An investigation of the functional significance of the pyramidalis muscle using shear wave elastography

The pyramidalis (Py) is a small triangular-shaped anterior abdominal wall muscle, the function of which remains uncertain. We used brightness-mode (B-mode) ultrasound to determine Py prevalence and tested the hypothesis that Py contributed to stiffness of the linea alba (LA) using shear wave elastography (SWE). Py prevalence was observed and documented using conventional B-mode ultrasound in 20 participants (7 male). LA stiffness was measured using SWE with Py relaxed and during Py contraction in 10 of the participants (3 male, mean [SD] age 26.3 ± 8.0 years). Py activity was controlled using neuromuscular electrical stimulation (NMES), while lower rectus abdominis activity was monitored using electromyography (EMG). Py was present bilaterally in 15 of 20 (75%) participants, unilaterally in 2 (10%) and absent in 3 (15%). A statistically significant difference (p < 0.002) in the mean LA stiffness was observed when comparing SWE measurements of the Py at relaxed (15.7 kPa ± 6.1 kPa, 95% CI 11.4-20.1 kPa) and during contraction (24.2 kPa ± 10.8 kPa, 95% CI 16.5-32.0 kPa). No statistically significant difference in lower rectus abdominis activity was observed between the presence and absence of NMES (p = 0.517), indicating accurate and isolated Py stimulation. Our results indicate that Py muscle is commonly present (75% participants) and that Py contraction results in a significant increase in LA stiffness, supporting the previously unvalidated hypothesis that the function of Py, at least in part, is to increase LA stiffness and contribute to the biomechanics of the trunk.