‐abdominal pressure. During both maximal voluntary isometric trunk flexion and extension, transversus abdominis activity and intra‐abdominal pressure remained constant, while all other abdominal muscles showed a marked reduction during extension. When maximal isometric trunk flexor or extensor torques were imposed upon a maximal Valsalva manoeuvre, transversus abdominis activity and intra‐abdominal pressure remained comparable within and across conditions, whereas obliquus internus, obliquus externus and rectus abdominis activities either markedly increased (flexion) or decreased (extension). Trunk twisting movements showed reciprocal patterns of activity between the left and right sides of transversus abdominis, indicating an ability for torque development. During trunk flexion‐extension, transversus abdominis showed less distinguished changes of activity possibly relating to a general stabilizing function. In varied pulsed Valsalva manoeuvres, changes in peak intra‐abdominal pressure were correlated with mean amplitude electromyograms of all abdominal muscles, excluding rectus abdominis. It is concluded that the coordinative patterns shown between the muscles of the ventrolateral abdominal wall are task specific based upon demands of movement, torque and stabilization. It appears that transversus ahdominis is the abdominal muscle whose activity is most consistently related to changes in intra‐abdominal pressure.