Three‐dimensional architecture of human medial gastrocnemius fascicles in vivo: Regional variation and its dependence on muscle size

Abstract

Fascicle architecture (length and pennation angle) can vary regionally within a muscle. The architectural variability in human muscles has been evaluated in vivo, but the interindividual variation and its determinants remain unclear. Considering that within-muscle nonuniform changes in pennation angle are associated with change in muscle size by chronic mechanical loading, we hypothesized that the regional variation in fascicle architecture is dependent on interindividual variation in muscle size. To test this hypothesis, we reconstructed fascicles three-dimensionally along and across the whole medial gastrocnemius in the right lower leg of 15 healthy adults (10 males and 5 females, 23.7 ± 3.3 years, 165.8 ± 8.3 cm, 61.9 ± 11.4 kg, mean ± standard deviation) in neutral ankle joint position with the knee fully extended, using magnetic resonance diffusion tensor imaging and tractography. The 3D-reconstructed fascicles arose from the deep aponeurosis with variable lengths and angles both in sagittal and coronal planes. The fascicle length was significantly longer in the middle (middle-medial: 52.4 ± 6.1 mm, middle-lateral: 52.0 ± 5.1 mm) compared to distal regions (distal-medial: 41.0 ± 5.0 mm, distal-lateral: 38.9 ± 3.6 mm, p < 0.001). The 2D pennation angle (angle relative to muscle surface) was significantly greater in distal than middle regions, and medial than lateral regions (middle-medial: 26.6 ± 3.1°, middle-lateral: 24.1 ± 2.3°, distal-medial: 31.2 ± 3.6°, distal-lateral: 29.2 ± 3.0°, p ≤ 0.017), while only a proximo-distal difference was significant (p < 0.001) for 3D pennation angle (angle relative to line of action of muscle). These results clearly indicate fascicle's architectural variation in 3D. The magnitude of regional variation evaluated as standard deviation across regions differed considerably among individuals (4.0-10.7 mm for fascicle length, 0.9-5.0° for 2D pennation angle, and 3.0-8.8° for 3D pennation angle), which was positively correlated with the muscle volume normalized to body mass (r = 0.659-0.828, p ≤ 0.008). These findings indicate muscle-size dependence of the variability of fascicle architecture.

Coronal As Well As Sagittal Fascicle Dynamics Can Bring About a Gearing Effect in Muscle Elongation by Passive Lengthening

“…The fiber tracking was initiated bidirectionally at a 0.1-mm step from a seed point randomly assigned within the muscle belly segmentation and repeated until 5000 fibers were extracted. The stopping criteria of fiber tracking were as follows: fractal anisotropy <0.1 or >0.5, the angle between two consecutive steps >30°, fiber length <10 mm or fiber length >200 mm, or if the tracked fiber reached outside the MG segmentation (24,35,36).…”

“…The fascicle orientation is essentially in 3D, i.e., not limited to the sagittal plane (21)(22)(23). Recently, we showed the medial gastrocnemius (MG, known as a highly pennated muscle) fascicles in humans in 3D using diffusion tensor imaging (DTI) and tractography in vivo (24). It was found that fascicles ran with angles in the coronal (parallel to the aponeurosis) as well as the sagittal plane of the muscle belly (24).…”

Impairments in muscle shape changes affect metabolic demands duringin-vivocontractions

“…Ultrasound is a two-dimensional imaging technique, but fascicle orientation is not limited to the sagittal plane. Using magnetic resonance imaging analysis, Takahashi et al [38] showed that GM fascicles present an angular orientation in both the coronal and sagittal plane of the muscle belly. Since fascicle rotation could occur in both planes, threedimensional gearing is expected to be larger than twodimensional gearing, and its effect on metabolic energy expenditure could be even more significant.…”

Distribution and length of muscle spindles and their 3D visualisation in the medial gastrocnemius of male and female rats

“…The extrafusal muscle fibres of the medial gastrocnemius muscle have a greater diameter in male rats than in female rats (Mierzejewska‐Krzyżowska et al, 2014). Furthermore, male rats have a larger pennation angle of the medial gastrocnemius than that of females (34.6° vs. 27.3°, respectively) (Manal et al, 2006; Takahashi et al, 2022). Additionally, Chow et al (2000) found that the length of muscle fibres was greater in male individuals than in females.…”