Functional significance of the pyramidalis muscle

Short CL¹, Morrison T², Algate K¹, Gladman MA¹, Crotti TN¹, Barras CDJ^1,2,3

¹School of Biomedicine, Faculty of Health and Medical Sciences, Discipline of Anatomy and Pathology, The University of Adelaide, South Australia, Australia
²Jones Radiology, South Australia, Australia.
³South Australian Health and Medical Research Institute, Adelaide, Australia.

Objective: Pyramidalis (py) is a triangular shaped muscle of the anterolateral abdominal wall (AAW). Whilst our understanding of the AAW is comprehensive, the significance of py remains uncertain, with modern-day anatomists regarding it as vestigial. Despite advances in analytic measures of muscle function, to date, there has been no functional study of py. The aim(s) of the current study are to investigate a functional role for py. We hypothesise that py increases linea alba (LA) tension to maximise biomechanical efficiency of the AAW. We also investigate a relationship between py morphological variability (unilateral / bilateral presence, or absent) and differing levels of trunk rotational torque force.
Methods: Shear-wave elastography (SWE) compared LA stiffness in healthy participants (n=8) with and without py contraction. Electromyography (EMG) recorded py muscle activity during functional movements of the trunk. Force strain gauge (FSG) compares trunk rotation peak torque force and trunk rotation endurance in healthy participants, with and without morphological variability.
Results: Mean LA stiffness with py at rest was 16.4 kPa ± 6.4 kPa and with py activity of 26.2 kPa ± 11.1 kPa (p < 0.003). EMG analysis demonstrated increased amplitude and frequency of py during functional movements of the trunk. Current investigations are determining the significance of both increased LA stiffness and the contribution of py activity in trunk biomechanics via FSG analysis.
Conclusion: Py activity is associated with LA stiffness, demonstrating a functional role as part of the AAW, challenging historical presumptions of vestigiality.

Keywords: Pyramidalis; electromyography; force strain gauge.; function; shear-wave elastography

Ethical statement: This research has received approval from the Human Research Ethics Committee (Approval Number: H-2022 – 139). The above experiments were performed with the understanding and consent of the subjects.

Funding statement: This project is supported by an Australian Government Research Training Program Scholarship, and the Royal Australian and New Zealand College of Radiologists (RANZCR) Research Grant – Clinical Radiology.