Architectural anatomy of the human tibialis anterior presents morphological asymmetries between superficial and deep unipennate regions

Abstract

The tibialis anterior muscle plays a critical role in human ambulation and contributes to maintaining the upright posture. However, little is known about its muscle architecture in males and females. One hundred and nine physically active males and females were recruited. Tibialis anterior muscle thickness, pennation angle, and fascicle length were measured at rest in both unipennate regions of both legs using real‐time ultrasound imaging. A linear mixed model was used with muscle thickness, pennation angle, or fascicle length as the dependent variables. All models were carried out with and without total leg lean mass and shank length as covariates. Causal mediation analysis was computed to explore the effect of muscle thickness on the relationship between fascicle length and pennation angle. There were no significant differences between dominant and nondominant legs regarding muscle architecture. Muscle thickness and pennation angle were greater in the deep than the superficial unipennate region in males (1.9 mm and 1.1°, p < 0.001) and women (3.4 mm and 2.2°, p < 0.001). However, the fascicle length was similar in both regions for both sexes. The differences remained significant after accounting for differences in leg lean mass and shank length. In both regions, muscle thickness was 1–3 mm greater in males and superficial pennation angle 2° smaller in females (both, p < 0.001). After accounting for leg lean mass and shank length, sex differences remained for muscle thickness (1.6 mm, p < 0.05) and pennation angle (3.4°, p < 0.001) but only in the superficial region. In both regions, leg lean mass and shank‐adjusted fascicle length were 1.4 mm longer in females than males (p < 0.05). The causal mediation analysis revealed that the estimation of fascicle length was positive, suggesting that a 10% increase in muscle thickness would augment the fascicle length, allowing a 0.38° pennation angle decrease. Moreover, the pennation angle increases in total by 0.54° due to the suppressive effect of the increase in fascicle length. The estimated mediation, direct, and total effects were all significantly different from zero (p < 0.001). Overall, our results indicate that the architectural anatomy of the tibialis anterior shows sexual dimorphism in humans. Tibialis anterior presents morphological asymmetries between superficial and deep unipennate regions in both sexes. Lastly, our causal mediation model identified a suppressive effect of fascicle length on the pennation angle, suggesting that increments in muscle thickness are not always aligned with increments in fascicle length or the pennation angle.

3D Ultrasound based Determination of Skeletal Muscle Fascicle Orientations

“…7). This agrees with measurements on the TA of women (n=45), but is in contrast to men (n=64) (Martin-Rodriguez et al, 2023). In our study, due to the small number of participants (6 male, 4 female), we cannot examine these differences statistically.…”

“…Furthermore, determination of pennation angles from 2D images is highly dependent on the correct alignment of the ultrasound transducer (Bénard et al, 2009;Bolsterlee et al, 2016) as it should optimally lie in the fascicle plane. In addition, pennation angles show a high variance within a muscle (Schenk et al, 2013) and regional differences in pennation angles may occur (Martin-Rodriguez et al, 2023) which is difficult to capture and record with 2D ultrasound.…”

Determination of muscle shape deformations of the tibialis anterior during dynamic contractions using 3D ultrasound

“…Such differences in findings may be explained by the different methods used in the studies or differences in TA architecture of the examined subject groups. The TA has a complex bipennate architecture and exhibits sexual dimorphisms in humans ( Martin-Rodriguez et al, 2023 ). Furthermore, the TA presents morphological asymmetries between its superficial and deep unipennate regions.…”

“…Furthermore, the TA presents morphological asymmetries between its superficial and deep unipennate regions. For example, the superficial and deep TA regions differ in thickness and pennation angle ( Martin-Rodriguez et al, 2023 ). Depending on their architecture, muscles and muscle compartments can deform differently in their width and thickness when shortened ( Eng et al, 2018 ).…”

The relationship between muscle thickness and pennation angle is mediated by fascicle length in the muscles of the lower extremities

“…In moderated mediation, the influence of a third variable (moderator) on the mediation effect is explored, adding complexity to the relationship analysis 11 . Our laboratory has recently identified through CMA that the FL of the tibialis anterior muscle seems to have a suppressive effect on the PA, suggesting that increments in MT (i.e., set in 10%) are not always aligned with increments in FL or the PA 12 .…”