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

Abstract

Muscle morphological architecture, a crucial determinant of muscle function, has fascinate researchers since the Renaissance. Imaging techniques enable the assessment of parameters such as muscle thickness (MT), pennation angle (PA), and fascicle length (FL), which may vary with growth, sex, and physical activity. Despite known interrelationships, robust mathematical models like causal mediation analysis have not been extensively applied to large population samples. We recruited 109 males and females, measuring knee fexor and extensor, and plantar fexor MT, PA, and FL using realtime ultrasound imaging at rest. A mixedefects model explored sex, leg (dominant vs. non dominant), and muscle region diferences. Males exhibited greater MT in all muscles (0.1 to 2.1 cm, p < 0.01), with no sex diferences in FL. Dominant legs showed greater rectus femoris (RF) MT (0.1 cm, p = 0.01) and PA (1.5°, p = 0.01), while vastus lateralis (VL) had greater FL (1.2 cm, p < 0.001) and PA (0.6°, p = 0.02). Regional diferences were observed in VL, RF, and biceps femoris long head (BFlh). Causal mediation analyses highlighted MT’s infuence on PA, mediated by FL. Moderated mediation occurred in BFlh, with FL diferences. Gastrocnemius medialis and lateralis exhibited FLmediated MT and PA relationships. This study unveils the intricate interplay of MT, FL, and PA in muscle architecture.